I tried with a new diode. It works fine. Reversed the power and the magic smoke escaped.
I've used a IN4742A zener diode with a maximum current rating of 21 mA.
When running the motor at 12 volts and letting it get up to some RPM, is it possible it generates more than that (21 mA) when the power is suddenly turned off?
But even with the new diode destroyed, the motor works with polarity reversed and not reversed, just very poorly. I'm guessing there is some resistance in the diode in both ways after it has burned.
Hi,
Oh, so it was the diode. So the motor itself is ok then.
The diode will see whatever current the motor runs at when run at 12v. If it is 50ma, then 50ma, if 1000ma, then it will see 1000ma. If it is 50 percent duty cycle then it could see a 50 percent duty cycle at 50ma or 1000ma as above but the average would be 1/2 of that, or 25 to 500ma. So the diode could EASILY have been overpowered. The diode wuold get hot, so you could try a new diode and see if it gets hot.
HOWEVER, you dont use a 12v zener to clamp something that runs at 12v anyway. You use a higher voltage zener such as 14v or 16v or even 20v. Also, you cant just use just one zener if you reverse the current to run the motor backwards, you'd have to use two zeners in series, back to back, with the two cathodes together or the two anodes together.
Alternately, you might be able ot use a full wave bridge and clamp the motor voltage right to the power supply voltage and ground, depending on how you are driving the motor and what your power supply looks like.
So what you should do is measure the current of the motor when it runs at the normal 12v and we can then calculate the average diode current, or come up with another method to clamp such as the bridge rectifier if you specify what kind of power supply you use to run the motor.
If the motor draws more than about 160ma then the diodes could blow again, so something else has to be done.
It would also help to know where you are getting the PWM signal from.