# Problem with current limiter

I designed a current limiter in a sim and it works perfectly in the real world. It gives me an output of 0-5v for 0-50 amps in one direction.

The problem however is that as soon as the direction of the controller is changed, the voltages goes much higher in the other direction. So the op amp doesn't match both ways if I've made it clear?

I've included my circuit below. I am a little stumped as to what do really do... I could always make it so that the limit output voltage are matched to the higher side, but then as soon as the direction is changed again the readings will be too low on the lower side.

So the reading from one direction of the h-bridge is 5v at 50A which is perfect, but then if the direction of the h-bridge is changed the voltage at the output of the op-amp is 5v at only 10A.

Any help is appreciated!

I've abandoned this project as it's flawed by induction issues

• I'm not entirely clear on how this circuit limits current. Could you post pics of your actual circuit IRL? Where do the gates on the H-bridge go? I'll assume that the current that you wish to limit is the current flowing through the bridge, correct? What exactly do you mean by "my voltages go higher in the other direction?". Do you mean that the output of the op-amp isn't proportional to current when the current flow in the bridge is switched to the other leg? – Platytude Jan 25 '19 at 5:17
• Is uses the sense resistors to read the current going through the circuit, the actual h bridge doesnt have a lot to do with the current limiting. Then the tiny voltage from the sense resistors feed into the op amp and gets amplified to a higher voltage so the micro can read it. But when the direction of the h-bridge is changed, the circuit seems to get more voltage from the sense resistors in one direction than the other! and thats my problem – Dean Hill Jan 25 '19 at 5:20
• Hmm, that's definitely odd behaviour, the circuit should definitely be agnostic to which FET leg is on, and which is off. When the direction changes, does the higher voltage persist? Or is it transient? Also, what is the load for the circuit? Motor? Lighting? – Platytude Jan 25 '19 at 5:25
• no...it persists, its a motor controller that I want to add a limiter to – Dean Hill Jan 25 '19 at 5:27
• I don't know what any of that means dude.. im a beginner – Dean Hill Jan 25 '19 at 5:33

Whilst it the sensing circuit should be agnostic as to which upper/opposite lower FET pair of the HBridge is active there is a chance of two possible things going on. Before getting into that though let's first review where the load of an HBridge is connected:

For proper operation of the HBridge it is normally required that either pair of FETs (upper left and lower right) or (upper right and lower left) be on whilst the other pair be off. Electronic breaking of a motor load can be implemented by turning on just the lower two FETs but let us not get into that here.

Problems can come up with your current sensing circuit under one of the possible two scenarios.

1. The impedance of your load on the HBridge changes a lot when the polarity of the current through the load changes direction.
2. The switching control of the four FETs in the HBridge is not being done correctly and one side of the bridge has both the upper and lower FET on (or partially on) at the same time allowing an excess of extra current to flow past the load and into the sensing resistors.

So you will want to study and make measurements of your circuit to make sure that one or the other of the above situations is not happening.

• The MOSFETs are being driven by an ir21844s. It's a commercial controller, I've only reverse engineered the actual current limiting circuit on it. It had the output of the first op amp feed into the second op amp with a trimpot to adjust the current limit. – Dean Hill Jan 25 '19 at 20:17
• Here's the weird thing... If I change the polarity of the motor on the outputs of the bridge I get the same results... The reading from the current limiter is 5v at 50a which is perfect, but when I change direction the readings go incorrect again giving me 5v on the output when only around 10a is drawn! – Dean Hill Jan 27 '19 at 6:01
• I just realised.. is it possible that the regeneration the. Ontroller has is somehow affecting the readings? – Dean Hill Jan 27 '19 at 6:03