Author Topic: Dave Jones' dummy load with added features.  (Read 24217 times)

kibi

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Dave Jones' dummy load with added features.
« on: April 15, 2012, 03:41:03 PM »
Hi

I have been meaning to build one of these for a while. I finally got around to it this week.
I have mostly used parts that I already had in stock. The only things I bought specifically for this project were the 10 turn pot, heatsink and meter.
The 10 turn pot is very expensive, but necessary and makes the device so much simpler to use, so worth it in my opinion.

The circuit is based on Dave's design on Youtube alongside his "World's simplest soft latching power switch". The soft latching switch is included mainly because I think it's cool and is made with "jellybean" components that anyone who owns a soldering iron should have in stock :)

Dave's dummy load video : http://www.youtube.com/watch?v=8xX2SVcItOA

Dave's "World's Simplest Soft Latching Power Switch Circuit" video : http://www.youtube.com/watch?v=Foc9R0dC2iI

One feature I really wanted to include was the ability to set the load value prior to connecting the load. It's very difficult to judge where you are with a 10 turn pot. You could just wind the pot down to 0 and then work up from there, but it's a bit cumbersome I think.
 I found an old junk PCB with a Hamlin reed relay on it. This particular relay has C/O contacts and has a 5v coil with the flyback diode built in. I consumes 25mA, but this is on momentarily, so not a problem for my battery powered design. The relay switches the meter between "set" and "read" values via a momentary push button on the front of the case.
A last minute feature I added was a pair of 2mm jacks to connect a DMM to if you want to monitor the voltage of your load at the same time. Not strictly necessary, but it offers rather a convenient place to connect standard 2mm meter leads.

I thought about including maximum power protection, but with the current limited at 2.5A there will be no problems as long as you don't exceed 50V.
The MOSFET's will blow up at 60V anyway, so you have got a whole set of other problems if you do exceed the maximum voltage. The power is well within the limits of the MOSFET pair if you stay below 50V.
I suppose a maximum voltage lockout could be employed, but I couldn't be bothered and I just wanted to get this thing done. I can't imagine myself going beyond ~25v anyway, so there is no problem for me.

I have based the circuit around the Texas TLV2374 quad rail-to-rail op-amp that I have in my parts bin. It's old, but it's good. I even managed to use all four op-amps in my design. :)
The actual layout of the circuit and the use of strip board could be improved upon especially in an amplifier such as this with high currents involved, but it worked in the end.  8)

So, first off, design the case. It's a Hammond case that I bought for another project, but found it to be unsuitable in the end, so I used it for this project.

Here all the apertures are cut and holes drilled and threads tapped as necessary. A tungsten carbide drill bit snapped in one of the holes, so it's there to stay, (me being a dumb-ass).





Power resistor (10W) and a pair of FQP50N06L MOSFET's are bolted down and wired up. The FQP50N06L is one of my personal favourites. :)





The "wiring" is complete and ready to be tested.




I then performed some testing and it didn't go too well. It worked, but it oscillated like a good 'un, 750KHz to be precise! Not too surprising really, with all those long wires and a badly laid out board, but short of sitting down and designing and fabricating a board, I cured the oscillations with a 10nF across + and - of the comparator section. It then settled down and worked well.

Here is the completed project.

On the left front of the case is the set/read button. Without a load connected, you hold down the button and dial in your preferred load current.




On the rear are the load connections, 2mm sockets for meter leads and the power button.





I have included the circuit diagram that I drew if anyone is interested.


« Last Edit: December 12, 2014, 08:21:21 AM by kibi »

MJLorton

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Re: Dave Jones' dummy load with added features.
« Reply #1 on: April 17, 2012, 07:01:26 AM »
Brilliant...I actually need one of these myself....if I ever get some free time in the near future I'll come back and use your design....thanks for posting.
Play, discover, learn and enjoy! (and don't be scared to make mistakes along the way!)

runem

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Re: Dave Jones' dummy load with added features.
« Reply #2 on: May 08, 2012, 05:08:32 AM »
Very nice project, kibi  :)
Some time ago I made a somewhat extended version of Daves dummy load with a single BUZ31L mosfet attached to a CPU heat sink with a fan. 25 Watts dissipation is not a problem at all and a CPU heat sink can be bought on ebay for almost nothing, compared to the ones you buy from a typical electronic store. I used one of the sections in a LM324 for a temperature controlled fan, so I don't have to listen to the fan noise all the time when I test things over a longer period.
It has indeed served me well and I have used the device many times.

Torrentula

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Re: Dave Jones' dummy load with added features.
« Reply #3 on: June 07, 2012, 06:12:44 AM »
I really like the casing, very clean!

Where did you get the lower half (the one with the display and the pot) from?

@runem: Do you have a link to a crcuit that you used to accomplish the temperature control of the fan?

Elia

runem

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Re: Dave Jones' dummy load with added features.
« Reply #4 on: June 07, 2012, 03:57:21 PM »
Of course you can. It's nothing out of the ordinary. I ''lied'' when I said i used the LM324 in my previous post. It's actually a LM339. You need to play around with the 6k8 resistor from the output to the non-inverting input, that controls the hysteresis, if you want to use LM324 or LM358. The switch that keeps the fan constantly on is probably a little bit misplaced. Connect it from the junction between the pot and resistor, to the op-amps positive rail instead.

Spikee

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Re: Dave Jones' dummy load with added features.
« Reply #5 on: June 13, 2012, 06:35:23 PM »
Where can you get that case , It has an ready made window for a display :O ?

kibi

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Re: Dave Jones' dummy load with added features.
« Reply #6 on: July 14, 2012, 02:05:33 PM »
Where can you get that case , It has an ready made window for a display :O ?

It is a Hammond case. Similar to this one: http://uk.mouser.com/ProductDetail/Hammond-Manufacturing/1455A802BK/?qs=813lsRKI3SBzCjkXiay4eN3vWRX1GtNvn1G8u0rl%252bcE%3d

It does not have any apertures cut in it, I cut the window and drilled the holes myself.

Amarbir[Lynx-India]

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Re: Dave Jones' dummy load with added features.
« Reply #7 on: June 23, 2013, 06:14:27 AM »
Kibi ,Lorton And Others

       There Was No Activity In This Thread Even After Lorton Making Many Videos On This  .Kibi I Have a Repair Lab Where i Get All Type Of Adapters For Repair .This is What i Am Looking At

Input Voltage : 0 To 30 Volts DC Would Do
Max Current Load : 5 Amps

                          Could you Please Suggest Design change For The Same  .I Also Repair Lots of Laptop , LCD Adapters And Hence i Think My 5 Amps Requirement Should Be OK i Guess  .

kibi

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Re: Dave Jones' dummy load with added features.
« Reply #8 on: June 23, 2013, 12:53:30 PM »
You can use this design, no problems.
To get the 5A that you want, you'd have to either use bigger output transistors, make sure they are logic level ones, or add more of the same MOSFETs with the appropriate series resistors in parallel. That would also give you the higher current capability that you are after.

mianchen

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Re: Dave Jones' dummy load with added features.
« Reply #9 on: October 03, 2013, 03:44:40 AM »
I like your battery powered design. I too made one of these dummy loads after watching Dave's video, but it's powered by a 5V power plug and uses a 2N3055 instead of MOSFET (what I had at the time was a bunch of TO-3 2N3055). It works ok for me but only sinks up to about 2.5A and an extra power cable is annoying. I will build another one base on your idea :)

Please ignore the cute little frog stuck on the case by my wife :)