Alternate methods for driving DC motor

[Jon Slaughter]

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

 

[Tim Wescott]

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

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

 

[Mook Johnson]

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

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)

 

[Robert Adsett]

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)

There are a couple of disadvantages in using linear control. The first
and most obvious is the power loss (and resulting heat in the linear
components). Another related effect is the possible torque loss at low
speeds. The PWM technique has a current multiplication effect at PWMs
< 100%. For example the current in the motor at 50% PWM will be ~2X the
current from the power source. Of course these are more pronounced at
higher powers.

Of course PWM comes with it's own disadvantages such as noise from the
PWM switching and the need for caps to handle the ripple current.

Robert