Need a MOSFET H-Bridge, questions about this schematic

wayneh

Joined Sep 9, 2010
16,102
1) Is "A" or "B" the control, er, forward/reverse?
Yes. :p
But seriously, in this setup it's crucial that A and B are never both high. If both are low, that's OK and the motor is off.
2) Can I use the 12v for the motor to control A or B?
Yes, the same 12V could be used as long as the MOSFETs are OK with 12V on their gate. Many are fine at 12V but I'd check the data sheets.
3) What is "EN"?
Enable. If that's held low, the motor stays off.
4) Why the 1N4148 diodes?
So that the EN only acts by pulling both gates low. If EN goes high, it doesn't matter.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
252
Thank you!
While waiting for a reply, I decided to breadboard the circuit, then lost confidence
with the schematic, and came up with the following. I plugged a little 12v motor in
with a direction indicator on the shaft. Works nicely.

Instead, the N-chanel gates are drained by
the 10k resistors, I suppose I could have gone lower value. If the switching time was
too fast, the gates might not have a chance to drain...


upload_2019-3-5_15-32-41.png
 

MisterBill2

Joined Jan 23, 2018
3,883
Really, that "enable" point, if it was held low, would indeed keep either side from switching on, and if the diodes were not perfect it might lead to both sides switching on. And you certainly do need to avoid switching both sides on and have a fuse in the bridge supply voltage line.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
252
All good advice, thank you.

If I had a 3-position, 2-gang rotary switch, I could connect the EN to the center contact of the second gang,
and the FWD/RVS to the #1 and #3 contacts of the first gang. With the EN switch position passing to ground, it would drain the gates of their charge, preventing shoot-thru when changing direction.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
3,883
All good advice, thank you.

If I had a 3-position, 2-gang rotary switch, I could connect the EN to the center contact of the second gang,
and the FWD/RVS to the #1 and #3 contacts of the first gang. With the EN switch position passing to ground, it would drain the gates of their charge, preventing shoot-thru when changing direction.
You need to be very careful with rotary switches because there are two types, shorting and non-shorting. In a shorting type switch the contact is made to the next position before it is broken from the previous position. If there is not much chance of a stray voltage getting to the gates I would not bother with the enable function. I do suggest changing the two gate pull-down resistors to 1K ohms so as to reduce the effect of any stray voltages. And if you are using a double pole switch anyway you could have the second section connect the gate not driven to the common (negative) supply side. That would assure that the MOSFET would stay off.
 

MisterBill2

Joined Jan 23, 2018
3,883
The purpose of the typical H-bridge is to remove the power part of the switching function from the control part. Yes, a mechanical switch is simpler unless the control point is far from the motor part.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
252
I appreciate all the angles shared here., folks. Lots of variation on a theme, I'll use 1k for the pulldowns, and
short opposite gate, just to be safe. This should get me started, thank yous.
 
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