I am trying to build a device which uses a 6-12 v 8 amp dc motor controlled by a 2 channel rf relay, and powered by a 12 v battery. i need the motor to run forward and reverse, but in reverse i need the speed to be approx. 25% of the forward speed. i have a pwm module that i use to get the reduction, but i cannot get the pwm to work on reverse only, as i am getting pwm in both forward and reverse.i have destroyed 2 relays already trying to figure this out @ 10 bucks apiece. now i am thinking of wiring 1 relay hooked to the motor using straight polarity, and 1 relay hooked to the motor reverse polarity, with a diode on the positive side of each relay. i had the device working well without pwm, but reverse needed throttled down and when i added pwm things went to hell. am i looking at this the wrong way? can reverse speed reduction be achieved another way? thanks
An H-bridge driver is needed to replace the relays - they burn out ;-) when used this way. (@mdlrodgers above)
You can look for a RC car 'esc' (electronic speed control) from tower hobbies then drive with a single PWM that is switched 37.5% or 100% (50% is zero).
Something like this can also work https://www.olimex.com/Products/RobotParts/MotorDrivers/BB-VNH3SP30/open-source-hardware.
You will need to configure the signals to get it working - you could even use a 555 timer and switched resistors to get the pwm signals.
There are many other similar products from Sparkfun, Ladyada ebay , DX etc
I would suggest (and have the junk box to prove it) that you start with a motor control module capable of handling double (or more) your intended/measured current. Reason is the back EMF you get when changing speed changes the battery voltage perceived by the controller.