I designed this circuit for those interested in controlling motors that require a lot of Amperage, but have gotten frustrated over trying to create a MOSFET H-Bridge. I present to you a simpler design (yes it may take up more space) that uses only one MOSFET. This design allows the user to have two motors run forward, reverse, and have an electronic break. A single logic level N-Channel MOSFET (or a bunch hooked up in parrallel to increase the amount of current the circuit can with stand) allows for this H-Bridge to be PWM compatible! This circuit can also be re-engineered quite simply to use only three pins on a microcontroller instead of five, however doing this will not allow you to have an elctronic brake. Problems and Comments below :)
Is this the flaw you spotted in the original circuit? (Reference the updated one). Thanks again for helping me spot this problem! having the robot go full speed in reverse and not expecting it would have been potentially dangerous, thanks!
I spotted that too. Haha yeah, I can picture the relay spasming out when the power is applied, very loud and annoying and probably would wear down the relay.
This is a great design. I have a couple fo comments:
1) If you use a DPCO relay per motor instead of two SPCO, you save yourself a pair of digital pins.
2) Rather than the FETs all in parallel, why not split them into two groups and PWM them for speed control? That worked really well for me here. (Schematic included.)
3) I couldn’t give you the theory, but something says to me that you can’t parallel up the FETs like that. I could be confusing it with something else, but I have a notion one of them will sink all the current.
The theory I think youre thinking of is only relevant to bipolar technology, which becomes more effecient as it gets warmer, therefore all the current would go through one Bipolar transistor. The good news is, for MOSFETs the hotter they become, the less effecient they are, therefore creating more resistance and no longer becoming the least resistant path. This allows for you to put MOSFETs in parrallel and as one that is hot “cools” off, one that is already “cool” bears the load As for the DPCO relay, I want my robot to be able to coast and be able to aply an electronic break this circuit here can actually be converted into acting into what the Circuit would be with just a DPCO relay by hooking up the relays and NPN transistors differently As for the FETs in parrallel, they are just like that for simplicities sake, making it much easier to wire. Thanks for the input, it helped be broaden the view of what this circuit can be capable of with a few adjustments!
Of course. I’ve forogtten all that theory. rakes didn’t bother me as my motors were worm geared! I was thinking, though: if you have teh control set up for N channel FETs, is putting P channel FETs on the high side of the H a real chore? I seem to recall, though, that P FETs can’t source so much current. And they’re more expensive.
Anyway, if you’re sticking with relays, have you considered coding for hysteresis around zero? Otherwise there’s potential for your relays to swich many times at low demands, specially if your controller has any kind of intelligence.
The good thing about changeover relays as you’ve used, is no worries about shoot-through!
Success! This circuits works amazingly we’ll. I built a modified version for only one motor and set up the relays so only one pin was needed for controlling the motors direction and another pin was used for controlling the mosfet. It successfully drove around a 20 amp motor with a significant load. So I must say it is quite a successful and super reliable design!
this circuit here can actually be converted into acting into what the Circuit would be with just a DPCO relay by hooking up the relays and NPN transistors differently 
