# 0-3v variable (dc) to contol a 12v motor

#### benster

Joined Nov 17, 2004
2
:unsure: Hi,

I need some help in finding a way to control a 12v 1380mA motor from a variable 0-3v input.

I don't know much about electronics but I can solder and follow kit instructions or a simple circuit diagram.

Is there an easyish way to do this, maybe with an external plug pack and a way to read the variable voltage so as to control the 12v from the plug pack to the motor? A friend of mine suggested using multipul "windowed comparators" but this seems like a clumbsy way of doing it and not very smooth on the motor.

I havn't had any luck searching a dozon or so electronic circuit websites.

If anyone could help I'd really appreciate it as it's the last step in finishing of my homebrew force feedback steering wheel.

Benster!

#### nerd256

Joined Nov 21, 2004
23
If would be at all possible, could you receive input via PWM? This would be extremely easy to drive a half or full-bridge of mosfets. You could also build your own PWM generator using available ICs specifically made for the purpose.

I think what you're trying to do is control the current to the motor which must be done with a transistor. In that case, an op-amp voltage follower coupled to a darlington would be the simplest design. This has the disadvantage of heavy power-dissapation in which PWM would be better.

#### Maxx

Joined Oct 30, 2004
43
Originally posted by nerd256@Nov 22 2004, 01:05 AM
In that case, an op-amp voltage follower coupled to a darlington would be the simplest design. This has the disadvantage of heavy power-dissapation in which PWM would be better.
Run the darlington as an emitter follower then have a couple of resistors as a potential divider across the output to give you a 4:1 ratio to feed back into the inverting (-) input of the op-amp. your 0 - 3 V control voltage would be supplied to the non-inverting (+) input of the op-amp.

The overall gain of the circuit would be about 4x set by the feedback resistors, you could even use a pot or a trimmer to calabrate the motor speed if you wanted to.

I agree tho with nerd256 if you can use PWM this would be a much more efficient way of doing things and generate a lot less heat in the transistor

Sorry i couldn't include a diagram of the op-amp circuit suggestion but hopefully you will get the idea from my breif description.

#### benster

Joined Nov 17, 2004
2
Originally posted by Maxx@Nov 22 2004, 01:50 PM

I agree tho with nerd256 if you can use PWM this would be a much more efficient way of doing things and generate a lot less heat in the transistor

[post=3702]Quoted post[/post]​
Maybe I could convert the variable 0-3 volts with one of these circuits:
http://www.4qdtec.com/pwmmod.html

But how do I get it to control a 12v 1380mA motor, since the input voltage is 0-3 volts and most likely a lot less current?

I found this site (http://www.discovercircuits.com/P/pwm.htm) maybe there are some good curcuits that can do this, I'm just not educated enough to figure it out on my own... If you could point me in the right direction I'd be very appreciative.

Thanks people, for all your help so far.

Benster