Alternate methods for driving DC motor

- J
- Jon Slaughter
  
  Contact options for registered users
posted
16 years ago

Tue, Mar 11, 2008 2:22 AM

I'm curious at to the alternate methods to use for driving a DC instead of PWM.

Could one use power BJT's as an amplifier also? I was thinking that I could use 1 BJT and 4 relays where the BJT would control power to the motor and the relays would control polarity(direction which doesn't change very often). Not sure if this means that that the power delivered would be uniform or not though. (although I can't see how it would be that much different from PWM)

Just curious because I have a bunch of BJT's and relays laying around but no full bridge drivers that are large enough.

Thanks, Jon

- T
- Tim Wescott
  
  Contact options for registered users
Vote on answer
posted
16 years ago

Tue, Mar 11, 2008 3:28 AM

Build an H-bridge from discrete BJT's. Use the relays in case you need a hammer.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

- M
- Mook Johnson
  
  Contact options for registered users
Vote on answer
posted
16 years ago

Tue, Mar 11, 2008 11:33 AM

make ser you put a diode across the collector (cathode) and emitter (anode) assuming you have NPN transistors. This will steer any inductive currents when the motor is turned off.

Using linear control is doable but will dissipate a lot in heat in the transistor depending on the current and voltage levels involved. You will need a power transistor and a large heatsink and maybe a fan to keep it cool. Check the Safe operating area for the BJT before using in this application.

You can do linear control in a few ways. You can make a emitter follower to set a constant voltage (lower than the supply voltage) that will control the speed (with poor load regulation) or you cna use the BJT as a constant current source to control the max torque (with pretty good torque regulation)