Constant current dummy load - home etched, single sided

by george graves on Thu May 10, 2012 11:54 pm

I threw this together. Single sided, home etched. Based on Dave's plans:

http://www.eevblog.com/2010/08/01/eevbl ... y-testing/

I used ExpressPCB software, just cause it's so much quicker to quickly whip up a board for home etching(eagle can be a pain at times - footprints and all). Then I usually print it off as a PDF, and tweak the pads in illustrator for home etching (very much needed to make bigger pads, and such for the toner transfer method)

http://dl.dropbox.com/u/2295566/BLOG/dummy%20load%20v1/dummy%20load.pdf

http://dl.dropbox.com/u/2295566/BLOG/dummy%20load%20v1/ExpressSCH.pdf

The only change I really made was running the opamp at 8 volts (7808) for a bit more range. It seems happy at 1 amp. Two amps and things start to really heat up. ;D I'm planning on making a second version down the road. Something arduino controlled with DACs, ADC, multiple mosfets, maybe a fan and a LCD. But this will do for now. All I really need is 500ma - 1000ma.

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Re: Constant current dummy load - home etched, single sided

by john444 on Fri May 11, 2012 12:44 pm

Very nice!

The photos look as if you did not wire the current-sense resistors in a 4-terminal connection.
(I am at work so I could not open the links)
You can avoid several potential problems with 2-terminal resistors.

CC loads can be really handy.

What did you do to have such clean boards?
I have not been very satisfied with the results when I used the toner transfer method.
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Re: Constant current dummy load - home etched, single sided

by adafruit_support_mike on Sun May 13, 2012 12:13 am

That's some quality geek banned right there..

Good boards, good build, good photography. It's a thing of functional beauty.
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Re: Constant current dummy load - home etched, single sided

by johngineer on Tue May 15, 2012 11:50 am

Very nice, George! I like your R_sense resistor bank -- reminds me of one of those vertical parking deck elevators! :)
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Re: Constant current dummy load - home etched, single sided

by sklarm on Tue May 15, 2012 5:30 pm

Nice job.

Constant current power supplies are helpful devices. I released a open source hardware kit last month called Da Pimp which works off the same principle. It is used for battery testing, charging and desulfation. It would also be good for power long strings of ultra bright LEDs. I'm surprised there are not more projects like these. Mine puts out a constant .5A when given 120VAC (US) or 1A when 220VAC is used. This is a capacitive charger design. Safety third.

http://screwdecaf.cx/dapimp.html

dapimpwithcase.jpg
Da Pimp with Case
dapimpwithcase.jpg (46.06 KiB) Viewed 12815 times
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Re: Constant current dummy load - home etched, single sided

by freaklabs on Tue May 15, 2012 10:57 pm

Hi George.
The project looks excellent. I am now inspired to build a dummy load too for an AC energy monitoring project I'm doing. Thanks for the post!
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Re: Constant current dummy load - home etched, single sided

by skylen on Wed May 16, 2012 8:56 am

Nice work. BTW, did you leave the extra op amps floating or did you hook them up as followers?
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Re: Constant current dummy load - home etched, single sided

by IsotopeJ on Thu May 17, 2012 1:39 pm

Hi George,

You may have already figured this out, but the limiting factor in this circuit isn't the opamp supply voltage. The range is more dependent on the sense resistor. For instance, with a 1 ohm 5 Watt resistor, you're going to dissipate I^2 watts. You'll exceed 5 Watts at 2.24 Amps. Any higher than that you'll burn up your resistors.

Now, before I move on, one consideration is that the OpAmp must be able to supply enough bias voltage to the mosfet base to reach saturation. An amp like the LM324 can go from 0 to Vsupply - 1.5 volts, so you need to look at the datasheet for your mosfet to get the saturation voltage and make sure to drive your opamp at Vsaturation + 1.5 minimum.

For the sake of argument, let's say you're still using a 1 ohm resistor and expect up to 6.5 volts across it, giving you 6.5 amps. At that current your sense resistor will need to dissipate over 42 watts. Yikes. The way to reduce this is to use a smaller value for your sense resistor. In fact, I built this circuit using 40 half-watt 10 ohm resistors in parallel for 0.25 ohms. This way I am able to sink currents up to 8.9 amps. I also added a toggle switch that bypasses 30 of these resistors to leave me with a 1 ohm sense resistor. This way I can switch between a "high" range (about 8ma to 8.9 Amps) and a "low" range of about 2ma to 2.2 Amps.
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Re: Constant current dummy load - home etched, single sided

by bearmos on Mon May 21, 2012 9:14 am

Constant current can be extremely useful! I needed one that was fairly beefy for doing some electroplating. I went the TO220 route for the resistor and FET and added some forced air cooling. Components are slightly warm to the touch running at 8 Amps.

Here's a shot of the board
constCurrentCCA.jpg
constant Current board
constCurrentCCA.jpg (93.09 KiB) Viewed 8369 times



...with the ridiculous heat sink (salvaged from a Pentium 2 I think):
constCurrentAssembly.jpg
constant current with heat sink
constCurrentAssembly.jpg (84.24 KiB) Viewed 8369 times


The page below discusses some of the tradeoff's listed in the posts above for component selection (super simple schematic is there as well):
http://twilightrobotics.com/prototyping/constantcurrentsource
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Re: Constant current dummy load - home etched, single sided

by simonclark on Fri Jan 11, 2013 4:41 am

george graves wrote:I threw this together. Single sided, home etched. Based on Dave's plans:

http://www.eevblog.com/2010/08/01/eevblog-102-diy-constant-current-dummy-load-for-power-supply-and-battery-testing/

I used ExpressPCB software, just cause it's so much quicker to quickly whip up a board for home etching(eagle can be a pain at times - footprints and all). Then I usually print it off as a PDF, and tweak the pads in illustrator for home etching (very much needed to make bigger pads, and such for the toner transfer method)

http://dl.dropbox.com/u/2295566/BLOG/dummy%20load%20v1/dummy%20load.pdf

http://dl.dropbox.com/u/2295566/BLOG/dummy%20load%20v1/ExpressSCH.pdf

The only change I really made was running the opamp at 8 volts (7808) for a bit more range. It seems happy at 1 amp. Two amps and things start to really heat up. ;D I'm planning on making a second version down the road. Something arduino controlled with DACs, ADC, multiple mosfets, maybe a fan and a LCD. But this will do for now. All I really need is 500ma - 1000ma.

Image

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Image

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Nice board that you created . nice photography also .each and every wire visible properly .
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