H-bridge circuit

Posted on Feb 7, 2014

Develop a cheap high-current circuit and still use PWM. Specifically, did I get the flyback diodes right I placed the high-speed Schotkky clamping diodes in the circuit to protect the MOSFET from the CEMF generated by the motor as the power is switched on/off via the PWM signal. This will help keep the FET from generating to much heat and destroying itself. Also, the Optoisolator is a fast-switching type that will help

H-bridge circuit
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ensure the MOSFET is switched on/off quickly. The opto has a fast rise/fall time. The relays are set up in an H-bridge configuration while the FET is used for the PWM signal. If anyone finds errors in this design please post them. Updated the schematic for those that want to save an I/O pin. There is a Hex Inverter/Buffer circuit (U1) that feeds the inputs of the Optoisolator (U2). If you look at the wiring for the Hex Inverter you will notice that the output of the second inverter feeds the input of the first inverter. So, when a logic 1 is placed across pin-3 it is inverted into a logic 0 which turns off the Reverse Relay. A logic 0 is also placed at the input of the first inverter which gets converted to a logic 1 on its output and turns on the Forward Relay. By using the inverter circuit you will no longer have the capability for dynamic breaking. In other words, one of the relays will be active as longs as powered is applied to the circuit. Disabling the PWM signal will keep the motor from turning. I have used something very similar with RC cars that don`t need to go in reverse much. only I use a single DPDT relay for direction with it being energized for reverse. The only thing that I can see, and this depends on the intended design, is that you have a HARD electrical brake to the motor when the FWD and REV relays are in the same position. This can cause abrupt stops if your intention is to free-wheel. But as I said...

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