Servo motor conversion for simple DC use

[Nile]

As seen in the Servo control on the cheap thread, this is my method of using servos as cheap point motors.
I don't claim any originality here, this was inspired by an article in Model Railroader.
Most of this is copied from my club's forum (members only), from a guide I wrote a few years ago.

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This is a guide to how to convert a servo motor to operate from a DC power source (such as a battery) rather than a control unit (such as that made by MERG).

The servo I am using is the Corona CS-928BB,which comes with a number of actuators.

First,place some sticky tape over the joint at the gear end of the body.This is to stop the gears falling out.

Next,remove the 4 screws holding it together and remove the end cover.

 

Unsolder the 2 wires to the motor -

Pull the PCB out of the body and unsolder the 3 wires inside -

Be careful not to melt the body when doing this,there is not a lot of room inside.
Unsolder the 3 wires and put the PCB aside -

 

[Nile]

continued...

 

Separate the 3 wires slightly -

 

 

Solder the middle wire to one of the terminals inside the body.This is to stop it flapping about and causing trouble.
Solder the other 2 wires to the motor,it doesn’t matter which way round -

 

 

Refit the cover and put the 4 screws back in.That’s the end of the modification.

The gears limit the movement to a total of 180 degrees of rotation.This is more than enough for us,but it does mean the motor needs to be centred before we can use it.To do this,fit a long actuator to it and gently turn the motor anti-clockwise until you reach the end stop.(It is possible to damage the gears if you turn it too far,too hard).
Refit the actuator so it is at 90 degrees to the body -

 

 

Now rotate the actuator 90 degrees clockwise -

 

 

The motor should now be in its mid position.Photo shows the range of movement possible.

 

 

The way I have used it is to use the cross shaped actuator,with one of the long arms cut off. The 2 smaller arms can be used to limit the movement.

[Nile]

This shows one way of using it (hint, it's upside down).The arm can move about 15 degrees either way.Power is only applied long enough to move the arm,by push button or non-latching switch.The microswitch on top is for frog switching.

Other servo motors may come with different actuators,so may need a different method.The internals should be similar.

 

This diagram shows how I have wired it up. Ideally there should be a fuse inserted between the buttons, as it is possible to short circuit the batteries if you accidentally push both buttons. A sprung toggle switch would avoid this, but I have loads of buttons available.

The voltage from a single battery is enough to move the motor, with a slow motion action.

 

I conducted some tests on a motor that I'd removed the end stop from the gears, so it could rotate continuously.

 

Voltage          Free running Current          Stall Current

 

1.0 V                   70 mA                              245 mA

1.5 V                   70 mA                              360 mA

2.0 V                   70 mA                              470 mA

5.0 V                   92 mA                           > 500 mA 

( the PSU current limit was set to 500mA, it was also very hard to stall the motor without damaging the gears at 5V )

 

the minimum voltage that would turn the motor was around 0.5V

[daveyg]

I posted this in 'Servo control on the cheap'

[CourthsVeil]

Interesting concept this. Since yesterday I use it on my 2-points micro layout – works a treat!

Here's the wiring:

 

 

 

Since I couldn't obtain on-off-on DPDT-switches I added the two push button on-switches. The second pole of the DPDT switches may be used to juice the respective frog (not shown).

 

Regards

  Armin