Problem with Constant Current Dummy Load design

[dannyf]

Just use an opamp that can go close enough to ground.

You only need the input to be R2R. The output doesn't need to be R2R (at least with regards to ground).

[mrflibble]

Just use an opamp that can go close enough to ground.

You only need the input to be R2R. The output doesn't need to be R2R (at least with regards to ground).

Correctemundo.

[Conrad Hoffman]

This is a case where I wouldn't try to use a split supply. You need enough supply voltage to get the MOSFET turned on, often more than a couple volts. You also don't want to take the opamp input negative. A 9V battery is ideal and lasts a long time, especially with a MOSFET because the gate current is near zero. You have to read the fine print to find opamps where the input stage includes ground. When they say "rail-to-rail" they often mean the output, and you find they think a couple hundred millivolts on the input qualifies too. It doesn't! The values I showed will work for most common devices- I think I used IRF540 or similar. Any effort I made to deviate from that design- eliminating the 100k and using local capacitive feedback around the opamp, or other slightly different topologies, met with instabilities, oscillation or discontinuities in the control range, so I'm not surprised at the troubles being had here. It looks so ridiculously simple, but the number of pitfalls are surprising. Be careful that the current flow in the MOSFET loop doesn't alter the input voltages of the opamp- layout (connection points really) is important.

[Monkeh]

Hi guys,

I have posted this before......but hopefully it'll be useful here......from my own design notes for my own load. I still get instability but only at much higher loads/voltages. My design basically follows Dave's but with the following components. PS. Don't faint at the cost of the mosfet!

MOSFET:
These are N-type Linear Power Mosfets, the IXTK46N50L (500v, 46A). Conventional mosfets can be used but you would be limiting the range of voltages/loads the Dummy Load would be able to cover due to running the mosfet in it's linear mode. The IXTK mosfets overcome these limitations by extending the transistors’ FBSOA. I.E. ETI (Electro-Thermal-Instability) as a result of positive feedback within the mosfet when used in linear mode. The IXTK is ideally used in programmable loads, battery chargers & current regulators etc.

And I thought I used a fairly expensive FET (IXFH110N10P)...

[dannyf]

I used IRF540 or similar.

If you don't need that much current capabilities, the incredibly capable, incredibly obsolete, incredibly available and incredibly cheap IRF510 would be a great mosfet to use in this type of applications: check out its gate capacitance.

[Conrad Hoffman]

Just to muddy the waters a bit, the original circuit used a lowly uA741 and split supplies. They grounded the input divider so the inputs wouldn't go negative. If I were going to split a single supply, I'd split it unequally, maybe -2 and +7. That would take care of the inputs for any old opamp, and still give enough voltage to turn on the most stubborn MOSFET, without needing more than 9V total input.

edit- The circuit can be fast. I've used this as the constant current load half of a class A headphone amp and it worked just super.

[MrAureliusR]

Holy smokes... a lot of new information to absorb.

When I asked which parts I should ground reference, I was referring to the split supply scheme. you end up with a positive, negative, and 'virtual' ground between. I was wondering which parts of the circuit needed to be referenced to this 'virtual' ground.

The information that mrflibble has been giving is what I was looking for -- why it is oscillating in the first place.

Still, the behaviour I'm getting is that turning the pot has essentially no effect on the current. There's a very, very narrow range in which it does change the current -- from just under 2 amps to just over. I can actually see the current following the input at that stage, but once it gets down to 1.8A it just stops there no matter how far I turn the pot. Once again I'm working on this too late at night, I'll have to have another go at it once I get up tomorrow. 

I will post a schematic of what I've built, and the few permutations that I've tried so far, tomorrow. Hopefully that will help you guys to help me.

Once again, thank you all for your support and help. It's invaluable -- I'm trying to read as much as I can from other sources but you guys are able to explain things in a way that 'click' with me. 

[mrflibble]

Still, the behaviour I'm getting is that turning the pot has essentially no effect on the current. There's a very, very narrow range in which it does change the current -- from just under 2 amps to just over. I can actually see the current following the input at that stage, but once it gets down to 1.8A it just stops there no matter how far I turn the pot.

If you are still using an LM741 opamp and a 1 Ohm sense resistor I think I can see how that would happen. Short version: try an LM324 instead and it suddenly just might be able to go a lot lower in current.

I will post a schematic of what I've built, and the few permutations that I've tried so far, tomorrow. Hopefully that will help you guys to help me.

Yes please, that way we don't have to guess.

[sleemanj]

Coincidentally I've been working on a dummy load this last couple weekends trying to get it working satisfactorily.

So a few lessons and traps for young players I learned the hard way...

Because I wanted to make it from stuff in the drawer, I stuck with an AZ324 op amp (because I have a bunch from somewhere), and IRF610 mosfet (actually two of, but due to not being well balanced only one is probably doing most of the work, if I redesign the board I'd have two sense resistors and drive each mosfet separately, without a heatsink one very definitely is doing almost all the work).  If I'd just got a better op amp much of this wouldn't have been a problem.  The mosfets are not logic level, so everything is powered from 12v so they can hit 10Vgs if necessary.

After much filddling (MUCH fiddling) I've got it good-enough-for-my-purposes, there's some oscillation happening when scoping it if you purposefully forget you ever looked at it on a scope it still all works :-)  It's hitting within a percent or so for most of the range 0 to about 1.5A (0 to about 12W), and much of the range is a spot on (depends exactly where I "cal" it).

I started out with a 0.01R 3W sense resistor, but that's just too small for the op amp and getting rid of errors was a problem (I might try again later now I have some corrective ability).  So I moved to a 0.1R.  In keeping with the bodged together nature, my 0.1R is a stack of 10 1R 1206 resistors of unknown (and probably very poor) quality which I further trimmed with another 1.18R (which is a stack of 4 resistors itself) - of course if I'd used better resistors I wouldn't have had to trim, but when using standard resistors careful trimming of your sense resistor(s) is essential especially down at these levels.

Don't underestimate the resistance of your wires and component legs, they can make for a substantial unexpected "error" depending on where you measure!  For a while I had my stack of resistors hanging off about 10cm total of around 24AWG wire... and much head scratching about "wrong" gains ensued - if you're measuring across your sense resistor, make sure you're measuring as close to the same point as your op-amp is going to see!

I originally put in a small capacitance on the opamp output (to ground, from looking at other designs with regard to oscillation damping) but removed this as the oscillations were atrocious with it, this is of course very op-amp dependant.  Similarly I tried with some various other cap suggestions already discussed here, but they were not really necessary, getting rid of the output capacitance cured the worst of the evils.

I anecdotally find that if the DUT is starting to sag that oscillations do increase a bit, but I havn't looked hard at that, but make sure you're "calibrating" at a current that you know your "DUT" can supply without any problem otherwise it's just a game of "guess which is bad, the DUT or the test equipment".

Like you are, to combat the terrible offset voltage induced errors I resorted to using a negative supply so they could be better nulled (well...).  Slapped together with a 555 driving a voltage inverter at about 50kHz (because just that's the fastest I could get with the surface mount caps I had on hand) from the 12v input, and a zener regulator to give me about -3.6v which is supplied to the negative rail of the opamp (12v to the positive).  A (biased so it's just a little above and below ground) potentiometer with 200k on the wiper feeds into the opamp I care about (the output voltage - eh, I'm amplifying the sense output by 10 to get 1A=1V, and feeding this into another amp on the 324 to do the compare to the set voltage which is also 1V=1A, just because it makes the numbers nicer) - see http://www.ecircuitcenter.com/Circuits/op_voff/op_voff2.htm

Similarly I found that the bias current also must be compensated for, a 1k resistor between 0v and the non inverting input did the trick - see http://www.ecircuitcenter.com/Circuits/op_ibias/op_ibias.htm

Because of my gain requirements (10x as mentioned to get 1V per A reported out of a 0.1V per A sense resistor) it does mean I have now 2 calibration points, the gain, and the offset null.  I found the best way to calibrate it was to connect a supply (DUT) through a multimeter showing current, allow it to warm up, then disconnect the DUT and zero using the offset adjustment, reconnect the DUT and adjust the gain to synchronise what's "reported" and what it should be, check/adjust offset again, check/adjust gain again... after 2 or three cycles it gets "homed in" such that no more adjustment is necessary and hopefully you've got the offset killed (at that temperature, angle of tongue and phase of moon).


Attached annotated schematic (excuse the crudity), ignore the percentages on the resistors it's just the default.  VNeg and VOff are not shown, they are respectively the negative supply and the wiper (with 200k in series) of the offset potentiometer which I made separately (after finding it was necessary).  Ignore the 3 funny lookin' components not connected to anything bottom left, they are artifacts of some 0-Ohm jumpers I used routing the PCB.