I have a circuit for a 12v dc motor with a battery and a PWM controller. the system sort of is in this order battery–>PWM–>motor. I want to add in a switch to reverse the polarity to switch directions of the DC motor at will. Without the PWM, its a pretty simple operation and straightforward, but I was wondering how to do it now with the PWM. Do I put the switch after the pwm before the motor or between the battery and pwm? I wasn't sure how the PWM would affect the system direction and didnt want to ruin anything in my system while implementing the new switch
Electrical – adding a switch for direction reversal in 12v DC motor w/ PWM controller system
dc motorpwm
Related Solutions
Putting a large electrolytic capacitor across the motor obviously won't work in reverse, but even if that wasn't a problem it's still not a good idea.
When PWM turns the bridge transistors on they will charge the capacitor as quickly as they can, but when turned off it will discharge more slowly through the motor. This has two undesirable side effects:-
It will destroy the accuracy of your PWM. With the capacitor 'filling in the gaps' between PWM pulses the motor will spin faster than expected at lower PWM ratios, but less so when heavily loaded.
High current spikes will occur as the capacitor charges, putting stress on the bridge transistors and increasing electrical noise in the circuit. There may also be audible effects.
You can eliminate these problems by inserting an inductor in series with the motor/capacitor. However if your PWM frequency is low enough to be audible then the inductance required will be quite large (since you can't use an electrolytic capacitor the amount of capacitance you can apply to the filter is limited, so the inductance must be larger). Also the inductor must pass the relatively high DC motor current without going into saturation, and its resistance must be low to keep losses down.
If space is not a problem then simply use a choke with appropriate inductance and current rating. Then the only problem will be keeping it quiet!
The LC filter's cutoff frequency should be much lower than the PWM frequency, eg. if the PWM frequency is 1kHz then the filter should cut off at 500Hz or lower. If you used a 10uF capacitor then you would need about 10mH of inductance.
Something like this should work. Note that the limit switches are different- one is normally closed and the other is normally open. If you have a relay with three form C contacts you can avoid using the diodes.
simulate this circuit – Schematic created using CircuitLab
The two contacts are used to reverse the motor in the usual way, but one contact does double duty as a self-hold for the relay. The diodes prevent SW2 from shorting out the supply when the relay is de-energized and the limit switch to engage the relay is activated.
You can put a bridge rectifier across the motor to absorb inductive spikes from the motor inductance.
Best Answer
There are a few ways of doing this, but the easiest is the one to insert a DPDT relay in line with the motor.
Depending on the polary of the output, you might need to connect VCC instead of GND on the contact of the Relay, but the concept remains the same.
With a simple switch or a GPIO you can drive the relay and change the direction
simulate this circuit – Schematic created using CircuitLab