DC Motor. When manually pushed my card goes on... How ?!

[weeb0]

Hello,

I have a little problem that I don't know how to fix, if fix is needed.

I have a small control board that control a motor, a pump and a linear device (like car electronic door unlock device). It can happen that the end user would turn the motor manually. When the motor is turned in reverse polarity, I have a schottky diode (flyback) that short the motor and everything is fine. When the motor is turned in the normal way, up to 12V 100mA is sent in my circuit and the card is powered on. I don't understand how this can happen since there is a mosfet that should closes the circuit.



I could add a diode to isolate the 12V but I would lose 0.6V and it is not something that I want.

Or I could think that there will have a 12V voltage on my board (unsure how this can occur) and it should not cause any harm to the board ? Maybe add a zener somewhere to be sure the voltage would not increase more than 16V

Any suggestion ?

Thank you

 

[KlausST]

Hi,

maybe it is caused by the FET internal body diode.

Klaus

 

[schmitt trigger]

As Klaus indicates, it is caused by the body diode.
You have two solutions:

1) Simple one, add a schottky diode to isolate the motor. If you properly select the device, it only drops about 0.2 volt.

2) Add a PFet instead of the schottky. With a low RDSon device, you will drop less than 100mV. But you will require a NPN and a couple of resistors to level shift the microcontroller's output to 12 volt.