Single input h bridge circuit for motor control.

Go To Last Post
7 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi All,
I've to control 2 motors using PWM. I used an h-bridge to switch the directions of the motor. Thus, 2 pwm outputs are needed for one motor, thereby needing 4 pwms for 2 motors.. but i'm restricted to 2 pwms on my board (the MuC on the board used is the ATMEGA32)

Thus, i've used a single input H-bridge circuit, where one pwm output is sufficient for controllin both forward and reverse directions of the motor..

My prof said ive to control the changing of directions using software.

Any ideas on how to do that using just one PWM per motor?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Supposing you are not allowed to modify the uC board, start designing your H-bridge with only one PWM input and an additional input for direction control. If your H-bridge is already built with two PWM inputs, it wouldn't be hard to add a demultiplexer at it's inputs and transform it this way.

The H-bridges with 2 PWM inputs may be easily destroyed by hardware and software design errors when both inputs are activated if no protection is built there. Using H-bridge with one PWM input and a direction control input is always safer.

Dor

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

To expand on the above, what happens when you accidentally allow both the upper and the lower transistor to be on at the same time is a very nasty almost-short-circuit between power and ground. The only thing limiting the current is the RdsOn of the two transistors, which for power FETs is likely selected very low, milliohms probably. 12V, 100 mOhms, that's 120 A right there. Burrrrn, baby!

There are loads of fun variants of the H bridge, and even more when you start wanting to brake the motor, and measuring it's power, voltage and a whole host of other aspects. Fun fun!

As said, look into how you would incorporate a direction control signal to your H bridge. Look, think, and if you need more help, post a follow-up question then.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

thanks both of u!!
cant work with the motor until another 3 days, ll post follow up questions then.. my prob right now is to make sure that there's no short as u mentioned.. and control one motor with just one pwm.. thereby oughta use the single input h bridge circuit.. which has already been built.. fiddlin that by usin software is somethin i oughta think abt a lil more.. coz i dont exactly know how it works! :(
Thanks for the replies.. ll post specific questions asap!

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

The first question I'd have is what the voltage and current requirements of the motor are. If they are low enough, there are single chip solutions like the L293. If they are higher, there are driver chip/power MOSFET combinations that would work. If they are very high, then things get messy. Without knowing what the motors will require, it's not really possible to suggest much.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Oh sorry,
Well the motor is a permanent mag, brush type motor.. with a voltage ratin of 36 volts and current ratin of 2 amps.. this is why it gets messy.. but i've managed to control that.. my issue raises wen i've to control both the forward n reverse direction using just one pwm output!! I need ideas for that!!
Thanks! :)

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:

my issue raises wen i've to control both the forward n reverse direction using just one pwm output

If it's a DC brushed motor then the way you reverse it is to switch the polarity of the applied voltage - PWM only helps with the apparent magnitude of the applied voltage - not its polarity.