I am using Mitsubishi FX-5U PLC for controlling Sg-90 Micro servo(hobby) which rotates 0 deg for 1 ms PWM and 180 deg for 2 ms PWM.
The PLC's output is in sink mode hence I can't connect it to servo directly.I am using and Op-amp OP07CP by Texas in inverting configuration(please find the attached diagram)

I have shorted the supply ground and op-amps common terminal and then used as gnd for the servo.I have tried using both different supply so that their must be sufficient current available for servo to work.

I have verified the Op-amps output on Oscilloscope and verified so that it exact matches with the Arduino's PWM output. The motor runs neatly on the arduino board and external supply(by shorting their ground).But i am not getting why its not working on the op-amps outbut when:

  1. Vcc is 5.5 v
  2. all ground and common are shorted and connected to motor
  3. control signal verified on oscilloscope is given

on switching power supply the motor gives small jerk(means the circuit is complete).Then it is not moving at all.I have kept the wave frequency much below 50Hz still not getting o/p.

servo datasheet:
Weight: 9 g

• Dimension: 22.2 x 11.8 x 31 mm approx.
• Stall torque: 1.8 kgf·cm
• Operating speed: 0.1 s/60 degree
• Operating voltage: 4.8 V (~5V)
• Dead band width: 10 µs
• Temperature range: 0 ºC – 55 ºC
enter image description here

  • \$\begingroup\$ You don't mention what your 5.5 Volt Power Source is. I was able to find here addicore.com/Addicore-SG90-Mini-Servo-p/113.htm that the running current is 220 mA and the Stall current is 650 mA, is your Power Supply capable of providing that? \$\endgroup\$
    – Tyler
    Commented Jan 9, 2017 at 18:49
  • \$\begingroup\$ The supply is anable of providing upto 5Amp current at maximum.I have also verified supply working without using arduino supply .I think as @Andy aka has suggested their is problem with min voltage levels of output signal. \$\endgroup\$
    – user135446
    Commented Jan 10, 2017 at 10:21

1 Answer 1


The PLC's output is in sink mode hence I can't connect it to servo directly.I am using and Op-amp OP07CP by Texas in inverting configuration

Get rid of the op-amp and use the open collector ouput from the PLC with a resistor to +5 volts.

please find the attached diagram

On that circuit you show +/-15 volt rails and this could have damaged the servo before you even tried toggling the PLC pin. It looks like the op-amp would deliver a signal between 2.75 volts and 6.3 volts when the PLC pin is toggled so at best you are unlikely to be meeting "0" logic levels.

  • \$\begingroup\$ You are absolutely right, I got the point just now that op-amp signal's minimum output voltage will not be zero volts.But i didn't got why servo will get damaged as it is isolated from +/- 15 V supply and provided 5.5V source with 5A sourcing current.But can you please explain how to use PLC's output in some detail(any diagram is appreciated,as my analog skills are terrible).Thanks for your help......!!!! \$\endgroup\$
    – user135446
    Commented Jan 10, 2017 at 10:13
  • \$\begingroup\$ OK, try it out and if that definitely solves it please come back and confirm and consider accepting this answer. \$\endgroup\$
    – Andy aka
    Commented Jan 10, 2017 at 10:15
  • \$\begingroup\$ If possible,Please upload the image for open collector use of output or suggest the links for reference. I am not getting how to directly use PLC's output for the sg -90 servo . \$\endgroup\$
    – user135446
    Commented Jan 10, 2017 at 12:24
  • \$\begingroup\$ You said "The PLC's output is in sink mode" and I have assumed this means open-collector: upload.wikimedia.org/wikipedia/commons/thumb/c/c0/… - there is just a single transistor with no collector resistor i.e. it can sink current but not source current. \$\endgroup\$
    – Andy aka
    Commented Jan 10, 2017 at 12:34

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