# PWM Motor frequency

#### samh93

Joined Oct 4, 2011
36
hey guys, I've built a prototype of my pwm circuit and it all works fine. The problem is that the motor whines at lower speeds which I could guess is too low a frequency. I want to increase the frequency but I'm not really sure how in this case. Could anyone help me? Like what equation do I use to work out the frequency?
Here's my circuit drawing and my prototype:

Thanks, Sam

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#### RRITESH KAKKAR

Joined Jun 29, 2010
2,829
use uC.....

#### shortbus

Joined Sep 30, 2009
7,078
Changing the pot and C1 values is how its done. The formula for the values is in the 555 data sheet. 18kHz - 20kHz is what your looking for. Just at the top end of the hearing frequency, no real need to go higher.

#### Bernard

Joined Aug 7, 2008
5,285
Using same pot, try 500pF for C1. If pot is at extreem limit,then it's about .1MΩ X .01 = .oo1 sec or inverted= 1 K Hz. Try it with .0005 μF.

#### samh93

Joined Oct 4, 2011
36
Using same pot, try 500pF for C1. If pot is at extreem limit,then it's about .1MΩ X .01 = .oo1 sec or inverted= 1 K Hz. Try it with .0005 μF.
so .1MΩ X .0005 = .00005 which inverted = 50,000 = 50KHz. Isn't 50KHz a bit too much for my circuit?

Also thanks for replying, really appreciate it.

Sam

#### samh93

Joined Oct 4, 2011
36
ohh I see my mistake, its 20KHz not 50, my calculations were wrong. I'll get it tested and get back to you all.

Sam

#### samh93

Joined Oct 4, 2011
36
Hey guys. I've gotta say you're all genius'. Thanks for all your help, my motor now runs quietly and it works better than ever .

Thanks everyone

Sam

#### shortbus

Joined Sep 30, 2009
7,078
@samh93, hope your still around Glad you got the problem fixed, in the future you might want to consider another PWM circuit.

While the one you made works, when you adjust the on-off time you are also changing the switching frequency. That may not be a problem for some things but it can be for others. By adding a few extra (inexpensive) components you can have a true fixed frequency PWM control.

One of the moderators here, Bill Marsden, has it in a blog. Here is the link to it-
http://forum.allaboutcircuits.com/blog.php?bt=678 Figure 5.3 is the one I'm talking about. Hope this helps you