Electronic – Driving a servo with MOSFET


I am trying to build a small battery powered device containing a servo. I would like to be able to turn off the servo to save battery life. I have read previously that MOSFETs can be used to do this, but I am having trouble finding example circuits that are detailed enough (missing resistor values with no way to calculate them) and to be honest I am not too sure what sort of circuit I am looking for (I have never used any FETs before). Can someone please give me a nudge in the right direction?

potentially relevant info:

  • code running on a mega88 @ 3.3V
  • 4.8-6V servo connected straight into the 6V battery pack (I would like to change this)

Best Answer

You didn't mention how much current you require. Here is a quick guide --

For most switching applications the important parameters are the voltage rating (BVdss), the maximum drain current (Id(on)) and the gate turn-on voltage.

For a 6V battery you want a breakdown voltage at least 6V. Make this a bit higher in case switching produces transient voltages. Since the majority of FETs have voltages of 20V or greater this should not be a problem. Choose a 20V or 30V FET.

Choose a maximum drain current above what the servo requires. The maximum drain current is usually limited by the thermal performance of the system not the device. How much current do you need? How large a device can you use? Do you have room for a heatsink?

To use the FET as a switch in a 3.3V system you want a logic level device. This will insure that the device is fully on (lowest on resistance) at 3.3V levels.

For circuitry I will usually put a pull-down resistor on the gate so that the gate is never floating. For some applications I will place a zener diode across the gate for transient protection.