Electronics > Projects, Designs, and Technical Stuff
MOSFET and OPAMP Load tester / constant current load
lordvader88:
Hey Mark 1 problem I'm having is on startup. I can have the circuit turned on, with the output set for zero, and the mosfet drain supply is turned off. The LM358 is on max output. Then hitting the power switch for the drain supply, the current often jumps really high, and around a bit in general and doesn't respond at all to the poti. setting. That's alarming, I had my PSU limitied to 0.5A and is jumped right to it.
Once I can get it settled down, then it behaves fine, and seems very steady and low noise, at static loads.
So the last 2 times I started it with the gate unconnected, aka connected to the external input switch. Just about to troubleshoot a 12V 5A SMPS brick for a monitor, so I'm hesitant to connect the load tester, in case it shoots to max.
For now, I only added a current limit of down to 70% max of any setting, just using the other 1/2 the opamp. I should change it down to near zero%.
lordvader88:
Wow so the IRL540 I'm using is a logic level mosfet, how did I miss that right on the datasheet. Well I burned out 1 so far, IDK how, it wasn't violent or smokey. Either way mine are from cheapest on ebay, so they might be fakes too, it might have burned out around 1-2A for all I know, like my fake 2n3055
MarkF:
I have lost track of what your circuit looks like now. An updated circuit diagram would be helpful.
If there is no load connected to the MOSFET drain (i.e. drain supply is OFF), even a small positive voltage on the (+) op-amp input will drive the MOSFET gate to the max supply voltage of the op-amp because the (-) op-amp input will stay at zero volts because there is no current available to flow through the sense resistor. This may be caused by the input offset voltage of the op-amp.
If your current limiting is done in the 10:1 voltage divider, the pot (R4) may act up at either end stop. If it opens, there will be no pull down for the (+) input and you will get some unknown gate voltage (probably max turn on). You could put a 100K resistor across the pot as a protection (100K being high enough not to interfere with operation).
What is the op-amp supply voltage? The max IRL540 gate/source voltage (VGS) is 10V. If your supply voltage is 12V, the VGS max can be exceeded.
Have you put a scope on the circuit? Is it oscillating?
Does it act up if you always turn ON the load voltage (i.e. drain voltage) before turning on the op-amp voltage?
Give a read through this long thread for some background:
https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/
lordvader88:
--- Quote from: MarkF on June 15, 2019, 02:45:32 am ---I have lost track of what your circuit looks like now. An updated circuit diagram would be helpful.
If there is no load connected to the MOSFET drain (i.e. drain supply is OFF), even a small positive voltage on the (+) op-amp input will drive the MOSFET gate to the max supply voltage of the op-amp because the (-) op-amp input will stay at zero volts because there is no current available to flow through the sense resistor. This may be caused by the input offset voltage of the op-amp.
If your current limiting is done in the 10:1 voltage divider, the pot (R4) may act up at either end stop. If it opens, there will be no pull down for the (+) input and you will get some unknown gate voltage (probably max turn on). You could put a 100K resistor across the pot as a protection (100K being high enough not to interfere with operation).
What is the op-amp supply voltage? The max IRL540 gate/source voltage (VGS) is 10V. If your supply voltage is 12V, the VGS max can be exceeded.
Have you put a scope on the circuit? Is it oscillating?
Does it act up if you always turn ON the load voltage (i.e. drain voltage) before turning on the op-amp voltage?
Give a read through this long thread for some background:
https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/
--- End quote ---
Ok thanks I lost track of this working on other stuff. My circurt basically was like your's, with 10V, except I used a 2.5 volt reference into one 1/2 the op-amp, and with the other side I multiplied it by x1.7 to x2, so up to 5V from there. Then the rest is the same, I made 5600 and 620 values. And I'm using the IRF540. Geez I never imagined they are also only 12V you say...This was probably the project that got me to buy them ages ago, so I'd have logic level fets for someday.
It seems to work fine, nothing to worry about on the scope when it's working . It's just at the start, where between the op-amp and gate starting, it shoots right up, possibly to max (but my PSU can't do 2.5A so IDK). And the potenitometer pretty much does nothing, so itt's the op-amp gone crazy.
So when you turn on your loadtester, you always have the DUT already on first ?
Well regardless I have now the current limiting working on my PSU so I can try the LT again.
MarkF:
I need to do some more checking on mine to see if I can reproduce your issue.
I seem to remember something strange when I power the Load before the SUT.
But can't say the current spikes up and not responding to the pot.
I ask @Jay_Diddy_B on his thread and he responded with a change.
His suggestion and his Load control the current via the (-) input. Opposite from most Loads.
I have not had time to try it yet.
https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/msg2488077/#msg2488077
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