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[SOLVED] Modified PWM Circuit Design Help Required

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hshah8970

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Hello everyone.

I will be implementing the circuit found here to drive a DC motor through Pulse Width Modulation.

In addition to the Vcc, I have a 'surplus' DC source / battery. What I want to do is power the motor through this surplus battery every time the output of the PWM is zero.

I have thought about using comparator opamps but the problem is that opamps draw the current from the Vcc supply and not the supplies connected to the input. I want to utilize the power of the surplus source in order to reduce the zero regions in the PWM waveform.

I am thinking along the lines of using comparators with SPDT (Single Pole Double Throw) relays but I need helping in an exact circuit, since I haven't used relays before.

Thanks to anybody who responds.

Humza.
 

1) What do you mean by "reduce the zero regions in the PWM waveform"?
2) Generally, PWM outputs are high when the driver should be supplying power. If you really want to supply power when it's LOW, then you can just add an inverting stage.

As far as performing the level-shifting that you require, one simple solution would be to use your PWM output to drive a transistor or MOSFET. The collector(drain) would connect to the negative side of the motor; the positive side of the motor would go to your battery.
 

@barry: Thanks for your response.

Let me try again to explain what is required.

I have a motor and it's being powered by a PWM signal with a 50% duty cycle. What I want is a SPDT relay to monitor the PWM waveform. When the waveform is Low and the motor is not getting any power, I want the relay to break the PWM contact with the motor and connect the motor with another DC supply that I have. When the PWM waveform goes back high, the motor should go back to being powered by the PWM.

The aim of all this is to reduce the time during which the motor is getting no power. In other words, the zero region of the net waveform being delivered to the motor would be reduced, without modifying the original PWM signal in any way.

Humza.
 

I'm thinking you don't quite understand the point of PWM. PWM is PULSE WIDTH MODULATION, it does NOT have a fixed 50% duty cycle as you say. The variation of duty cycle (pulse width) is what is used to control the motor speed. It sounds like what you are proposing is to have the motor powered part of the time from one voltage, and part of the time from another. Why are you using PWM, if you think it's a constant 50% duty cycle?
 

@barry:

I understand the point of PWM and I know we can adjust its duty cycle. In my experiment I will not be changing the duty cycle of the PWM.

And you are correct: my objective is to have the motor powered on part of the time by the PWM and part of the time by another DC source.

This is an experiment that I'm working on. The point of the experiment, so far, is to see how I can accomplish this objective.
 

I think I'd use a simple pulse train detector (C-R) through a
comparator to a NPN sink driver and use that to energize the
coil of a relay, whose NC position attaches the PWM to the
load and whose NO position attaches the battery instead.
 
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