PWM question

Thread Starter

mazur50

Joined Feb 5, 2007
28
Is it posiable to do PWM with no IC.

I want to make a varable motor control but not use a ic or 555 timer.

I would like to just use a pot and a few caps and a mosfet. do you happen to know and circuit diagrams.

thanks
Mike
 

rwmoekoe

Joined Mar 1, 2007
172
why don't you wanna use a 555? it's the simplest oscillator ic there is. you don't wanna get down to transistors do you?
do you have any suggestions?
 

Thread Starter

mazur50

Joined Feb 5, 2007
28
I would like to make it as simple as possiable so i can learn different things. I have already done one with the 555 timer.
 

beenthere

Joined Apr 20, 2004
15,808
You can use the pot to control voltage on the FET gate to pass more or less current through the motor. But that's not anything like PWM. Some means of generating pulses with variable widths defines PWM.
 

bloguetronica

Joined Apr 27, 2007
1,359
I would like to make it as simple as possiable so i can learn different things. I have already done one with the 555 timer.
In my opinion, using discrete components is a good way of learning, but is is the worse way of making things as simple as possible.

"Reinventing the wheel" is a good choice if you want to do serious learning.
 

Thread Starter

mazur50

Joined Feb 5, 2007
28
just trying to learn how things work. trying to get the basics out of the way before i dive into IC and Cmos and pics.
 

scubasteve_911

Joined Dec 27, 2007
1,202
The basis for most PWM circuits is a triangle wave generator followed by a voltage comparator. You would need to create discrete versions of these two components and ensure they will work together.

Discrete Triangle wave generator:

http://www.diyaudio.com/forums/showthread/t-57885.html

Framework of a comparator:

http://home.cogeco.ca/~rpaisley4/Comparators.html

You should learn the circuits, how they are constructed and the logic behind them. Otherwise, learn how a FET and a BJT works, and expect a steep learning curve. Engineers stand upon a tall and historical epistemology.

Steve
 

Gadget

Joined Jan 10, 2006
613
A few years back I had a need to control the speed of a 2 volt motor (a mirror ball rotator), and as most I/C's wont work at that voltage, I used a modified 2 transistor multivibrator circuit. It worked and still works. I never checked the waveform, But it's probably fairly awful, and might be a bit hard on the output device if a high current motor were used. I think the circuit went something like this..... I havnt put any values, as I can't remember, and cant be bothered to re-work them out.



(opps, ignore the polarity of the motor symbol)... ;)
 

SgtWookie

Joined Jul 17, 2007
22,201
I tried simulating that circuit with various values and various transistors, but no joy.
 
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