MJLorton Solar Power and Electronic Measurement Equipment Forum
New proof of concept ideas, projects, inventions => New proof of concept ideas, projects, inventions => Topic started by: birrbert on April 11, 2013, 04:48:24 AM
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Hi Forum!
Please allow me to present you my first bigger project: making a variable DC power supply for hobby electronists. In this topic I would like to present to you what I built. Though this is not a how-to or tutorial, I would be glad to help anybody with information about how I solved different issues during the build.
The heart of the power supply is a J-31 DIY kit from the Polish company named Jabel. I chose this because it had all the necessary components (I didn't want to spend too much time researching and hunting transistors or ICs). Main components are: LM324N (http://www.datasheetcatalog.org/datasheet2/9/0oa8seftq8d6peigox0lrx6e9wwy.pdf) - Low Power Quad Op Amp, BD243C (http://www.datasheetcatalog.org/datasheet/boca/BD244A.pdf) - NPN Silicon Power Transistor and BC337 (http://www.fairchildsemi.com/ds/BC/BC337.pdf) - NPN Epitaxial Silicon Transistor.
Schematics and complete parts list:
- PDF: http://www.jabel.com.pl/files/instrukcje/J-031.pdf
- JPG: http://i.imgur.com/Zu6rDvh.jpg
Next, I needed an AC to AC transformer to power the whole thing. I went online to a forum and found a hefty (11 cm x 11 cm x 6 cm) 220 V to 24 V (4 A) toroidal transformer. It actually has two 24 V secondary lines, but I'm using only one.
Other components I bought: two 10-turn wire-wound potentiometers, a simple power meter with an LCD that can measure/display Voltage, Amperage and Wattage, silicone test leads with banana plugs, copper croc clips, female banana sockets (thank you again Franky!), a case and some cables, a couple of switches, crimp-on connectors, nuts and bolts.
To-do list:
- Today I received a nice radiator (20 cm x 5 cm) that I will mount in the back and hopefully it will be able to cool the power transistor. On the photos you will see a small one which couldn't do the job. During my test with a small light bulb which consumed only 0.3 A, the radiator got so hot, that it burned my finger... twice.
- I ordered a DC to DC step down converter which should arrive in 3 weeks time. This is needed to power the LCD (during my test I powered it with an external 12 V adapter). I will see how I can connect it; my plan is to drill some wholes on the PCB right after the capacitors and solder the input wires of the converter there.
- Installing fuses (0.8-1 A on the primary side and 3 A on the secondary side of the transformer).
Upgrade:
- I'm planning to upgrade the power transistor. I would like it to be able to supply 3 A at 30 Volts, but I have no idea whatsoever what kind of transistor I need. It would be great if you could help me with that.
So, I'll let the photos and the short (5 min) video speak for itself. I hope you like! :)
Video: http://youtu.be/jsEM_Bf-yDs
Photos (Part 1): http://www.flickr.com/photos/birrbert/8614658320/in/set-72157633152662250/
Fullscreen lightbox: http://www.flickr.com/photos/birrbert/8614658320/in/set-72157633152662250/lightbox/
Photos (Part 2): http://www.flickr.com/photos/birrbert/8638210242/in/set-72157633214377290/
Fullscreen lightbox: http://www.flickr.com/photos/birrbert/8638210242/in/set-72157633214377290/lightbox/
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I found a few websites today where the more experienced DIYers swapped the standard components that come in the kit package with better ones. I haven't tried any of them yet since I don't fully understand the reasoning, but I will list them below in hope that somebody will shed some light.
- T2: BD243C changed with 2N6341 (http://www.onsemi.com/pub_link/Collateral/2N6338-D.PDF) or 2N3055 (http://www.circuitstoday.com/wp-content/uploads/2009/03/2n3055.pdf) (probably the most important upgrade).
- T1: BC337 changed with BD140 (http://www.datasheetcatalog.org/datasheet2/f/0c695lq7ut3hjriqzff0kfs5i8wy.pdf).
- C1: 2 x 1000 uF 40 V changed with 2 x 4700 uF 50 V or 1 x 10000 uF (better filtering, lower ripple).
- R14: 1 Ohm 5 W changed with 0.1 Ohm or 0.22 Ohm 10 W.
I guess that there are formulas that show the benefit of these higher performance components vs. the standard ones, but is there somebody who could help me understand the math or the theory behind it?
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Hi birrbert,
Could you post links to a forum where this change was described? I guess it's somewhere on elektroda.pl
I can help you in this topic.
Regards,
blankfield
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Hi blankfield!
Here's the first website I found: http://www.edaboard.com/thread239896.html
Your hunch was good! There is an elektroda.pl site too: http://www.elektroda.pl/rtvforum/topic1234478.html
Here you will find in the chapter called 'j-31 -> Zasilacz laboratoryjny 0-30V' a bunch of links to topics where they discuss about upgrading this kit. I've been Google Translating all night last night, but it's very hard to understand, because the translation is not good.
And the last one: http://forum.elportal.pl/viewtopic.php?t=9817
Here the topic starter guy wants to find something about transformers, but he presents other details in the first post.
I'm looking forward to your thoughts! Thanks!
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Output stage of your power supply should be like this:
(http://img713.imageshack.us/img713/8850/oneue.jpg)
Q4 to (Q1||Q2||Q3) is Darlington pair it allow increase of current output { Ie=Ib*(1+(Q4 hfe * Q123 hfe))} in worst case hfe of 2n3055 will be about 20A/A so Ib of Q4 should be about 12,5mA {5A/(20*20)} BD140 is capable of this.
If you can use more capacitors with lower values than one big, simple method to reduce ESR.
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Great posts gents...this will be a project that I take up in the near future too.
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@birrbert: i think your circuit has a Current limit LED, so why not take it to the front panel?
This is mine, just in case it inspires anyone:
(https://lh3.googleusercontent.com/-NdisYc3uAkg/UWSV-rbzePI/AAAAAAAACv8/BhkbxVPZnc0/s902/IMG_2675.JPG)
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Brilliant dr_p....love the analogue mA meter!
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Hi!
I'm sorry that I haven't replied for so long. I had to travel abroad for a week to resolve some business.
dr_p, the circuit does have a current limit LED which I will take out to the front panel, but first I have to figure out what to do with the power transistors.
blankfield, thanks for the schematic, though at a first view I find it hard to understand (I'm still learning how to correctly read a schematic). Where exactly do I have to connect those components? Where do I start and end? Do I need to modify the PCB?
I will need one 3k3 Ohm resistor, one BD140 transistor, three 2N3055 transistors and three 0.1 Ohm resistors, correct?
Another question: do I really need that shunt resistor (0.1 Ohm, 10 W)? I'm asking because the LCD that I have can measure up to 5A.
Thanks! Looking forward to continuing this project! :)
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dr_p, that PSU looks cool with the analogue meter. Looks a bit strange having the seven segment displays next to it though. 8)
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It seems that I'm a bit stuck here. Maybe I should buy a new kit, because I made mistakes (broke a few pads) on the current one.
What I still don't understand is the connection methodology of the new transistors. On the PCB there are three wholes where the original power transistor goes and three other wholes where the smaller transistor goes. I believe I have to use those, but how?
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blankfield, thanks for the schematic, though at a first view I find it hard to understand (I'm still learning how to correctly read a schematic). Where exactly do I have to connect those components? Where do I start and end? Do I need to modify the PCB?
I will need one 3k3 Ohm resistor, one BD140 transistor, three 2N3055 transistors and three 0.1 Ohm resistors, correct?
Another question: do I really need that shunt resistor (0.1 Ohm, 10 W)? I'm asking because the LCD that I have can measure up to 5A.
Hi sorry about delay I'm too busy lately.
You can reduce this resistor to 5W, this resistor must handle I^2*R of power 5^2*0.1=2.5W.
Another mistake is T1 transistor should be NPN BD140 is PNP, sorry about this it is my mistake, replace with BD139.
Bellow you have connection diagram, you need to make connections with wires externally. PCB is to small to accommodate the new elements and changes.
Good luck:)
(http://img109.imageshack.us/img109/1160/beztytuu1oa.jpg)
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Excellent post blankfield! Thank very much and sorry for disturbing in busy times!
I decided to buy a new DIY kit since it's affordable and make a new, clean system (I broke a few pads on the current one). I'll order the new parts as well and connect everything up. I'm hoping to get back here with good results. :)
PS: just a small correction: R4 3.3k on you complementary diagram is actually R5 3.3k on the main diagram, right? also, R14 1 Ohm 10 W will be changed with 0.1 Ohm 10 W or 5 W resistor, correct?
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Yes R4 is the same R5 resistor, R14 should be grater than 2.5W so 5W or 10W will be good.
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Hello Ladies and Gents!
I stopped a little bit with the construction because of the situation at work, but now I'm about to continue. I don't have all the new components yet so I'd like to ask a couple of questions in hope that somebody can shed some light.
1. If I do everything according to blankfield's suggestion how many Amps of output can I expect from the power supply? How can that be estimated?
2. Is there any reason to consider changing the LM324N integrated circuit with something better or it's good and won't burn?
3. What value should the three resistors be? The ones that divide the load equally on the three transistors? Do they need to be also 5 W or smaller value is also OK? I'm just wondering because space is small in the case.
4. How about diodes? Should I buy something better than the default ones?
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Hello Ladies and Gents!
I stopped a little bit with the construction because of the situation at work, but now I'm about to continue. I don't have all the new components yet so I'd like to ask a couple of questions in hope that somebody can shed some light.
1. If I do everything according to blankfield's suggestion how many Amps of output can I expect from the power supply? How can that be estimated?
It will largely be limited by the transformers output VA. If you are using 5W resistors P=I^2*R, 5 = I^2*0.1 transpose the formula for I = sqr(5/0.1) = 7A each trans, may want to beef the shunt resistor to a 20W
2. Is there any reason to consider changing the LM324N integrated circuit with something better or it's good and won't burn?
324 seems to be ok
3. What value should the three resistors be? The ones that divide the load equally on the three transistors? Do they need to be also 5 W or smaller value is also OK? I'm just wondering because space is small in the case.
Start with 5W see what happens
4. How about diodes? Should I buy something better than the default ones?
Why?
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Hi Strada916!
Thanks for the info! The transformer is second hand. It was used in an audio amplifier and it can handle 4-5 Amps easily without getting hot (the guy who made it told me).
Now, I'm a bit confused about the shunt resistor. blankfield wrote that I can reduce it to 5W, but you are saying that I need a 20W. What's the truth? :)
The value of the shunt resistor depends on the value of the three other resistors that divide the load equally on the three transistors? Sorry about the noobish questions; I honestly don't mean to offend anybody; I just want to get a clear explanation how it works.
Regarding diodes, I asked that question only because those which come in the package I'm not sure how many Amps can they handle. I will double check and just make sure that they can handle at least 5 Amps.
edit:
Question nr. 2 was about the LM324N. I read the description of the J31 kit and even though Google Translate does a terrible job in translating from Polish to English I found out that the upper limit of this IC is 33 V. This is actually incorrect, because the datasheet says 32 V. What's the problem with that? My transformer is 25 VAC, but after the diodes I'm getting 36 VDC (measured with multimeter). This 36 V is about right if we multiply 25 with 1.41 (I'm still learning what this factor number is). I have to confirm that with measuring at the legs of the IC, but still, even though 4 extra Volts might not seem too much, I think I need to be aware of that. Plan A to substitute LM324N with something that can handle higher Voltage or plan B would be to get a lower Voltage transformer, e.g. 20-22 VAC, I guess.
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Actually you can replace R5 with a short and use one of the other resistors for current limit. It will save on the one resistor. Use 5W units, and as good a tolerance as you can get, 5% or better, and take the connections for current limit from any one of them. As they share the current you can use one for sense. They will work for up to 5A per resistor, or 15A total without problem, though you will be better off using a lower current like 1A per resistor to reduce the change in resistance with heating.
The 324 will do fine, it is a good unit in this case, simple and robust. The bridge rectifier can be upgraded, a 25A unit will run cooler and not need a big heatsink at all.
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If the diodes you have are 1N4001 or simular then yes get some higher current ones. Also the above circuit will be happy with 15A flowing, however the shunt resistor may melt due to heat at that current. Since you transformer can only output 5A. I would not worry. I see you concern about 324 as your transformer has a higher output voltage. Maybe put some diodes in series to drop the voltage for the IC only.
P = I^2*R this formula is your friend 8)
20W = I^2*0.1
rearrange to find I, I = sqrt(20/0.1) = 14Amps but you always under rate by 50% So there is plenty of head room.
1.41 = sqrt of 2, its derived from the sin wave being positive humps and no longer going negative .
I think the 3 X 2N3055 is over kill, two to be safe, you could even get away with just the one. as the transistor can handle it. Although it would get fairly hot at 5A on its own.
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The 3 2N3055 is actually fine for 5A, as the SOA of the transistors is pretty poor, and 3 sharing will have enough SOA for it to survive a short at full output indefinitely with a big enough heatsink. As well the gain of the transistors will be higher at lower collector current, though you are always able to use better more modern transistors with higher voltage capacity and gain, using these will still be better with 3, as the heat can then be transferred from the dies easier with 3 separate paths.
At 15A a 2N3055 is only guaranteed to have a gain more than 1, it can go up to 10 or more at 3A, reducing the base current required from the driver.
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Reading your replies I started to get the hang of it. :)
To start with I'm going to stick to the original J31 scheme as much as I can, meaning that I will install the shunt resistor, with the change that I'll use a 0.1 Ohm/20 Watt instead of 1 Ohm/5 Watt. This is simpler for me and if it doesn't work out then I'll do SeanB's method of "short and use one of the other resistors for current limit".
I will upgrade the bridge rectifier.
Thanks for the tips about resistor tolerance! In the case of power resistors (5W and above) I couldn't find lower tolerance than 5%. I will use cemented carbon film resistors. All the other resistors I will swap with 1% tolerance metal film resistors (I have a friend who sells them at the price of 1 Cent/piece).
tbc
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Thanks for the tips about resistor tolerance! In the case of power resistors (5W and above) I couldn't find lower tolerance than 5%.
http://uk.farnell.com/vishay-dale/lvr05r1000fe73/resistor-precision-0-1-ohm-1/dp/1108091
Ignore the tempco, it lies.
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Hi Monkeh! Thanks for the heads up, though at first I will go for the cheaper version. Well, these ones are not badly expensive, but I have to pay a lot for shipping. We'll see. By the way, temperature coefficient is explained in the datasheet.
The good news for today is that I received a big heat-sink with 2N3055H transistors on it. I was lucky to find this and be able to buy it for a low price. Plus, I got a couple of cool bridge rectifiers. Check out photos by clicking the numbers below!
1 (http://i.imgur.com/vqfiuzW.jpg) | 2 (http://i.imgur.com/hGqr5Ga.jpg) | 3 (http://i.imgur.com/ZoaIdhL.jpg) | 4 (http://i.imgur.com/4Dm4RK2.jpg) | 5 (http://i.imgur.com/CTiSP68.jpg) | 6 (http://i.imgur.com/VMZ0wTh.jpg) | 7 (http://i.imgur.com/a8BcTh5.jpg).
Only God knows how I will be able to attach this beast to the back of the case. Dimensions: 24 cm long, 10 cm high, 4.5 cm deep.
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By the way, temperature coefficient is explained in the datasheet.
I know, that's why I said ignore the site.
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What are the two transistors in the centre of the heat sink?
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What are the two transistors in the centre of the heat sink?
http://www.svntc.com/TPDF/121.pdf
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What are the two transistors in the centre of the heat sink?
http://www.svntc.com/TPDF/121.pdf
wow. Since they are already on the heat sink can you use them instead of the BD139?
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Yes, you will just need one extra TO3 and one TO66 isolation kit for them and the other 2N3055 that is not isolated. As well remove all the devices, clean the heatsinks and the bases of them and re-apply the heatsink compound and mount them again, the old compound probably has dried out by now.
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Congratulations! 8)
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Thanks! Mersi! :)
But, to be honest, I couldn't finish it without your continuous help. So, thanks guys! I really hope that I can complete the unit in a reasonable amount of time.
edit:
What kind of heat-sink compound do you recommend? I found too many types (with silicone, without silicone, silver based, etc).
edit2:
I went for some very cheap silicone based compound. During next week I'll be getting all the new stuff I bought: a new J-31 kit, insulators, thermal compound, isopropyl alcohol, capacitors, resistors, etc. The list is long, but as they arrive, I'll get to work. For TO-66 I found for a good price on eBay, but that might take a bit longer to arrive to me than all the others.
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Hi Folks!
Just a quick update on this project which has come to a longer halt, unfortunately. I had to travel abroad for the past two weeks and as I returned I had to start some renovation in my apartment so that kept and still keeps me busy. Plus I've got a kid on the line, so in about a month I'll be a father. :)
The good news is that I have all the necessary parts. In addition I bought some nice TO-3 adapters, pin headers, connecting terminalsl all I need now is one or two free afternoons to put everything together.
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edit:
What kind of heat-sink compound do you recommend? I found too many types (with silicone, without silicone, silver based, etc).
edit2:
I went for some very cheap silicone based compound.
I think you made a good choice there. I have bad experience with the silver stuff and mica, because the first and last time I tried, the silver particles somehow found their way through the mica insulation sheet and made a strange metallic fume i couldn't understand where was coming from in the beginning.
Of course I wouldn't hesitate using silver paste if the component can be mounted directly on the cooling element.
Good luck on your project!
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Hi! I'm back! ;)
I am in the need of some clarification: on blankfield's complementary schematic is Q4's Emitter connected to Q1, Q2 and Q3's Base? I mean, Q4's Emitter has to be connected with the other three transistor's Base? And then Q1's Emitter connected to Q2's and Q3's Base simultaneously, plus in the same time Q2's Emitter connected to Q3's Base?
Or I'm misunderstanding the schematic? I see some dots there; they represent junctions and where there is no dot then those lines go to their own separate ways?
That's in mind so confusing as I wrote it down... :o Please clarify! I have everything laid out on the table and wiring up the power transistors and the resistors would be the final step.
edit:
I think I figured it out by trying it live this way: a) connected Q4's Emitter to Q1's, Q2's and Q3's Base by wiring three bases together b) the Emitter of Q1, Q2 and Q3 connected one by one to a resistor c) wired Q1's, Q2's and Q3's Collector together to power them from the positive rail. Then I hooked up the three resistors to the big one and this big one got wired as it would have normally been wired on the original PCB.
But I'm disappointed. The system is working, but everything is running very very hot. No change whatsoever compared to previous attempts, except for the fact that it can deliver higher currents. As a load I used a 24V/100W light bulb. With the current limit set to 2 Amps, after just 4 or 5 minutes the radiator was burning. The KBPC1510W (http://www.cnelectr.net/diodes/pdf/kbpc15005w.pdf) also. I put a cheap thermometer's probe between the wings and the temp rose to 60 degrees Celsius, but I'm sure it was hotter than that. I don't know how to continue. I worked hard in the last three days to wire most of the components nicely and make it work, but I expected better results. Or I shouldn't have?
I made it as modular as possible, so I can change components without having to redo everything. I might try a fan tomorrow, though the whole thing is big already; I wanted it to run passively, so I don't know if I want to make it even bigger by adding fans.
Photos attached (the last one is high res). Every constructive thought is highly appreciated! I'm about to give up, but I spent too much time and money to stop... I guess. Thank you!
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So, the thing is that I can't get away without active cooling. I tried that today. I put a 12 V DC fan just a few centimeters from two power transistors that are closest to each other and everything looked fine. Bot transistors got hot while powering the light bulb, but the temperature seemed to be nicely controlled by the added airflow. I'm glad that at least it works. I went up to 4.2-4.5 Amps, but at that point I realized how hot the bridge rectifier became. Now it's placed just behind the LCD so it gets no extra cooling and it got so hot that it was almost melting the plastic around it. By the way, I swapped the 15 A bridge with a 25 A one, but I'm afraid I'll have to mount that too on the radiator. Or maybe find a more efficient type that runs cooler.
Next week I have some other stuff to care of, but I'm planning to finalize the power supply by the end of the week. In the meantime, every additional thought from you - regarding this project - is highly appreciated! Two more high res photos attached. Thank you!
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Those bridges are meant to be mounted to a heatsink in normal use, preferably a big one if you are drawing full load. You need a sheet of aluminium there to mount it to, probably best to bring it around and mount to the heatsink as well. If you use Shottky diodes you will get less heat, though you will need 4 devices, or 4 PC power supplies to rob the output rectifiers out of if they have a high enough voltage, generally they have a 45V device on the 12V rail that will be in a TO220 package, typically rated at 30A. Use both halves in parallel and make a bridge out of them. Otherwise you can buy them from RS or other suppliers, at 2A they will not need a heatsink, though you can use one if you want.
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birrbert, hello from a fellow Romanian :)
If you think it would help, I could mail you a VSIB620 (200v 6A bridge rectifier) : http://www.vishay.com/docs/84656/vsib620.pdf
I could possibly also add some anodized aluminum heatsink, the one you see in the picture, which would work for this style of bridge rectifer... it's a 7.6c/w heatsink so with a bit of thermal paste, it should still keep the rectifier at about 70-90c when you drive it at 4-5 Amps - it would be hot, but safe to run.
Otherwise, i have some big chunky UF3007 ultra fast around and I could spare 4... but they're only 800v 3A diodes
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Remember what goes in must come out. So. You apply 30 something volts at the input. Or on the collectors of the transistors. Vin - Vout x I is what is dissipated into the heat sink. for example. Vout is 5 and Iout is 2amps the you are dissipating 25 x 2 50 watts in heat. Does that make sense?
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The downside of linear power supplies is that you simply cannot skimp on the cooling. You will need some big chunky heat sinks to keep it cool. It seems to me, from the size of it, that yours is generally too small in size, has not the sufficient surface and so on.
I would try to get hold on a heat sink from a junked 100W (or something like that) audio amplifier, with lots of thin fins on it that gives plenty of surface. Some scavenged CPU coolers with fans should also be worth a try. I believe you can by them pretty cheaply on e-bay. Or, you could also try a small liquid radiator and let the thermal convection do the "pump job", and then have a silent system. I have such a radiator (Thermaltake CPU cooler) here and you can have it for free if you pay the shipping cost, but unfortunately I don't have the cooling block anymore.
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you might want to solder the transistor connections as just wrapping bare wire over the pin is not a good idea as it can fall off and there would be some volt drop there too.
Reason for edit
suggested to use modern transistors, turns out you are.
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MJ15003 might be a better power transistor, it has half the junction to case thermal resistance than the 2n3055, and better electrical characteristics, although the mj is a little more expensive around $5US plus shipping also you would not need three as they can handle more current too. ;D
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Hey guys! I just checked in quickly and am very glad to see so many replies! I can't read them in detail, because my daughter was born today. Our very first child, her name is Sára (Sarah in English). :)
This means that I have to pause the project again for two weeks or so, but when I come back I'll resume and promise to finish the power supply.
Hello mariush! Thanks for the offer. I might test that b.r. I will send you a message later.
Strada916, thanks for the explanation. The dissipated heat make total sense. Thanks for the tip about the transistor too.
runem, the reason I got this radiator is because it had the TO-3 holes. I'll see how I can mount two 80 mm fans on it and if it's efficient and it looks cool then I'll stick with this. Of course, I'm always looking for improvements so I would appreciate if you could send me photos/description about the Thermaltake CPU cooler and the cooling block (just to know what to look for).
Only the best and hope to be back soon!
edit:
Speaking of bridge rectifiers, this question got stuck in my head: what influences how hot or cool a bridge rectifier will run? Is it the current rating? Is it the type/package? Are there know types that keep their cool more than others under stress?
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Congratulations birrbert! :)
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Congrats for Sara !
Speaking of bridge rectifiers, this question got stuck in my head: what influences how hot or cool a bridge rectifier will run? Is it the current rating? Is it the type/package? Are there know types that keep their cool more than others under stress?
It's the forward instantaneous voltage drop and the forward instantaneous current. Let's take for example, the part mariush mentioned ( http://www.vishay.com/docs/84656/vsib620.pdf )
On page 3 you have Figure.3:
(https://lh3.googleusercontent.com/-zM6ZQJVMs_Y/UhSgw9o6LCI/AAAAAAAAC5U/3xkrjgsEv6c/w442-h390-no/Fig.3.jpg)
You are loading your power supply with a continuous 5A of DC current. So the AC side is loaded with 5A RMS (AC) as well, being it's a linear supply (almost all the input current goes to the output). The AC current is jumping up'n'down according to the 50Hz sine, the voltage drop on each diode varies, etc. If we look at the graph and assume a current of 5A DC (constant), we get a voltage drop of 0.62V (at 125C).
But I want to consider the worst case scenario:
(https://lh6.googleusercontent.com/-b3pVo63YeSY/UhSlIoNX9CI/AAAAAAAAC5w/eBgJk9Nuo54/w958-h117-no/Clipboard03.jpg)
Even though the test scenario is at 3A, I don't think I'm making a mistake by considering a constant 5A at 0.95V drop on each diode. That should be more than needed.
At each one time current passes through 2 diodes of the bridge, so the total power dissipation in the bridge is: 2 x 0.95V x 5A = 9.5W
So what heatsink to choose? Well, Dave made a video about that:
http://www.youtube.com/watch?v=8ruFVmxf0zs
Long story short, having a look in the datasheet also reveals this:
(https://lh5.googleusercontent.com/-v5OYtyo60y0/UhSky3XJohI/AAAAAAAAC5g/MaSXETg1hm8/w958-h201-no/Clipboard02.jpg)
Add the Thermal Resistance from Junction to Case and the Thermal Resistance from Case to Ambient and you get R(JA) - from junction to ambient, expressed in degrees C per W. In this scenario 3.4 + 7.6 = 11C/W
So you dissipated 9.5W earlier so the junction temperature will rise by 9.5 x 11 = 104C above ambient, so probably around 130C.
Now we look at the datasheet again:
(https://lh3.googleusercontent.com/-6AGJvD0bOFY/UhSoJ2UOjjI/AAAAAAAAC6M/GgksqrPp_XU/w433-h390-no/Clipboard04.jpg)
and we know that at 130C the bridge can pass 5A maximum. So it's fine to use and we expect a lower junction temperature than 130C, since we over rated the calculations. We then expect a much lower heatsink temperature, because of the thermal resistance between junction and case (also heatsink). Still, probably too hot to touch (70-80C). Incidentally, it's just like mariush said :)
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Of course, I'm always looking for improvements so I would appreciate if you could send me photos/description about the Thermaltake CPU cooler and the cooling block (just to know what to look for).
Congrats with a new family member birrbert!
You were asking for some pics and here they are! If you mount the radiator horizontally the hot water will always rise. Sorry, I don't have the cooling block, but it shouldn't be to difficult to make something yourself, or have someone with a mill to machine one out of an aluminum block for you.
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Thank you very much everybody! Both the mother and child are well. :)
What do you think about installing the cooling fans?
A) Should I bother with temperature based cooling?
B) Should I consider to let the system vary fan speeds as temperatures go higher or lower?
C) Or should I simply buy some nice silent fans, wire them directly and let them be on/blow/spin non-stop?
Maybe option C is the most realistic at the moment. Although option B would be professional, I would need to set up a complex system with some micro controller and stuff, right?
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You get fans with a built in thermistor for temperature based speed control, or simply grab an old PC power supply with the temperature sensing fan and grab the tiny board with the thermistor and a transistor out of it and use that.
Nice cooling block there, even without a pump it will do well, just use radiator antifreeze in it and it will do fine, though the transfer rate will be much reduced by not having a pump to move it, it will be a lot lower in heat transfer capability with only a thermosiphon driving it.
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Back again with results. I mounted two old, 80 mm, 12 V, DC fans on the radiator and they work like a charm (third photo). I took the bridge rectifier too on the radiator and now it stays cool even without thermal paste. Basically everything that's on the radiator (three 2N3055's, one 2N3054 and the bridge rectifier) reaches a maximum temperature of 32 degrees Celsius. On the first photo you can see 28.3 degrees Celsius, the temp of one power transistor. Things change a bit on the second photo, because I lowered the voltage to 10 V and the temp of the same power transistor rose to 36.3 degrees Celsius; I say it's still reasonable.
OK, now what's up with the rest of the components? The values above were the result of a quite long (~1 h) stress test: I used a 24 V/100 W Philips incandescent light bulb as load. The maximum power drawn was around 100 W (23 V/ 4.3 A). The transformer got quite warm (reached 50 degrees Celsius at a given moment). The 20 W resistor went up to 51-52 degrees Celsius. The hottest of all was the 5 W resistor on the LCD: 70 degrees Celsius.
Plus here's an interesting situation: as you see on the second photo, I brought down the voltage to 10 V, current went down to around 2.64 A and a few moments later I heard a high pitched noise. It was coming from the small 100 uF/ 63 V capacitor. I touched it and it burned my finger. I don't have an explanation to this behavior which is not present at higher output voltages.
It seems to me that all major components get hot in certain situations. I might need to drill holes on both sides of the case and mount one more fan on the top cover to blows cool air inside.
Finally, I would like to point out one more thing! This variable power supply has a big disadvantage: it can't stabilize the voltage. :( As I reduce the current with the help of the current pot, the voltage decreases too. Quite sad that I just noticed that. I don't know if there's a reasonable explanation to that, but in my head the voltage should stay still as I lower the current. Or I'm missing something?
As always, your thoughts are welcome! Cheers!
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As you limit the current the voltage across the load will naturally decrease. The load is a relatively constant resistance, so if the voltage is say 12V at 1A it will only drop 6V with 0.5A though it.
The capacitor getting hot means it was either defective, had too high a ripple current through it ( which means the power supply is oscillating, check with a scope or by using a DVM set to AC volts and fed via a 220n isolating capacitor from the output), was a pirate part or was put in backwards.
Good to see the fans do help, though when mounting them it will be better to have a larger gap between the fan and the heatsink, a top and bottom spacer sheet glued to the heatsink and the fans spacing them about the thickness of the fans will increase airflow across the heatsink and lower temperature a lot more, probably by about 10C at the highest power dissipation. You might want to have a few holes in top and bottom of the case for airflow inside for cooling as well, or add a further fan to blow air through the case as well.
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Hi Sean! Thanks for the short explanation about voltage drop while controlling the current, though I'm gonna have to read more about that to fully understand it.
The capacitor was definitely not put in backwards; the polarity is correct. It could be faulty. To be honest, I snapped it off my previous J-31 PCB. With the current kit/PCB I received a 100 uF / 35 V cap and I thought what the hell, the higher the voltage the safer to use, so I desoldered it from the previous PCB and soldered it on the current one. It's either this or the power supply is oscillating. Since I don't have an oscilloscope, I will take it to a friend and check it there.
Regarding fans, I actually left space between them and the radiator. Please see the attached side shot; there is a 19 mm space. I will place a 120 mm or even bigger fan with low rpm on the top to blow fresh air inside the case. Poor power supply will look like the beast of Dr. Frankenstein when I finish it. :o
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Not as bad as some I built, but looks like it will be with you for a long time.
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The first is always special. ;D
What's your opinion about the resistors below?
- OHMITE - 15FR100E - RESISTOR, 0R1, 1%, 5W (http://www.farnell.com/datasheets/66208.pdf)
- BOURNS - PWR220T-35-R100F - RESISTOR, POWER, 0.1R, 0.01, 35W (http://www.farnell.com/datasheets/1697196.pdf)
At a first glance they seem more suitable than those huge cemented resistors that I bought, because the 5 W Ohmite is much smaller and and 35 W Bourns can be mounted on the radiator.
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The TO220 units definitely as they can use the heatsink you have and the existing fans. As they are close to the transistors anyway you will have shorter wiring.
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I'm a bit confused about the value of the resistors. blankfield recommended 0.1 Ohm ones, but if I=V/R, then wouldn't this type of resistor increase the minimum current or mess up something else? ???
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You are measuring voltage(for current limit) "before" those 0.1ohm emitter resistors, so they don't influence things. Well they do slightly - they cause an extra voltage drop, but the op-amp takes care of things, opens up the transistor some more, and in the end a 1A current will still drop 100mV on the current shunt.
But keep them low value, because you lose output voltage on those babies. If you were to use 3 pieces of 10 ohm resistors, you effectively add a series 3.33 ohm resistor to your load. That means that at 3A you drop 10V on just those resistors. So your 0-15V power supply is now only capable of 5V output at 3A, but if you draw less current, there's less voltage drop on the resistors and it's capable of more output voltage. Also, If they are too big in value they limit the maximum current capability for the same reason - too much voltage drop on them, no more voltage left to drive the output at the desired current.
I hope I'm not getting you confused.
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Not fully understood yet, but I'm trying. In the meantime the latest parts arrived and I soldered/wired everything together. Please check attached photos. So, it's not nice, I know, but I tried to do my best. I definitely didn't plan that last PCB with the 5 W resistors. I will be doing wire management later, but with these PVC insulated cables it's hard. Maybe I'll try silicone cables next time, but those are expensive...
It's still not 100% ready, because the main switch is not wired yet, there's no power for the fans, the back case cover has to be installed too and I put no fuses. Please advise in this regard. What kind of fuse and fuse holder do you recommend? Should I put on both places: the primary and secondary of the transformer? I'm a bit tight on space.
At least now I can work on the solar cell phone charger. Yay! ;D I already charged some Li-Ion batteries. :)
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Hi Ladies and Gents!
Almost three months since my last post, yikes! ???
A lot has been going on, hence the huge time-out... family stuff, renovations in the apartment and work all at the same time = no chance to tinker with the power supply. I believe Martin is one person on this forum who roughly knows what I'm talking about.
But, winter came this week with temperatures around -10 degrees Celsius, which hopefully brings along days of peace and quiet. I'd like days like that anyway, because for me Christmas is not Christmas when there's rush; instead it should be all about family, nice mood and force-meat rolls in cabbage leaves (http://www.bubblews.com/assets/images/news/281216218_1357633850.jpg). :)
I'm about to order a few parts off eBay, small but important parts and assemble the PSU for a review on an oscilloscope. I think that once I hit that point I'll have a working unit (Thanks to all of you, again and again!), but in the same time I'll know what kind of changes need to be done to improve it.
Thank you for your patience and I promise to keep you posted more regularly from now on. Cheers!
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Those look very nice, I miss them.
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I installed and activated DesignSpark. After 5 hours of work I came up with this (please see attached files; exports of the work, but I am happy to share the .pcb file with anybody upon request).
The original design helped a lot, but I had to make quite a few changes to it to represent my personal build. This is just the main board. I have to make two smaller ones too (i.e. bridge rectifier with the filtering capacitors, plus the one with the four power resistors).
Now, the big question is, whether this is something Martin had in mind for the oscilloscope competition?
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as a very rough rule, I use 1mm (40mil) of track width per every 1 Amp. If I recall, your design goes up to 3-4A, so either go 3-4mm of track width on the final current path, or (if the PCB is done) add solder or even a 0.5mm copper wire to the track, to lower it's resistance and current caring capability.
I see you thickened the traces a bit, but: trace leading to trimpot PR1 doesn't need to be that thick, it's not caring current. However, the input capacitors traces are caring current, since they supply power when the 50Hz sine is near 0V. Also, slightly larger traces for the output cap, even if it's only 50uF.
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Mersi dr_p! I will make changes accordingly. I'm still trying to figure out how to make things more precise in DesignSpark. I didn't like the 3D view; it would need more details like connecting pins and such.
As you may have noticed, I changed the default colors to colors that are more pleasant to/forgiving with the eyes. The default combination of black background and vivid colors was making me dizzy.
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Hi!
It occurred to me just now that I didn't specify what the input is for this power supply. So, where it says input there it's already clean DC voltage entering the circuit. Rectifying and smoothing takes place elsewhere. I'll wrap everything up hopefully in a video and post it in the giveaway topic... sooner rather than later. It's gonna be cool, count on that. :)
Cheers!
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Hello!
Just to make sure nobody missed the video, here's the link again: http://www.youtube.com/watch?v=T2hxFi8Rmno
Thanks for all who supported me! :)
Now that I have my own oscilloscope let me "bombard" you with questions. Here's the first one: after checking the attached screenshot what is your thought? It's showing a signal while the power supply is turned on, output is turned off and of course oscilloscope probe is connected to the output cables. If I touch the metal parts where the probe and positive output cable meet then the Voltage rises; if I take away my finger then the signal goes back to this size. I feel that something is not right here, but I have no idea what. When I turn the output on then this sine wave signal disappears and the oscilloscope is showing the output DC signal (i.e. a straight line).
edit:
I just checked because I was curious... I see the same signal when the probe is connected to nothing... It's just that the amplitude is low, but when I "zoom in" all the way to 20mV per division I can see it clearly. I hope I'm not making a big thing out of this, but is this normal or what?
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Mains pick up on the open circuit output. Quite normal. add a 1k resistor across the output and it will mostly disappear. This is mains voltage being capacitively coupled into the 1M or 10M scope input impedance, most likely from inside the power supply.
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I'm trying, but I don't understand this. If I completely remove the probe from my oscilloscope, even then there's this signal shown; it's barely visible but it's there. If the probe is connected to oscilloscope then the amplitude is higher, if the probe is connected to my power supply then the amplitude goes even higher, if I touch the metal parts with my finger then the amplitude is even higher. I watched Martin's episodes about using an oscilloscope, but he didn't have this; neither on the Agilent nor the ISO-Tech. This has nothing to do with my power supply project, but still, what gives? Is there something wrong or it's just how the oscilloscope is?
I'm attaching two more screenshots. The first one I made simply with my finger on the tip of the probe; the signal was happily climbing up, then going down and so on randomly. The second I made in Single Shot mode; you can see the sine wave and when I turn the output of the power supply on then you see the ramp up followed by a rough but straight line.
PS: I'll go to bed now cause it's late and I hope I won't be dreaming of waveforms. :)
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Again, a bit off topic, but is it possible that I have a grounding issue? I should fix this somehow because in some cases it's affecting the Single Shot mode, i.e. it's triggering even when I'm not expecting it to.
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Ground the power supply output and you will not have this any more, as the power supply and the scope will have a common ground point. A floating power supply is good, but only if you need a floating power source if you want to have a ground at some point like at a half rail fed from a function generator or a scope probe. Otherwise it is a way to get mains noise into the powered device.
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OK! After reading some more (http://electronics.stackexchange.com/questions/78920/my-scope-detects-a-50hz-signal-when-the-probe-is-not-connected-to-a-circuit-is), I started to understand it. The probe is so sensitive that it picks up mains hum, as they call it, and that's why I see the 50 Hz sine wave. It's not always 50 Hz and not always a sine wave... sometimes it's just gibberish, of course. The source of mains hum can be, in my case at least: the power cable of the oscilloscope, the power cable of the power supply, the power supply itself or the power strip. Since my bench is very small, the oscilloscope can pick the signal up easily from any of these sources, even if the probe is not connected to the oscilloscope at all. Also, I checked by moving the probe around.
As far as I understood, this doesn't interfere when taking an actual measurement, but please correct me if I'm wrong. The other thing worth mentioning is that Martin didn't have this mains hum picked up by any of his oscilloscopes (Xytron, Agilent, ISO-Tech) and I don't seem to recall explanation in the videos.
I'll be back on topic soon with measurements about the output noise of my power supply.
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Last night's results with storyline below. It was interesting to see for example how the image changes as I vary the trigger level (i.e. first six images).
1. No load, trigger level set low
(http://i.imgur.com/fsfzwyw.jpg)
2. No load, trigger level a bit higher
(http://i.imgur.com/01wbBMw.jpg)
3. No load, trigger level more higher
(http://i.imgur.com/l5gudVJ.jpg)
4. No load, trigger level in the middle
(http://i.imgur.com/BRT1lLA.jpg)
5. No load, trigger level on the positive side
(http://i.imgur.com/Hi7O5TS.jpg)
6. No load, trigger level higher on the positive side
(http://i.imgur.com/ZX8tADI.jpg)
7. Rise time of the output turning on a 100 W incandescent light bulb
(http://i.imgur.com/RtzFdxT.jpg)
8. Output with light bulb running on full power
(http://i.imgur.com/Ct0Bx0V.jpg)
9. Output with light bulb and reduced Voltage to 21 V
(http://i.imgur.com/5SL8ISI.jpg)
10. Output with light bulb and reduced Voltage to 10 V
(http://i.imgur.com/4tMQOi7.jpg)
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What would be an acceptable noise level in the case of a power supply? How should I interpret my oscilloscope screenshots?
edit:
Another thing worth looking into is the following: when I power this 100 W light bulb the power supply can't push more Voltage through it than the bulb wants... or at least this is what seems to be happening. Let's say I turn the Voltage all the way up to 30 V on the power supply, I turn on the output, then bulb lights up, but the Voltage shown on the LCD drops to around 24-25 V. Normally if I power a lower Voltage bulb or an LED or any type of load then as I crank up the Voltage I'm pushing more current through it; this is why at a certain point the load gets damaged, but not with this bulb.
I should make a new video showing cases which I would like to understand and solve, because there are quite a few similar ones to the one I just described...
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Hello again!
I bought a couple of latching push buttons similar to this one:
http://www.ebay.com.au/itm/16mm-12V-Car-Silver-Aluminum-LED-Power-Push-Button-Metal-Switch-Latching-B-/390731993896?pt=AU_B_I_Electrical_Test_Equipment&hash=item5af9711328
But I can't figure out how to wire it so that the LED lights up when the power supply is on and turns off when the power supply is off. Because now if I apply 12 V through the positive and negative pins the illumination turn on and stay that way no matter what. Can you please help?
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The switch and the LED are separate items. You need to use the switch to light the LED, or at least connect the LED to something controlled by the switch. As you have discovered you can light the LED at any time by powering it. This can be used to your advantage by having a low current ( like 1mA) through the LED at all times ( 10k resistor to 12V) and simply apply 12V to the led to have it dim when not switched and bright when on.
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About oscillating: how strong was it? I have very similar circuit to this one:
http://electronics-lab.com/projects/power/001/schem.gif
and indeed the output of U2 is oscillating 20Vpp. It is filtered by C7 so no visible noise on output. But why this type of opamp circuit is oscillating in the first place and how to prevent from it?
Ax