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Topic: Custom H Bridge Design (Read 13782 times) previous topic - next topic


Mar 16, 2011, 12:42 pm Last Edit: Mar 16, 2011, 02:33 pm by Graynomad Reason: 1
I'm not even vaguely a MosFET expert but that's how I understand it. In the same way that you pull the gate to 0v to turn off an n-ch you raise the gate to +V to turn off a p-ch.

Anything less than the V on the gate and it starts to turn on.

Rob Gray aka the GRAYnomad www.robgray.com


Well, I had a chat with one of the technicians today. Basically, the following diagrams summed up the problem for me and the potential solution.

Fortunately, the mosfet driver I'd chosen will do the job apparently and, as long as I power it from the 12v power rail, will provide the 12v to the mosfets.

He also suggested I add 1 ohm resistors into the gate lines to prevent the mosfets 'ringing'. (no idea what he meant there, but I'll probably do it anyway).

I suppose my remaining question would be regarding the diodes. Do i need diodes of that rating?


I hold my hands up.

I'm beat.

I made up PCB boards, soldered one of the bridges up, put the 12v supply onto the bridge and one of the tracks popped.

The motor wasn't even running. Shorted again. Had the TC4427A in place as well - it should've been in default 'off'.

Don't know what I'm going to do now.

Is there a chance that I damaged the mosfets/diodes when I soldered them?


The source of the "control" FET and therefore the gates of the other two are floating. So the 5v control voltage has no reference, Vgs is not known so who knows what that FET is doing. If it doesn't turn on the other gates are floating as I said and who know what the other FETs will do.

I wish someone who knows more about FETs would step in, but meanwhile I'd pull the gates of the two power FETs down to 0v with a resistor to give that control voltage a reference.

And put a load between the two power FETs, if there's a resistor in there you can measure the voltage across it to see which way the FETs are switched, and you won't blow things if they are both on.

It's too late to think about this any more. See if that makes a difference.

Rob Gray aka the GRAYnomad www.robgray.com


Thanks for the suggestions Graynomad, but I think I'm going to cut my losses. I've learned alot about the subject area but I just lack knowledge of the overall subject to get it working in time.

Regardless, onto plan b. I've ordered diodes, transistors and resistors, and SPDT relays for making an h bridge that way. I've attached the schematic diagram I'm going to follow and (hopefully) succeed in making.

I'll let you know how it goes.


Did you ever try the h-bridge design in the below post?

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