Current limiting op amp circuit debugging
Current limiting op amp circuit debugging
(OP)
I have a current limiting circuit posted here-http://h ome.comcas t.net/~cat nipone/wsb /html/view .cgi-home. html-.html
It has an input voltage between 14 and 30VDC and is supposed to limit the current to 4 amps. Connected to the inverting input is a 200 ohm resistor and a 1 mA current regulating diode which places a Vin - 0.2V signal on that input. When the non-inverting input reaches that same voltage there is a0.2V drop across the current sensing resitor (0.05 ohms) which would be a 4 amp current draw through the FET and 2 ohm load. My problem is the current is not limited to the 4 amps but just keeps rising along with the input voltage to the non-inverting input. This circuit was modeled after the one on my photo's page of my website which is a working circuit. I have simulated the circuit with PSpice and it limits the current to 4 amps. The circuit is already on a circuit board so a complete redesign is out and the original designer has left the company. The circuit in my Photo's page has an 18V input and limits the currrent to 5 amps. Any help would be greatly appreciated.
It has an input voltage between 14 and 30VDC and is supposed to limit the current to 4 amps. Connected to the inverting input is a 200 ohm resistor and a 1 mA current regulating diode which places a Vin - 0.2V signal on that input. When the non-inverting input reaches that same voltage there is a0.2V drop across the current sensing resitor (0.05 ohms) which would be a 4 amp current draw through the FET and 2 ohm load. My problem is the current is not limited to the 4 amps but just keeps rising along with the input voltage to the non-inverting input. This circuit was modeled after the one on my photo's page of my website which is a working circuit. I have simulated the circuit with PSpice and it limits the current to 4 amps. The circuit is already on a circuit board so a complete redesign is out and the original designer has left the company. The circuit in my Photo's page has an 18V input and limits the currrent to 5 amps. Any help would be greatly appreciated.





RE: Current limiting op amp circuit debugging
1. Since the op amp is powered by the voltage drop across R2 and R1, it may not be high enough for normal operation. If the voltage between pins 7 and 4 is too low, the output will be indeterminate. In this case, the output may follow the V+ supply and keep the MOSFET on, which in turn would keep the supply voltage low. Perhaps tying pin 4 to the return would work.
2. Of course, make sure that circuit is actually built the way that you think it is. I've been bitten a few times by swapping the drain and source of a MOSFET and having the internal diode cause output to follow the input.
RE: Current limiting op amp circuit debugging
RE: Current limiting op amp circuit debugging
RE: Current limiting op amp circuit debugging
For troubleshooting purposes, it would be helpful if you could apply a fixed Vin and then tell us what the voltages are at all the pins of the op amp. It'd probably be best if you reference these voltages to the return path (minus pin of the Vin supply).
Also, when running these tests, is the load a simple 2-ohm resistor, or is it something more complex? If the load were the equivalent of an 8-ohm resistor, then the output should just follow the input.
RE: Current limiting op amp circuit debugging
RE: Current limiting op amp circuit debugging
Your inputs to the 3140 are within 0.2V of the positive rail and whilst this may not cause damage to the device it may cause unpredictable operation under such conditions.
Perhaps a rail-to-rail op amp would work better. Can't think of one right now, but one with similar characteristics to a section of the (quad) CA324 would work under these conditions.
RE: Current limiting op amp circuit debugging
I know you have a circuit board and don't want to re-design, however you may be able to fix it with minimal changes. As most op amps have a better range for negative common mode, including "below" the 0V rail, a better solution would be to put the sense resistor R2 in the V return (ground) leg of the circuit (you may be able to do this by placing it within the external wiring). Also, you need to swap the positions of the current source diode and reference resistor R3. Both op-amp inputs are thus around 0.2V above ground, however you would have to swap the inverting/non-inverting inputs to the 3140 so that it changes the drive voltage to the FET in the correct direction.
RE: Current limiting op amp circuit debugging
1. Apply 14V power supply as your Vin
2. Put the negative lead of meter on the negative lead of the power supply. On your circuit, that is NOT the V- pin of the op amp.
3. With the positive lead of the meter, measure the voltages at pins 2, 3, 4, 6, and 7 of the op amp. I realize that pin 7 should be 14V in this case, but it's always good to make sure.
RE: Current limiting op amp circuit debugging
RE: Current limiting op amp circuit debugging
RE: Current limiting op amp circuit debugging
Curious: what was the original 3-terminal regulator?
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Sometimes I only open my mouth to swap feet...
RE: Current limiting op amp circuit debugging
RE: Current limiting op amp circuit debugging
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Current limiting op amp circuit debugging