Relay H Bridge

Hello all, I have a simple 12vdc motor setup. Using 2 relays, one DPDT for forward and reverse. The other to send power to the switching relay. This diagram is just a quick one I through together. I have a pwm speed controller thats actually powering the motor. My main concern is where to place the diodes for the DPDT relay so the fly back voltage of the motor doesnt damage the relay contacts over time. This is for a solar actuator, so its only moving a few seconds at a time every half hour or so. I was given a diagram where there are 2 diodes on each side of the motor that are after the relay. I just want to be sure Im placing them in the correct locations. Not sure why I am struggling with this one. Probably because of building the actuator from scratch and not some preassembled kit. Everything is already built, but for the sake of simplicity I only put up the section on the diagram where I need a little advice. Thank you in advance.

by al514
February 20, 2023

Any attempt to use a plain old diode across the motor is doomed because the motor may be either polarity. However you are using a 12 volt motor so it should be possible to put 2 zener diodes rated. say 15 or 20 volts breakdown in series across the motor to limit the spikes to perhaps 18 or 25 volts

by Foxx
March 05, 2023

Correction: I should have said in parallel not in series. Sorry if I caused confusion

by Foxx
March 05, 2023

Yeah I have looked at so many Relay H Bridge configurations by now, and the big issue is the relays, I took a measurement with my scope while starting up and shutting down the motor and was getting over 2k transient voltage spikes, good thing I used and inductive automotive ignition probe or I would have fried my scope. But I went with a prebuilt mosfet h bridge instead. Now I just need to figure out the code for pwm Arduino control. Everyday is a learning experience. Thanks for your time though.

by al514
March 14, 2023

In the old days we learned the hard way to put a snubber across any inductive load to be switched on or off. This snubber was usually a .47 mfd capacitor in series with a 100 ohm 1/2 or 1 watt resistor and was surprisingly effective. Any more a diode, zener diode or other semiconductor is commonly used.

by Foxx
March 21, 2023

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