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Old servo as a DC speed controller?

Benny_h88

Junior Member
#1
I'm looking into making an ultra small ultra light fully controllable indoor scratch build plane. I've been playing with 3gram servos, I've stripped the servo apart and centred the pot so now I have a rc motor that runs forward and back at a controllable speed. Only problem is its not got the grunt it needs to pull the plane. So I was thinking of using the motor connections to drive a transistor, any idea how exactly i might do this?
 
#2
Im no genious but I have a lot of component level electronics experience so I'll give this a whirl. What is the purpose of driving a transistor? Transistors typically are used as either an electronic switch (for instance, 5V in "turns the transistor on" to pass 12V out to a load) or an amplifier.

Are you trying to make the servo more powerful? I doubt that will work very well. They are meant to be run at 5, or possibly 6 volts. Anything more than that will probably send it up in smoke. Secondly, they aren't meant to be a continous duty motor. They aren't meant to be spinning all the time so I'm not sure of the longevity. But hey, it's a 3gram servo that probably cost you next to nothing.

sconner
 

colorex

Rotor Riot!
Mentor
#3
Basically he is using the electronics of the servo as a variable switch - to convert from a controllable PWM signal input from the receiver to a 0-5v variable voltage - which he wants to hook up to a transistor to drive a bigger motor.

Why would it not work? Try it and let us know :)
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#4
It should work, assuming your load's current requirement is not in excess of what the board can source for the original motor.

If you want to switch/control anything heavy, you'll likely need a transistor driver circuit, which could be pretty tricky if you want it to be reversible.
 

colorex

Rotor Riot!
Mentor
#5
It should work, assuming your load's current requirement is not in excess of what the board can source for the original motor.

If you want to switch/control anything heavy, you'll likely need a transistor driver circuit, which could be pretty tricky if you want it to be reversible.
Yup - he would only need the 5V servo output for switching the transistor. The transistor would allow or cut a higher voltage source, say 11.1v.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#6
Weeeelllll, sorta no, but maybe yes.

Reread the OP, and it's a touch more complicated.

The problem the OP will run into is getting the tranistor biased such that a proportional output from the motor leads yeilds a proprtional output from the motor -- effectively, the transistor will *want* to be always on or always off. if the servo controller is pulsing the DC motor to turn the gears, this will work well, otherwise you've got to balance the circuit such that the trasnsitor is not like a floodgate -- open or closed -- but a water spiggot -- turn a little get a little, turn more get a little more.

Benny,

If you want an all-on/all-off control, you can use a circuit similar to this one (listed as "Basic NPN Transistor Switching Circuit"):

http://www.electronics-tutorials.ws/transistor/tran_4.html

hook up the servo's motor leads across the "Vin" and switch, and connect the main motor across the "relay". you'll likely still want the flywheel diode -- it'll protect your transitor from back EMF when you turn the motor off.

If you need proportional control, look into a receiver brick (many have brushed ESCs installed), or a reprogrammable brushless micro ESC -- if you can reflash these, you can edit the source, recompile, and flash to act like a brushed ESC.
 

rcspaceflight

creator of virtual planes
#7
Or you could buy a bigger servo than a 3 gram. Bigger servo = bigger motor = more pulling power.

Unless you already have a bigger motor you plan on using. Then go with what everyone else is saying.