Servos Surge on start up - My solution

When I started working with servo controllers I had the problem of surging on power up. If I had given it thought I would have constructed the doors on the heater box where they could have passed over center on closing without binding; but since I did make it that way, servos surging on power up trying to move against a closed door only damages servos or mechanism.

I searched for solutions. Others had the same question but no one really had a good answer. I thought I would pass on that which worked for me.

When I set up the Micro SSC I set up the servo power to be controlled by a Mosfet. When the circuit comes alive the SSC is powered but not the actual servo power. Once I have sent a command to the servo to move or hold a position, after a brief pause I then turn on the servo power.

In doing so I have eliminated the servo surging when it starts.

I thought that might be helpful so I thought I would pass it on.


Thanks for posting; I am glad that solution is working for you. By the way, it is generally not good practice to send signals to servos (and other devices) while they are not powered, though I have not personally damaged a servo that way.



What specifically is the problem with a servo? In looking at the servo circuit I am not sure what would actually cause a problem.

I guess I will be a test mule in that regard as they could not be used surging against a wall every time they started up.

One of the problems with sending signals to any device when it is unpowered is called “phantom” or “ghost” powering. The device in question is weakly powered via the clamp diode usually present on the input line. The clamp diode and other circuitry can be burned out, (with the input circuit burning out shortly thereafter) or if not, unintended actions can take place.

In case you cannot avoid sending signals to unpowered chips, a common practice is to put a resistor in series with the input line (say 10K ohms) to limit the current. For a high impedance input this resistor won’t affect operation under normal circumstances.

Servos are generally designed to withstand some abuse, so you are probably safe with your solution.


Another option would be to use a servo that allows you to enable motor (torque) separate from sending a position.
The Robotix AX-12A servos are nice, because not only can you enable/disable the motor torque separate from setting the goal position, but you can also read the current position back (even before you enable torque) which lets you know the open/close state of your lid or whatever.
The draw-back (if you call it that) is that you have to use a TTL UART rather than a PWM servo control board. The benefit is that you can control many servos with a single UART, as they daisy chain.