Hello, this is my first post on the forum. I am very much a novice in electronics but willing to learn. I'm working on a project to use a standard R/C system to control certain functions of a piece of power equipment that uses 12vdc motors. I'm feeding the PWM signals from each channel of the R/C Rx to a low-pass filter, then to an op-amp, and then to a window comparator that gives me two outputs to control an h-bridge. I need some help with the h-bridge. The existing speed control PCB for the FWD/REV uses a PWM +12VDC signal to the (-) side of the motor to control the speed in FWD only. The further you turn the speed control pot, the longer the negative going pulses of the PWM signal are. REV is full speed.

I think I need to invert this signal (SPEED_PWM) and connect it to the gate low-side N-channel FET? Is there a way to configure an LM339 comparator as an inverter? Is there a better device to do this? I will be ordering more parts.

When both A and B inputs to the h-bridge are OFF (HIGH), is appears that there may be an issue with SPEED_PWM being directly connected to Q4? Maybe I need to have an ENABLE signal for this. Any suggestions on a better way to do this? I can reconfigure the window comparator for ON= HIGH outputs if it would help.

I would appreciate any advice on the overall circuit, especially in regards to any missing capacitors and diodes. Vss is from a 12v battery being charged by an alternator, hence the 12-14.5vdc notation.

 

Your gate drive will create significant shoot-through currents when changing state, basically shorting the supply during the transition.
This one screams for a micro-controller, you could achieve much faster response and build-in switching dead-time to cleanly avoid the shoot through problem.
Use the timer resource to measure the pulse width, you could use this measurement to control your PWM output too.

 

Surely there must be a way to build a reliable h-bridge that doesn't involve a micro-controller. I really need to keep this as simple as possible.

 

How much current do your motors draw?

 

How much current do your motors draw?

40 amps max.

After doing some more research, I've decided to use a full h-bridge driver and all N-Channel FET's. Here's one that looks good to me:
http://www.intersil.com/content/dam/Intersil/documents/fn63/fn6382.pdf

Are there better choices?

When choosing the MOSFETS for this 40 amp h-bridge, is there anything to be gained by choosing a higher than needed max drain current rating? I'm thinking something in the 60-80 amp range would be appropriate (I'm planning on using plenty of heat sink on the FET's).

 

I generally use the IR2110, here is some examples using all N ch.
http://tahmidmc.blogspot.ca/2013/01/using-high-low-side-driver-ir2110-with.html
Max.

 

I generally use the IR2110, here is some examples using all N ch.
http://tahmidmc.blogspot.ca/2013/01/using-high-low-side-driver-ir2110-with.html
Max.

Thanks MaxHeadRoom. Your blog has some very useful and well explained info. I know you like the IR2110 but two will cost $8.20 from digikey while the ISL83202IPZ is $3.84. Is there a compelling reason why I would choose the IR2110?

 

Do NOT want to start an Arduino/PICAXE rant.

Arduino's have most of the supporting circuitry completed. So it is quick to develop with because much of the work is done for you. Also, being an old (literally) BASIC programmer, I found the conversion to C on the Arduino trivial.

Since you stated your proclivity for hardware, perhaps the PICAXE option won't be as daunting to you.