I recently made this simple circuit to power a small DC motor as part of a power feed mechanism on a milling machine. It uses 2 DPDT relays to decide the polarity of the motor and I have that running through a single mosfet driven by a 7555 timer to give me speed control over the motor with a PWM signal. The polarity is selected with a toggle switch that has 2 on positions and an off position in the middle similar to a rocker switch.
My problem is even though I have diodes across all coils and switch contacts, after only a few direction changes my timer dies so I lose speed control. Before I added the diodes across the relay contacts my circuit would die as soon as I changed direction but now it will survive for a little while and then go out randomly.
I don't have it shown but the timer is wired in a standard astable configuration with a single potentiometer to change the duty cycle.
The strange thing is that when I bench test the circuit with a smaller motor the circuit survives, so I don't know what else I can try. Every example circuit of this kind that I can find online has far less protection diodes than my design so I'm just stumped on what to try next.
Also there is a 10v zener diode connected from VCC to ground of the timer to hopefully clamp the voltage but that still didnt help.
Any help and suggestions are greatly appreciated.