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Topic: Custom H Bridge Design (Read 25470 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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I know this is a very old thread, but I've seen it referenced at least once. And there is a bit of this that cries out for clarification/correction.

Hello folk,

I'm trying to design a custom H bridge to drive my motors.

They draw atleast 6.5 amps, running off a 12v supply.
I keep seeing this particular mistake - it does not matter what the no-load current is, it does not matter if they draw "at least" some current. You want to know the stall current. The current drawn when full voltage is applied and the motor is stalled, not allowed to move. That is the worst case current.

Unless, in a particular configuration, it is possible that the motor may already be moving in the opposite direction. For instance, if you reverse direction without first breaking. Or if it is something that could already start moving and you attempt to reverse direction, like a robot or electric car on an incline.

I watched a friend of mine blow his motor drivers when his R2 replica started rolling backwards down an incline, and he hit Forward instead of braking first. He had bought motor drivers rated for only a bit more than his motor's stall current.
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When you have through hole components and a bread board you have the luxury of testing parts of the circuit before you do the full test.

I shutter at thinking that you can pass 6A through a beard board. There not designed for that.
Make sure your board isn't damaged before you assume your wiring is wrong.

Make half a H-bridge, see if you can make the motor turn. Then make the other half, make it turn the other way. Then connect them together.

The gates of the mosfets just small caps but with very high current rating, the resistor on the gate is to limit the current and protect whatever is sourcing the current.

If you tried to turn a large mosfet on with a arduino pin, the pin is only rated at 40mA, a lot of the larger mosfets I've seen have gate currents ranging from 1-15A (see spec sheet). Meaning you are maxing the pins current sourcing abilities and will likely kill it in short order. Use ohms law, voltage to gate/max-current-allowed = resistor.

The resistor will make the mosfet turn on slower and waste some power through the mosfet, but that's just how it is without a more complex design.

I'd post the design I've been working on but its all SMT parts, and it will be a few weeks before I know if its a flop or not.
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