My Variable DC Power Supply (30V/1A)

[birrbert]

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 - Low Power Quad Op Amp, BD243C - NPN Silicon Power Transistor and BC337 - 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/

[birrbert]

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 or 2N3055 (probably the most important upgrade).
- T1: BC337 changed with BD140.
- 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?

[blankfield]

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

[birrbert]

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!

[blankfield]

Output stage of your power supply should be like this:



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.   

[MJLorton]

Great posts gents...this will be a project that I take up in the near future too.

[dr_p]

@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:

[MJLorton]

Brilliant dr_p....love the analogue mA meter!

[birrbert]

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!

[steve30]

dr_p, that PSU looks cool with the analogue meter. Looks a bit strange having the seven segment displays next to it though.

[birrbert]

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?

[blankfield]

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:)

[birrbert]

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?

[blankfield]

Yes R4 is the same R5 resistor, R14 should be grater than 2.5W so 5W or 10W will be good.

[birrbert]

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?