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Reverse for DC motor PWM

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5ky

New Member
Hi. I've been playing around with PWM with my 555 timer and was curious how I would go about adding a "reverse"? Would I use a dpdt relay or an h-bridge or are there other methods? I'm not exactly asking for someone to just post me a schematic or anything, I'd just like to know how it's done, but if you have an example I'd love to see it :D

Thanks ahead of time.
 

5ky

New Member
ok well to show that I'm not just here to mooch some free work, here's my attempt at my relay version of the motor reverse

i actually built and tested it and it works but it's not as elegant as i'd like, are there any other ways to accomplish this without using a mechanical relay?

oh and i was curious as to where i'd need to put the snubber diode, would one across the relay's power terminals in reverse parallel suffice? i don't know how i could put any beyond that point since polarity will be switching when i hit the button for reverse

oh and I'd like to know roughly how electric golf carts do reverse, they seem to do some light dc braking, then switch polarity once the speed is slow enough, since i'm guessing a polarity switch going full speed would induce some serious current AND mechanical stress. i'm guessing to do something that fancy i'd need some kind of microcontroller like a PIC, yea?

but anyhow:

 
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dknguyen

Well-Known Member
Most Helpful Member
You use an H-bridge. DPDT relay would work for switching direction, but not for PWM speed control simply because they aren't fast enough and have a switching lifetime. A DPDT wired up for this task is basically the same thing as making an H-bridge out of 4 relays instead if transistors.

In most cases you would normally put the diode in anti-parallel with the inductance to supress flyback current. But for an H-bridge where current can flow through the inductance in either direction you obviously can't do this since it would be a short circuit in one direction. So instead you put the diodes in anti-parallel with the H-bridge MOSFETs. These provide a current path between the inductor terminals to supress flyback current, just like a diode in anti-parallel with the inductance, except the path won't conduct until the voltage across the inductance terminals exceeds the power supply voltage. In this way, it will not produce a short-circuit in one direction across the motor under normal operating conditions.

THis should answer other diode questions.
H-bridge secrets part 1

You'd need an MCU for soft-starts, soft-stops, and soft-reverse.
 
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5ky

New Member
You use an H-bridge. DPDT relay would work for switching direction, but not for PWM speed control simply because they aren't fast enough and have a switching lifetime. A DPDT wired up for this task is basically the same thing as making an H-bridge out of 4 relays instead if transistors.

In most cases you would normally put the diode in anti-parallel with the inductance to supress flyback current. But for an H-bridge where current can flow through the inductance in either direction you obviously can't do this since it would be a short circuit in one direction. So instead you put the diodes in anti-parallel with the H-bridge MOSFETs. These provide a current path between the inductor terminals to supress flyback current, just like a diode in anti-parallel with the inductance, except the path won't conduct until the voltage across the inductance terminals exceeds the power supply voltage. In this way, it will not produce a short-circuit in one direction across the motor under normal operating conditions.

THis should answer other diode questions.
H-bridge secrets part 1

You'd need an MCU for soft-starts, soft-stops, and soft-reverse.
thank you very much, i appreciate the info, i'll look into the link:D
 
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