Current isolation between two brushed motor driver outputs to get forward and reverse on motor

Thread Starter

degamble

Joined Jan 7, 2018
5
Hi, I have unique design requirement where I have two separate motor outputs which I need to connect to one motor. Each motor output from the motor driver is for a brushed motor which output 0-8VDC sourced from a small 2 cell lipo battery. I would like to use one motor output for forward motor direction then use the second motor output for reverse by swapping the polarity to the motor.

The motor at full power use ~1A. Only one motor will be on at a time. Plugging this in directly will just short the first motor output on the second motor output. I determined I need to disconnect one of the motor outputs which it is not in use to keep it from shorting.

I have tired various configurations to protect from the short. First I tried diodes, it allowed the motor to barely spin both directions but when they where on the diodes were nearly smoking and the current was 5A. Then I tired MOSFETs, both P and N channel which worked with out with one of the motor outputs connected but when I put it all together I had the same smoking results as the diodes.

The design requirement limits me to change the wires running from the motor control outputs, so I can't run a separate power line and PWM control. I have to use the two variable voltages outputs and then only add a circuit at the motor end to get forward and reserve direction.

Any suggestions would be helpful! Thanks in advance
 

dendad

Joined Feb 20, 2016
3,008
If you must use the outputs you have, use one to drive a DPDT relay to control the direction, and the other for the speed.
But, as AlbertHall says, you need a motor control bridge. There are many available for just this purpose.
 

Thread Starter

degamble

Joined Jan 7, 2018
5
Thanks for the support. I believe when I using P and N channel mosfets on each of the motor outputs to the motor I was making a H bridge but the gate control and forward voltage reverse might have been causing issues.

Is there H bridge IC out there which can follow my design requirements without needing a supply voltage and pwm logic control? I need two separate voltage outputs running to the same motor with polarity swapped with switches open when motor output turns on and off when off to protect the other motor output. Most of the common H bridges seem to have a separate power supply input and separate logic control.
 

Thread Starter

degamble

Joined Jan 7, 2018
5
Thanks for your support, here is a quick sketch of the setups I have tired. Both solutions tend to short one motor output with the other motor output resulting in ~5 amp power draw and the motor turning very slowly. I was worried the gates were being triggered by the diode so I tried p and N mosfet but still didn't work.

Here is the goal:
goal.JPG
And what I have tried so far, which all end up nearly shorting and pulling lots of current.

Dual BiPolar motor output to one motor.JPG
 

Thread Starter

degamble

Joined Jan 7, 2018
5
Thanks, I haven't tried simple relays, so maybe something like this? I supposed I would need them on both side of the output to prevent shorting. To be extra safe I was thinking about having the opposite motor cut power to the other motor's output so if they both end up on for some programming accident, all relays are off (I haven't drawn up that yet). Any relays part suggestions would be helpful. I'm guess when you say simple your are referring to the big solenoid style ones. A smaller solid state (fast) solution would be more ideal but I could make it work.


Relays.JPG
 
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