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I want to control the speed of a dc motor. It draws about 1 - 2A. I am powering it from a PSU scavenged from a PC, so i have 5v and 12v. I want to use PWM to control the speed of the motor, using both the supplies, with a switch, so I have 2 variable channels, a high speed one and a low speed one. However, on the 12v supply, I only want it to go up to the equivalent of 7v. How do i do this? It doesn't need to be that accurate.

Thanks

Depending on your preferred allocation of time/ financial resources, perhaps the best option would be to go to http://www.dartcontrols.com/PDF/15%20SERIES.pdf These are designed for 12 or 24 volt A/C input, but I'm sure that Dart tech service can tell you where to splice in the DC. The chassis mounted units are well under $100 and i've been pleased with the quality. They may have more bells and whistles than your need, but the PWM parts alone would cost you more to assemble than this.

=>Jim<=

Why PWM? Why both Voltage ranges? If you scavenged a power supply from a PC, I am assuming that you don't want to spend any money. A PWM drive will cost in excess of $100.00. You can design one yourself if you are so inclined. If you need examples just do a web search on "PWM Circuit". Is torque a big concern? If not, why not just use a power rheostat (potentiometer)or two if you really need dual voltage levels. If you need auto control, you could use Voltage Regulator IC's.

Good Luck!

You could probably do what you want with a 556 timer chip and a hexfet or similar. I have done this for a fishing motor controller back before they offered such things. A BJT would work but the
efficiency would be lower. The 555 will work at either voltage. If you need closed loop speed control, things get a little more complicated. Try a google for 555 pwm or 555 speed control. Or
if all else fails I could see what I've got in the dungeon. The manufacturers of 555s typically have pages and pages of applications thought up over 20 years.

Regards

cww

Hi,

Have a look at http://www.mmselectronics.co.uk/000200.htm This is a simple PWM input driver.

Regards,
Marc

Have a look at the National half bridge controllers.

hello!
read about choppers. thats all you need. generally, to step down a dc voltage, you can use two approaches:

1) use a resistor or a pot. In this case power is dissipated as heat, considering your current requirements, you will need a serious heat sink. Hence I prefer 2, (see below)

2) Choppers, involves switching of output at a particular rate to obtain an average value, the best alternative…since you want to use PWM, an intelligent chopper design might give you a compact circuit to do PWM and chopping at the same time!!

Best regards.
Zahir Saleh.

I would use an L298 dual full-bridge driver to drive the motor. Do a web search for the data sheet. The data sheet gives example circuits.
If you need different voltages, use an LM317 adjustable voltage regulator. Use digital control to change resistor divider (feedback voltage).

Jack Eskew
Microsmith, Inc.
301 W. Deer Valley Road # 3
Phoenix, AZ 85027
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Hi Andy,

Have a browse around the web for model train controllers or look in your local book shop for a book on electronics for model trains. Many of these are PWM and match the voltages and currents you're looking for. As an example try

http://www.sages.com.au/contest2000/ssau2009.zip

This has dual outputs as well as a parallel port interface for PC control. Just delete any bits that you don't need.

Regards

Peter Whalley
Magenta Communications Pty Ltd
Melbourne, VIC, Australia
e-mail: peter*no-spam*@magentacomm.com.au
delete *no-spam* before sending