I have a 5 Volts, 4 Amps power supply that gives power to a Raspberry Pi Zero W (120-230mA at 5.19V), and a servo motor with these specifications:

Operating Voltage Range (Volts DC)  4.8V ~ 6.0V
Current Draw at Idle                8 mA
No Load Operating Current Draw      240 mA
Stall Current Draw                  1,200 mA
Maximum Torque Range oz. / in.      42 ~ 49
Maximum Torque Range kg. / cm.      3.0 ~ 3.5

Sometimes it is able to perform its action, and some other times it stalls half-way with the voltage dropping in the 3.5 V - 4 V range. Even though my power supply should be good enough in theory, I thought that adding a 4 F (16 V) capacitor could alleviate the problem, but it does not seem to make any difference.

Do you think the voltage drops because I am using the servo to perform a task at the limit of its strength, or that the servo stalls because it does not have a good enough power source?

Unfortunately, I do not have any tool to measure the torque of the item the servo is rotating.

Servo Motor: Hitec 32085S HS-85MG Metal Gear Micro BB HS/HT Universal Servo

Please bear with me. I have some electronics notions, but I am no electrical engineer. If you think I posted this message in the wrong stack, please let me know. It just seemed the most appropriate one to me for this question.

  • \$\begingroup\$ @jsotola My power supply is 4 A. My RPi is drawing between 120 mA and 230 mA from it. \$\endgroup\$ – Jean-François Beauchamp May 28 '18 at 20:36
  • \$\begingroup\$ Where is the voltage dropping? If it is measured at the load end, use thicker cables, and maybe supply the Raspberry Pi from separate cables back to the supply. If it's dropping at the supply, then something doesn't conform to the specification. \$\endgroup\$ – Simon B May 28 '18 at 22:02
  • \$\begingroup\$ it is possible that the power supply is unable to supply current surges, only slowly increasing current \$\endgroup\$ – jsotola May 28 '18 at 22:07
  • \$\begingroup\$ @SimonB I measured the voltage close to the load. It is true that the servo is connected with a long flat ribbon cable (2'-3'). I will try a thicker one. \$\endgroup\$ – Jean-François Beauchamp May 29 '18 at 18:03
  • \$\begingroup\$ @jsotola I guess I can solve this with the proper type of capacitor, right? \$\endgroup\$ – Jean-François Beauchamp May 29 '18 at 18:03

You basically had the right idea to use a cap for voltage stabilisation. But a cap with 4F capacity is a supercapacitor, and those have a very high output resistance. So your Supercap is not able to deliver the current needed for the servo.

Please try to use a few 220uF aluminium electrolytic type caps. They have output resistance in the milliohms range and they should do the trick of stabilizing your voltage.

  • \$\begingroup\$ @Stefan_Wyss Thanks for the info! I had based myself on this formula calculate what type of capacitor I would need: C >= (I_max*T_discharge)/(V_beforedischarge-V_afterdischarge), from electronics.stackexchange.com/questions/122413/…. I was not aware of the resistance factor however. So I will try a 220uF or similar capacitor as you suggest. :) \$\endgroup\$ – Jean-François Beauchamp May 29 '18 at 18:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.