Electronic – Drive a 4.8V servo signal line using 3.3V


I have a servo with an operating voltage of 4.8V. I want to drive the servo from a MCU running on 3.3V. I haven't tried it yet, but was wondering if I needed to drive the signal line to 4.8V using a MOSFET or NPN transistor. Is it possible to drive a 4.8V servo using 3.3V signal? Will the performance be impacted?

If I need to drive it with 4.8V how should I connect it? The gate obviously goes to the signal (with the necessary resistance) but how do I connect the servo's signal line? Should I connect the signal line to 4.8V and the ground to the drain? I'm guessing this wouldn't work. Is this even possible?

Best Answer

It will probably work with 3.3V as a PWM signal level as long as VCC is at the rated 4.8V. The Vcc label, as opposed to Vdd, suggests TTL level logic where logic high is 2V to Vcc. Just try it with a 3.3V PWM and see what happens. The important thing is to have the servo and microcontroller GND references connected.

Edit: You can also use a nMOS or npn BJT using an inverting configuration. Connect the gate/base to the MCU PWM pin, the source/emitter to ground. The drain/collector is the output and must also be connected to V+ through a resistor. When the transistor is on, the output is pulled to ground. When the transistor is off, the output is at V+, minus the voltage drop across the resistor. Since this is a control signal and the current small, that voltage drop will be insignificant.

Note that in order to use a pMOS to avoid the inversion, you would have to pull its gate to V+ to stop current flow. However, since your PWM from the MCU is only 3.3V you won't be able to do that. You are thus limited to using a n-channel MOSFET.