H Bridge not working

Hi

I have made one H Bridge that works(probably not the best way to make one).

Now I have made the second one exactly the same as the first but have run into a problem and I have double checked everything.

Problem is I have voltage by the output even if both my logic pins are set to LOW or are unplugged which is not the case for my first one, so I think one of the mosfets are faulty.

How would I go about to test which one it is?

Thanks.

MOSFETs are subject to floating gate issues, so it might be a good idea to put a high value resistor from gate to ground to preclude stray voltages.

zoomkat:
MOSFETs are subject to floating gate issues, so it might be a good idea to put a high value resistor from gate to ground to preclude stray voltages.

There are pullups on the gates, so it's not a floating gate issue. With voltage applied and no input, Q2 and Q4 should be off, Q1 and Q3 on, so both A and B should be low.

@Undermentioned, to have a voltage across A and B with no input, one of the P-channel MOSFETs must be shorted between source and drain if there are no shorts on the board itself.
MOSFETs like to fail short-circuit. With the power off, measure the resistance between the source and drain of each of the P-channel MOSFETs, with the positive probe on the source. They should measure infinite ohms if they're good, a low resistance if dead.

Another point - I see no series resistors for the opto-coupler LEDs. I hope you have some in the connecting circuit.

MOSFETs are subject to floating gate issues, so it might be a good idea to put a high value resistor from gate to ground to preclude stray voltages.

I tried that, it didnt help, but thanks I didnt know that so learned something.

@Oldsteve, Well with your input I tested my P-channel MOSFETs and they are working, I then tested my N-channel MOSFETs but that didnt work then I tested it from gate to drain and found the one was giving me low resistance and the other one infinite.

Didnt know which one was faulty and which one was working so I replaced them both with IRFIZ48V and its working :slight_smile:

So testing The N-channel MOSFET from gate to drain which one is the faulty one now?

Another point - I see no series resistors for the opto-coupler LEDs. I hope you have some in the connecting circuit.

Haha no I dont, I should probably add some.

Thanks.

Undermentioned:
I tried that, it didnt help, but thanks I didnt know that so learned something.

@Oldsteve, Well with your input I tested my P-channel MOSFETs and they are working, I then tested my N-channel MOSFETs but that didnt work then I tested it from gate to drain and found the one was giving me low resistance and the other one infinite.

Didnt know which one was faulty and which one was working so I replaced them both with IRFIZ48V and its working :slight_smile:

So testing The N-channel MOSFET from gate to drain which one is the faulty one now?

Haha no I dont, I should probably add some.

Thanks.

Hmmm. Interesting. A shorted N-channel MOSFET should be low, so with no input, it would appear the same as a good one. ie No voltage between A and B. Only a dead (shorted) P-channel should have given the symptom you described.

Anyway, all’s well that ends well.
The MOSFET that measures short-circuit will be the faulty one.
You could make a simple breadboard circuit with a LED to test them both. A LED and resistor from the drain to +V, source connected to ground, then take the gate high to turn on, low to turn off. Don’t leave the gate floating though. A pulldown resistor from gate to ground is a good idea.
The bad MOSFET won’t turn off when it’s gate is taken low.

And add those resistors ASAP, or you risk damaging/killing the optocouplers and/or whatever drives them, from over-current.

Edit: By the way, Q1 and Q2 and OK2 are drawn upside-down on your schematic, too.

I tried that, it didn't help, but thanks I didn't know that so learned something.

Just experimenting in the past, I've starteted a motor running thru an N MOSFET, then removed the wire from the gate, and the motor kept running for a while. Be aware that DIY H-bridges some times have shoot thru that might damage the MOSFETs.

MOSFET gate oxide is easily damaged, power MOSFETs have no protection diodes (they can’t anyway),
so can be damaged by static more easily than many think. Typically this causes a small degradation
of performance and leakage currents, maybe followed by catastropic failure in circuit. If you have
a genuine device and its damaged, you are likely to be the cause. If its a counterfeit, who knows?

A good in-circuit precaution is a 15V zener across gate/source to prevent over-voltage at the gate.
You need hefty decoupling on the 12V power rail too, I see nothing.

BTW the circuit is a wrong design, severe shoot-through, very unsatisfactory, it will cause lots of
grief with massive current pulses every time it switches. Have you put an oscilloscope on the
H-bridge that works? It’ll show how poorly its behaving if you do.