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Can I limit the voltage to a motor using PWM? Answered

Hi, I am trying to build a robot with a dual motors controlled by a l298 chip and PWM. I have some really nice 9.6V NiMh battery packs that I would like to use. The problem is that the motors need 3-5v. Could I just use PWM to control the voltage of the motors by working out what duty cycle would give me the correct voltage and then supplying that to the motors? Would I need a capacitor to smooth out the voltage and reduce noise?

Thanks, David.

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rickharris (author)2013-07-06

No The essential idea of PWM is that the full voltage is applied - It is just switched on and off very fast to give a lower average over time. this makes the motor run slower BUT gives almost full torque.

You might be able to use a voltage regulator depending on the current requirements of the motors.

Far better look for more compatible motors or a different power supply.

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AdamR1 (author)rickharris2015-08-19

Rick, how does this affect the current? I'm trying to use PWM to lower the current to a heater device. The problem is that my heater is rated for 20A, but my PTC resettable fuse is only rated for 14A. When I set my max PWM output to half duty cycle and measure with an Ampmeter, it shows 8A, but for some reason it keeps tripping my PTC fuse. Is this still spiking the current?

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steveastrouk (author)rickharris2013-07-06

Yes, but the inductance of the motor could be used to iimit the current, if the drive frequency is high enough.

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rickharris (author)steveastrouk2013-07-06

Mmmm - Yes - room for a lot if's and buts in there - more likely to see the motor emitting it's magic electrical smoke!

Might not happen depends, on how much the OP values the motors.

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rickharris (author)rickharris2013-07-06

Why are builders so reluctant to spend a small amount of time on paper design and then buy the appropriate equipment.

Most designers and engineers I know spend days and weeks jiggling ideas around on paper before they get close to making anything at all. But when they do make something it usually works.

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frollard (author)rickharris2013-07-06

Hunch: many makers come across the parts from 'other' means, scavenging, etc...and have to make do with what they have. Yes, built to order is better but prohibitively expensive in many cases.

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rickharris (author)frollard2013-07-06

I guess so - I like things to work - Sometime I Fit things round what I have but when it gets important I buy in what I need.

Unfortunately I bought a Mazda sports car last month and part of the deal with my SWMBO was I would clear the garage. Sad face

Lot of things went to the tip or to other hoarders!

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frollard (author)rickharris2013-07-07

MX-5 or RX-8? I've got an 06 mx-5 convertible and I love it. best early-midlife-crisis decision I've ever made!

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David97 (author)rickharris2013-07-06

What about making a inductor and using a capacitor to smooth the voltage?

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David97 (author)David972013-07-06

The motor driver board has the diodes to cope with the emf

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rickharris (author)David972013-07-06

Give it a shot - Steve and I will be interested in the outcome. :-) Sometimes you have to try.

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steveastrouk (author)David972013-07-06

Congratulations.You independently reinvented the switchmode PSU !

That's what I was sayingt to Rick, you can PWM fast enough to use the inductance of the motor to limit the voltage and current.

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iceng (author)2013-07-07

PWM applied to a DC motor will lower the speed ( just like lower voltage )
However the motor torque is almost as high as the full power machine !

Take care not to overheat the motor because the slower self cooling fans
are much less effective and the RMS energy pulsing the motor causes
even more heating.

The way I really enjoy using PWM is to Smoothly crawl a full train into and
out of a station. No jerking like with a variac TR unit..

A

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