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strange 3 input DC motor help on its operation Answered

i have a servo from a mattrax hummer rc from radio shack (its PCB has broken)  so i opened the steering servo its has a small DC brushed motor, but what puzzles me is that it has 3 leads that control it, my question is how does that third lead work, it i attached to two capacitors and each lead to one of the other ends of the connection, a basic schematic and picture will be attached (if i figure out this uploading feature) also the servo was a proportional control servo with a sensor that indicated when the wheel has turned to maximum. any one have any idea how it worked and maybe even how to wire it to an Arduino with proportional control, also i don't think i works for noise suppression but any ideas 

11 Replies

DVSduckBest Answer (author)2012-06-29

The intent of the third lead is two fold. To use the capacitors as they were intended, simply ground the third wire. Do not forget that in the original configuration, both motor leads are most likely powered from a transistor bridge. Thus each lead is applied to power and ground as the toy is operated.

First is for noise suppression in trying to keep the PWM type signals from becoming a noise source for nearby radios the same way that capacitors are used on automotive ignition systems to keep them out of the radios.

The second propose of these is to limit voltage spikes as the PWM switches on and off. They could not apply either lead to ground because of the bridge needed to power the motor with the ability to be powered both ways.

The short answer is to do one of three things
A) Ground the wire to reduce electrical and radio noise
B) Ignore the third wire, (Roll it up and tuck it out of the way)
C) Cut it off and remove the extra parts
I vote for the first, but do as you want.

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ubdussamad (author)2013-02-18

IT'S a common technic for making a ground it avoids addtional leakage of the current and it also improves the performance of the motor!

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iceng (author)ubdussamad2013-02-19

IT has no affect on DC leakage but does shunt high frequency
commutator brush arc AC disturbances to ground.

Since the motor is generating the arc energy with or without the
capacitor, I don't believe it improves the motor performance at all !

IT'S for reducing Conducted and Radiated EMI electrical brush noise,
and this action is called suppression.

Suppressing broad band arc noise allows nearby radios 
to receive a cleaner signal.


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bannutechniclor (author)2012-07-25

only the two terminals work.(blue & yellow). the third (green) is just an extra it works when any of the other one of two is having inner shorcircuit. only blue & yellow to be connected and if thie polarity changed the motor moves forward Or reverse. just neglet the green. enjoy

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The nerdling (author)2012-06-25

still works with out that lead

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luig (author)The nerdling2012-06-25

it does actually but i was wondering if the third pin has a special purpose besides suppression?

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SaurIMX (author)2012-06-23

Third pin is PWM (pulse width modulation). it's connected to PWM port of the µC.

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