Overcharging a servo

Other Hardware Motors, Mechanics, Power and CNC
1

This is an experiment in which I tested to see what happens when I used a battery above the recommend voltage with a servo - specifically the tower pro SG90 servo. I figured the results could be useful.

I used the wiring diagram found on this article

servo_circuit_
servo_circuit_1920×1080 40.5 KB

and this code:

#include "ServoTimer2.h"

ServoTimer2 Servo1;

int servoPin = 2;



void setup() {

    Servo1.attach(servoPin);
}


void loop() {
    Servo1.write(2200);
    delay(1500);
    Servo1.write(1500);
    delay(1500);
}

For the first test, I used a 6v power supply (within the voltage rating) and it ran smoothly

For the second test, I used a 7.4v battery (multimeter read 7.7v) that I am using for a project. Ideally, I would like to use this battery to power the servo to minimize the number of power supplies needed. Here are the results:

(don't mind the sketchy battery wiring)

The key is in the audio. The servo could not hold a steady position and made a buzzing noise as the head jittered back and forth while trying to hold a steady position. But the servo still (kind of) worked. Also it didn't burn out and is still usable.

Just thought this was interesting, so I figured I'd share it here. Also if anyone has any advice on how to get the servo to work with this battery that would be greatly appreciated

2

Worst case, throw a pair of silicon diodes in series with the + leg of the diode - that'll drop your voltage at the servo by 1.2 V, more or less, and should help it 'behave'. Three diodes might be necessary, depends on what diodes you get, and what the typical current draw is.
IF it's a single servo, a 1A diode should suffice. If more than that, look for 3A diodes, 1N5401 IIRC.

3
  • What voltage is measured from the Arduino GND to +7.4v battery lead while this is happening ?
4

Use a switching voltage regulator (output 5 to 6V) capable of handling at least 1 Ampere per SG90, 3A per MG996R. Pololu has the best selection, e.g. Pololu - 6V, 3.3A Step-Down Voltage Regulator D30V30F6

5
  • 7.7v is too high, use 4.8 to 6v
6

It reads 7.7v, same as the battery

7

You can buy servos designed for 7.4V power. It is never a good idea to exceed the manufacturer's ratings.

8

Thanks for the tip! I will be using 3 servos

9

That's exactly what I need, thanks!

10

While the switching regulator is a better approach from a power conservation perspective, when in a rush, a quick and dirty lashup using two or 3 3A power diodes on the power feed to the 3 servos will work. YMMV, of course - and don't forget to be wary of burning your fingers on them, if you're working the servos hard.

11

Maybe. It just not a motor, there is an electronic circuit inside the servo. Even thought the motor may be OK with the higher voltage, the electronics may have been stressed and you may have reduced reliability.

12

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