Hey everyone,
I’m trying to control a HS-7940TH servo powered with 7.2V battery (from Pololu) using a 5V Arduino PWM output. However, the servo doesn’t seem to turn on or attempt to move when I power the servo with 7.2V but does work when it is powered via the Arduino’s on board 5V regulator. See image below…
The setup on the left works fine. The servo turns on and performs as expected.
The setup on the right does not work for some reason. The servo does not even attempt to move and the servo arm can be moved freely. Note: the servo is not under load. The Arduino isn’t resetting so I don’t think it’s an issue with the servo pulling too much current.
Is the 2.2V difference between the servo voltage and the control voltage too much? Would reducing the servo voltage to 6V work or am I stuck with 5V?
I want to power the servo with a higher voltage than 5V so that I can take advantage of the increased servo speed at those voltages.
Thanks!
Hello.
I am sorry you are having trouble getting your servo to work. I do not see anything obviously wrong with the way you have connected everything. Have you verified that your battery is fully charged? Can you use a multimeter and measure the voltage of your battery? If you have a benchtop power supply that can source several amps of current, you might try substituting that in your system to see if your batteries were the issue.
If your batteries are fully charged, can you post a picture that clearly shows your connections for the setup on the right in your image?
-Jon
Doh! Just noticed the battery voltage was low. I plugged it into the imax B64C charger and it charged the battery up. However, the battery voltage is now over 7.4V (high end of the servo operating voltage); will that burn the servo out? Should I discharge the batter to 7.4V?
It is a very bad idea to exceed the manufacturer’s absolute maximum voltage ratings on anything. You can put a diode capable of carrying a couple of amperes in series with the positive battery lead. That will drop the voltage by 0.7 or more volts, depending on the current draw.
That’s the operation voltage range not the maximum voltage rating. The spec sheet for the servo does specifically state what the maximum voltage rating is so I figured it might be something common to all servo’s with similar operation voltages. I’m very familiar with electronics just not with servos.
I took a gamble and hooked up the battery and everything seems to be working as intended now. Thanks everyone!
Servo operating voltages are typically reported as nominal battery voltages, so a servo rated to operate at 7.4V should work for a battery that nominally supplies 7.4V, even though its voltage can be higher than that when fully charged.
-Jon
