I've been trying to figure out a way to switch polarity of an output by using transistors so that I can operate a motor in either direction depending on the pin output from a PIC or similar. Basically I want a DPDT relay in transistor form.

I have cobbled together a few elements on the attached pdf which include a pair of NPN transistor switches, a pair of PNP transistors and a transistor inverter.

Basically a NPN and a PNP switch work together to connect pins to +V and 0V. When the state of the input changes (5V to 0) the inverter switches to the opposite pair which connect the pins to +V and 0V but the opposite way round.

I have run this in a simulator and it seams to work but I was wondering if there is a simpler alternative?



One alternative I have considered is supplying one pole of the motor with a constant +5V and having the opposite pole varying between 0V to 10V. This would give a Voltage drop across the motor of +5V to -5V. This seams simpler. Can anybody see any problems with this?

 

Google "H-bridge" and you'll find about 6.02x10^23 solutions.

 

Think what your after is called a H-bridge. A search on the forum will give many hits, and Gooogle will give many, many more. Mosfets would be a better choice if your talking more than a few amps. If only a couple of amps, there are IC's that are ready made to do it, the SN75441ONE is a good one. http://www.alldatasheet.com/datasheet-pdf/pdf/28616/TI/SN754410NE.html

Here's a good site that explains it better than me - http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/index.html

JaguarJoe types faster than me.

 

Now this is why I love All About Circuits. There is always somebody (unlike me) who knows what they are talking about.

Thank you very much, you've put me on the right path.

 

I have had a chance to look at the SN754410NE and it looks like you can power two motors from it which is what I am looking for.

Since all outputs will be at the same voltages, can I vary the motor speeds with a varying square wave on the inputs or would I need to use two ICs? I'm looking at using normal motors rather than stepper motors.

 

Yes, as long as you want both motors at the same relative speed one 555 timer circuit will run both. If you want two different speeds use two 555 circuits. One other thing is you can double up both bridges on one SN75441 to run a higher amperage motor.