Connecting a Servo Motor to the FP-PWM-520

[TelecomStudent]

I am connecting a Hitec HS-300 servo motor (http://hitecrcd.com/Servos/hs300.htm) to the PWM module but I am running across some problems. I've connected everything correctly and verified the output signals using a digital o-scope. The servo does not operate UNLESS you take a multi-meter and probe from Com to Vout on the given channel the servo is connected. Now we figured that the multi-meter is acting as a very high impedance so we measured the resistance of the multimeter probes and found the multi-meter acting as a 20k Ohm resistance. We grabbed a 22k Ohm resistor and put it from Com to Vout of the channel. This enables the servo to operate correctly, but we are still VERY confused as to why. The ser
vo motor isn't sinking extra current that would cause this problem nor does it have any special external circuitry requirements. We'd like to be able to come up with a better solution and a better justification for our temporary fix. Any help??

I have attached documentation for the servo motor and a diagram illustrating how the connections are made for an operational servo motor.

Thank You,
~Jason Lovel

[TelecomStudent]

Well after some more testing in the lab we found that the PWM was riding on a positive 0.8V - 1.0V edge, when it should be closer to 0V. The PWM signal should be operating at 0V -> 5.0V but in our case it was around 1.0V - 5.0V. We put a 4.7k Ohm resistor from the signal wire to ground to drop the 'low' voltage back down to 0V. It works fine now but still doesn't exactly explain the reason why the 'low' PWM signal is riding so high.

~Jason

[Aaron_G]

Jason,

In your diagram, you do not show how you are powering the FP-PWM-520's outputs. I am assuming that you have a 5 Volt supply wired to the V & C terminals based upon the spec sheet for the servo. The FP-PWM-520 has an internal impedance of 0.3 Ohms. The 0.3 Ohm impedance will cause a voltage drop in the module. Thus, depending upon the impedance and voltage requirements of the servo, you may be dropping too much voltage in the module itself.

Regards,
Aaron