Thread: Explain pulse width modulation

Basically, PWM is like turning the motor on, and off really fast. The motor always sees full voltage, but it only sees it a percentage of the time. So, if you turn it on for 10% of a second, it will only move at 10% full speed.

Get it?

Its doable if you use an H-bridge setup, for simple Clockwise, and counter clockwise motion.

First of all, what kinda of motors are they? Stepper? DC?
Do they go through gears to get down to 38rpm or is that the speed of the main shaft?

RPI th motros are DC and as far as I know they are geared. They are here:

http://www.hobbyengineering.com/SectionM.html

I'm using the Gear Motor 2 or 3 havent yet decided, but what
I plan to do is use a L293D Secret Motor Driver along with the a small motor on either side of the screen, then use a power window switch to control the up and down. It's designed with the DVD player behind the xenarc so the screen has to flip forward to load CD's and stuff. L293D Secret Motor Driver has a schematic that explains just how this setup works but because of the certain type of dyslexia I have, I have problem following linear diagrams, so I guess I ve have to find some one to help me put it all together

Ah yes, those are definitely geared motors. 38rpm is an implausibly low speed for a DC motor, so I wasn't sure why you wanted to use PWM (which is really only useful for DC motors). But back to your original post, kiltjim gave a pretty good description.
Here's an elaboration: imagine this is the voltage going to your motor: ____----____----____---- , alternating between full voltage and zero volts. This would be a 50% duty cycle, because the voltage is at full 50% of the time. A 25% duty cycle would look more like this: ______--______--______--. This happens so fast, and su much faster than the motor can possibly respond to the changes, that it just sees the input as a constant, approximate 50%, 25%, etc of the full voltage. Usually there is a capacitor between the outputs of the PWM circuitry to even out the spikes and drops a little bit.
I did my best for an explanation from memory, I'd recommend using google to find out more information if you really want to control the motors this way :P
You could also just find another gearbox to reduce the speed even more.

Thanx for the info RPI, do you happen to have a schematic of a circuit (simple as possible)??

If your looking for simple you might just want to try a simple voltage regulator. It will work the same way, it will through away some energy but it is sooooo much easier to build and understand seeing where your coming from here.

Get yourself a LM317T from radio shack. You'll need a variable resistor and a fixed resistor and thats it.

Good luck.

-Jeff

Originally Posted by rushnrockt

look up 555 and pulse width modulation, should be cheap.. although you will need an amplifier for that baby, so might not be so cheap after all

The dude probably only needs a voltage regulator. (317T) He said that he read that PWM can make it work, but he doesn't want to take circuits with calculas 501. PWM is great if you have a microcontroller drivin it but it isn't the only way to drive a motor slower.