Constant Current Dummy Load troubleshooting advice needed

[Jimmy the Squid]

I've gone ahead and built the constant current load featured in EEVBlog 102 - I think it's a right of passage. I've used the same parts as Dave - the LM324 OpAmp, and the MTP3055 FET, but I seem to have issues, described herein. I've read through the other posts here about this project, and I didn't find any mention of a similar issue.

The device is currently set up on my breadboard, powering the LM324 with 0 - 12 volts, and with a 7805 (and associated caps) bringing that down to 5 volts for the pot to control the thing. It "works a treat" as Dave would say - the current drain is easily settable and I have no problems drawing more current that I really care to with the thing (I didn't go over 1.5A for heat reasons). The problem is that above 150mA current, it does not stay "constant" if I change in voltage on power supply being tested. It tracks fine below that level, but higher than that point, and it increases/decreases with the voltage. The tested power supply is completely separate from the supply that powers the device, apart from common ground.

I'm asking for advice on what I should be looking at to make this problem go away (if that is possible). I've also followed Martin Lorton's series on the same device, and he doesn't seem to have a problem with the contant current piece. In fact when i fist saw him change the voltage without the current changing I said "Whoa, mine doesn't do that, what have I done wrong?"

My profile says it all - I'm a complete novice when it comes to this stuff with no formal education, so I'm at a bit of a loss as to what I should be looking into.

Many thanks for your advice!

[psycho0815]

I've been watching Martin's videos too, and i think i remember him having problems with oscilation at one point. IIRC he fixed it with a cap from the feedback loop to gnd. maybe you have the same Problem? do you have a scope to have a closer look at what the output of the opamp looks like?

[Rerouter]

or a better option, slow your loop down, its likely that it is free oscillating if you directly copied the dave-cad, but some small capacitance (10n) between the output and the inverting pin of the op amp and see how that goes for you,

[Jimmy the Squid]

Both great ideas. I'll try them out and report back tomorrow. Thanks a bunch for the input!

[Jimmy the Squid]

or a better option, slow your loop down, its likely that it is free oscillating if you directly copied the dave-cad, but some small capacitance (10n) between the output and the inverting pin of the op amp and see how that goes for you,

Yep, that worked. Attached are before and after scope measurements. Sorry they're a little blurry.  Sure enough, it was oscillating something fierce. A 10nf capacitor tamed it right down. I'll play some more with the values of the cap as I test at much higher current draw than the 200 to 250amp shown here.

[Fluxed Matter]

I just finish building this circuit on my breadboard. But I am having problems making the circuit work. I have been do some voltage checks and everything at the op amps is working except there is no voltage on the inverting input of the second op amp used to feed the gate on the MOSFET. The voltage should be about 1V per Dave's video but I am only measuring zero volts. The output of the second opamp is 3.87V constantly which means that the mosfet is on. I am using a PHP45N03LTA which I salvaged out of a dead PC PSU, and it is a logic level mosfet according the the datasheet.

I built a circuit using an LED and potentiometer to test the MOSFET. I used a 100K pot to vary the voltage at the Gate from 0-10. I connected the LED to 10V and to the Drain and tied the Source to ground. The LED started to turn on when the voltage at the Gate reached 1.3 and was pretty bright at 1.5V. So I believe that the MOSFET is working. 

I built this on the breadboard because I want to use to measure to current output of some transistor circuits that I have been experimenting with. These voltage measurements that I speak of are without any device to be tested hooked up to the dummy load. I guess I should have started a separate thread for this but this question fits the subject.

Thank you in advance for any help or suggestions.

[baljemmett]

I just finish building this circuit on my breadboard. But I am having problems making the circuit work. I have been do some voltage checks and everything at the op amps is working except there is no voltage on the inverting input of the second op amp used to feed the gate on the MOSFET.

[...]These voltage measurements that I speak of are without any device to be tested hooked up to the dummy load.

That'd probably be why, then -- isn't the voltage you're measuring the feedback signal, i.e. the voltage drop across the current shunt?  If there's no power source connected to the MOSFET/current shunt side of the circuit, you'll never get any current flowing at all so that voltage will always be zero!

Try attaching the load to a power supply (ideally current-limited in case there is actually a problem) and see if you get a voltage at the same point then -- it should vary to match your set current.

[Fluxed Matter]

Okay that is probably true. I was just trying to verify the circuit is operating correctly before I tried using it. Shouldn't the output of the second op amp be zero when the 50K pot is set to zero? I measure 3.87V all the time. In the video I am not sure when Dave is talking about the dummy load and he is showing the display if he had it hooked up to the device he was checking or if he only had power to the dummy load circuit.

I am still working on the display part but I will try connecting my circuit to be tested and see if the results change later on today if I get some time and post my results.

Thanks in advance. 

[m12lrpv]

When trying to determine why you're getting the op amp output that you are, you should check the inputs.

I found the same thing. My op amp was hitting maximum output without a load connected because of a very small voltage difference on the inputs.

[Fluxed Matter]

When trying to determine why you're getting the op amp output that you are, you should check the inputs.

Thank you, I will double check that. My initial check measured zero on both input, but maybe I missed something.

I am working on getting the display going at the moment so I don't have to use a meter.

[m12lrpv]

For me it was a case of "Millivolts Matter"