# DC Motors

J

I current have a 0-8V DC signal which drives a small 8V DC motor and I want to use this signal to run a 12V DC motor. But the smaller motor voltage ranges from 0-8V which in turn I want to control the voltage on the 12V DC motor in scale (variable), eg. small motor running at 4 volts and the larger motor should run at 6 volts. The 12V DC motor needs its own power supply. At the moment I am using the signal from the 0-8V to switch a small relay which inturn gives me 12V to my larger motor. My problem is that the small motor is variable and the 12V is not. How can I make them both regulate together?

G

#### Gene Climer

Q: I have a 0-8V DC signal 8V DC motor control circuit. I want to use this same circuit to drive a 12V motor. How can I do that?

A: One way is to design a 1.5 gain amplifier between your 8V drive circuit and your 12V motor. Another way would be to replace your 8V drive circuit with a 12V drive circuit.

P

#### Phil Mundy

This can easily be accomplished with a DC drive that will accept a 12 DC bus. The 0-8VDC signal from the master should be run through an isolation
device and scaled to represent the full 0 to 10 volt speed reference to the drive. My company makes both these devices but I am sure something similar is available in your part of the world.

Phil Mundy

J

Phil Mundy,

J

> Q: I have a 0-8V DC signal 8V DC motor control circuit. I want to use this same circuit to drive a 12V motor. How can I do that?
>
> A: One way is to design a 1.5 gain amplifier between your 8V drive circuit and your 12V motor. Another way would be to replace your 8V drive circuit with a 12V drive circuit.
>
Gene Climer,

Thanks for your reply. I was wondering if you could give me anymore information, or send me in the right direction, on design for gain amplifiers.

J

#### Johan Bengtsson

---------- Forwarded message ----------
From: [email protected]
To: [email protected]
Subject: RE: MOTION: DC Motors

Yes, it can be done, quite easily actually.

Well, I suppose circuits doing this can be found and others on the list might know where, but if you want to make one of your own this is what you need:

You need a CD voltage approx some 3-5V above your desired maximum output voltage, ie 15-18V.

you need one op, two resistors and one NPN power transistor capable of handling the power needed. You might need yet one NPN transistor if the power needed is really big.

Ok, now the circuit:

|+15-18V
| \ /
Input -----|+ \ | /
0-8V | \ |/
| >-----|
| / |\
----|- / | \|
| | / -\
| ---- |
*--------|R1|------*------- Output 0-12V
| ----
---
|R|
|2|
---
|
|GND

I hope you can understand my ascii graphic, it should be viewed with some fixed width font.

what you have is an op (any op will do) input signal is connected to op input + op output is connected to the base of you transistor the collector is connected to your voltage source
and the emitter is connected to the motor

The output (from the emitter) is feed back to the - input of the op by a voltage divider, this voltage divider (R1 and R2) is supposed to give 8V at the input pin when you have 12V at the output. Select for R1 approximately some 10k to 100k and R2 twice the value you have at R1.

The op should get it's voltage from the same 15-18V as the motor.

The power handled by the transistor will be worst at high load and low voltage (high current * high remaining voltage over transistor), depending in how your load behaves this can be att different speeds. I would suggest you select a transistor capable of handling the maximum current drawn by motor * the source voltage

If you need more than some 3-5A output you might need one more transistor, in that case it should be darlington connected to the other one, if that would be the case and you don't know how to do it mail me back...

/Johan Bengtsson

----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
----------------------------------------