Apologies in advance if my terminology is not exact, as I'm a very amateur hobbyist just starting out.

I have a DC servo motor that I bought used, rated at 24V maximum, with variable RPM depending on how many volts I supply to it. The motor works fine when I connect it to a 9V battery, but pulses and drains a connected power supply periodically (every second or so).

I bought a power supply so I could use the motor from an AC outlet (USA - 120V).

This is the power supply: https://www.amazon.com/gp/product/B01B1PRE42

Input Voltage: 110V/220V; 
Output Voltage: 24V DC 2A Output 
Current: 0~2A 
Power: 48W 
Shell Material: Metal case 
Protection: Shortage Protection. Overload Protection. Over Voltage Protection 
Safety Compliance: CCC/ FCC / CE 
Working Temperature: 0~45 degree 
Storage Temperature: -20~65 degree 
Ambient Humidity: 0~96% Non-Condensation 
Package Content : 1 x 24V 2A Switching Power Supply 

The power supply has a small green LED above the variable output screw. I tested the output with a multimeter when I connected it with 18/2 lamp wire to my outlet, and I verified (at least I think I did) that it is outputting between 9V minimum and 24V maximum, depending on the screw being fully counterclockwise or clockwise, respectively.

When the motor is connected, it spins for a brief moment (100 milliseconds or so) and the green led on the power supply goes out. Then the green led turns on again (it takes about a second) and the motor again spins and the led goes out, and this cycle continues endlessly.

I have checked that all my positive and negative connections between the power supply and the motor are correct, and the neutral AC to power supply matches as well.

What am I doing wrong?

EDIT: photos of the motor...

Motor photo 1

Motor photo 2

  • 3
    \$\begingroup\$ You haven't given any details of the servomotor, but it may be that the current it needs to start moving is more than the supply can handle. This style of supply usually has a fairly limited overcurrent capacity and will either fold back (reduce the supply voltage) or shut off completely once that overcurrent limit is exceeded. \$\endgroup\$
    – Phil G
    Jan 29, 2019 at 22:20
  • \$\begingroup\$ Thanks @PhilG, I wish I had more details on the motor, it was surplus from my local hobby store and is 15 years old (I got it real cheap!). I searched the internet high and low and could not find any specs. The only info I have is the variable RPM speed that the seller had. \$\endgroup\$
    – binarymax
    Jan 29, 2019 at 22:22
  • \$\begingroup\$ A photo would be better than nothing. Someone might recognize it. \$\endgroup\$
    – rdtsc
    Jan 29, 2019 at 23:11
  • \$\begingroup\$ Can you hook up a multimeter in series and measure the current? Even basic ones usually have a max-hold-feature. \$\endgroup\$
    – winny
    Jan 29, 2019 at 23:22
  • \$\begingroup\$ @PhilG has the right idea. Your supply is probably not able to handle the inrush current and is shutting itself off. \$\endgroup\$
    – Hearth
    Jan 30, 2019 at 0:12

1 Answer 1


Without seeing the motor specs I expect the servo can draw 10 x the rated load current when accelerating to reach full speed. Your supply can only support a slow ramp. This is normal for good DC motors. The current drops as rapid as the speed rises as current is a force that controls acceleration then rated load at rated RPM. Surge currents are always much higher than the static current rating.

This means the supply cuts out from overcurrent causing shutdown then recovery, so you get a jerky pulsed acceleration.

Your only options are ; use a slow ramp for speed up, get a bigger supply. SLA Battery option may be possible to support the surge.

  • \$\begingroup\$ A big enough capacitor will also do the trick. A very big capacitor. \$\endgroup\$ Jan 30, 2019 at 3:20
  • \$\begingroup\$ 10F cap vs 50kF SLA’s maybe. Ic=10Amin = 1V/s * 10F \$\endgroup\$ Jan 30, 2019 at 5:36
  • \$\begingroup\$ Thanks for your help! I had an AC/DC converter lying around from an old modem that outputs 12V and 1A. I sacrificed that and hooked it up, and after I turn on the motor it takes about 2 seconds and then starts spinning continuously. Is there any danger in using this power supply? It seems to work well for my purpose. The pause concerns me but I suppose that is the effect you mention in your answer and this is normal? \$\endgroup\$
    – binarymax
    Jan 30, 2019 at 14:49
  • 1
    \$\begingroup\$ Yes ok and normal for a weak or overprotected supply. The 24V voltage and much more current provides extra torque when needed. \$\endgroup\$ Jan 30, 2019 at 15:47

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