bidirectional PWM controller


I would need an PWM controller for a 24V 50W brushed DC motor. I made one like on this link, but it's working very bad, it work good when is alive but MOSFET's gone dead quick.

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Does maybee someone have somethink that could drive mine motor in both direction, so like this: When potmeter is in middle motors stops, and then going speed up when potmeter go to one side motor goes in one direction and opposite?

Thanks for help,


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The most obvious problem is that this circuit does not have protection, wheeling or ring-off diodes to keep the inductive kick from the motor from damaging the FETs when switched off. Any inductor, including a motor, when switched off will try to maintain the flow of current into the off circuit. This creates a high voltage spike that is the probable reason for FET failure.

The FETs have an internal body diode that is totally unsuited for this function. They are too slow and retain too much charge.

Place four fast diodes, reversed biased, from M1 and M2 to the plus voltage rail and to ground. This should protect the FETs from inductive spikes.

Secondly, insure that at no time are the FETs of either side of the bridge EVER on together. This situation creates a dead short from the power rail to ground and will blow FETs.

Driving the FETs are LM324 op-amps used as comparators. These are lousy amps and a worse comparators. They may not turn off fast enough to insure that no on time overlap occurs. It's hard to find this kind of stuff without good test equipment like a fast storage scope.

Try the diode fix first, then dig deeper if necessary.

Reply to
Bob Eld

I allready did place shotky on every mosfet, still burning, also I did try with some other gate resistors and so....problem is in dead time then....but this design end then.

Does anybody have something similar to this?

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That's a stunningly bad circuit, but then most circuits are stunningly bad.

These work nicely:

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but I'd add some clamp schottkies for luck.


Reply to
John Larkin

Um, this crcuit PWMs at 270Hz.

10K gate drive resistors.10mA gate drive currents. Give me a break.

As long as there's no actual shoot-through or gate overstress failure, and as long as the 324 can actually shut the Pchannel parts off, theres no reason why the circuit shouldn't work below 20V. It obviously does, to a certain extent.

Some kind of ripple current, though.

If all you're doing is introducing better drivers, you could probably retain the electronics to generate the control signals, providing that they're troubleshot before applying juice.


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It sure could use some ceramic low esr caps across the mosfet bridge. The mosfets have some antiparellel diodes in them already albeit mediocre ones. when the freewheeling currents attempt to go back into the bus you're getting a spike on the bus. A couple of caps with low ESR and ESL (think x7r ceramic) with a couple hundred uF electrlytic in parallel as a bulk capacitor and you might be in business. These go diretly across the mosfets (drain of the high side to the source of the low side.

That said hte circuit design is poor at best. The 10K gate resistors will be SLOW turnon and turn off causing heat and shootthrouh.

I'd scrap that and find a Brushed motor driver PWN IC (TI, onsemi, fairchild) that had real mosfet drivers in it. you'll be much better off.

Reply to
mook Johnson

I was just mulling over the possibility of substituting a better quad op-amp when I noticed that Q3 and Q5 will never be properly fully turned on since there's no high side drive. Heck, Vcc for the LM324 ins't even as high as V+ due to D3 and R15.

It's a truly very poor design.


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What about to omnited D3 and R15, and just leave cap?

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You mean cap to paralel on each MOSFET?

I try to put 70r also but is same.....

I tought, but I need it for about 10-15A here, if someone know some IC which can go inreverse maybee?

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that pig again.

there was a detailed discussion about that circuit a year or two ago. None of the skilled SED regulars had anything nice to say about it.

Dont bother repairing it, it will never work properly.

Cheers Terry

Reply to
Terry Given

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