Changing the duty cycle of a PWM signal

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    Cycle Pwm Signal
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Discussion Overview

The discussion revolves around controlling the speed of a computer fan using PWM signals from an RC transmitter/receiver. Participants explore various methods to modify the PWM signal's duty cycle and frequency to meet the fan's requirements, which necessitate a higher duty cycle than the receiver provides.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant describes the PWM output from the receiver as having a duty cycle of 10%-20%, while the fan requires 30%-100% duty cycle at 25kHz.
  • Another suggests using a 555 timer but notes it typically has a minimum duty cycle of 50% and proposes controlling it with a potentiometer turned by a servo.
  • Some participants discuss the possibility of inverting the PWM signal to achieve a higher duty cycle.
  • There is mention of using a 556 timer, which is two 555 timers in one package, as a potential solution.
  • One participant recalls using a specific circuit for PWM control with a 555 timer for a model train controller, suggesting it might work for the fan as well.
  • Another participant raises the question of whether it is possible to have different frequencies for PWM input and output without providing a schematic.
  • One participant suggests up-converting the frequency or using a local oscillator to control the width of the PWM signal.
  • Another participant shares their experience of using an old servo controller to achieve full throttle control of the fan, noting the challenge of shutting it off completely.
  • There is a discussion about the precision required for the PWM frequency and whether variations in frequency would still allow for effective control of the fan.

Areas of Agreement / Disagreement

Participants express a variety of approaches and ideas, with no clear consensus on the best method to achieve the desired PWM control for the fan. Multiple competing views and suggestions remain unresolved.

Contextual Notes

Some participants note the limitations of their knowledge and experience, indicating that they are still learning about PWM and circuit design. There are also references to the need for specific frequency and duty cycle adjustments, but these remain open questions without definitive answers.

Who May Find This Useful

This discussion may be useful for hobbyists and students interested in electronics, particularly those exploring PWM control for motors and fans, as well as those learning about circuit design and microcontroller applications.

Packocrayons
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I'm trying to control the speed of a computer fan using the radio input from an RC transmitter/ reciever.
The PWM output of the reciever is 1ms to 2ms high and 20ms low, so 10%-20% duty cycle. The computer fan requires a 30% to 100% duty cycle, at 25khz (about a 40uS pulse)
I was thinking of using a 555 timer (astable, monostable wouldn't work) but that's a minimum of 50% duty cycle. Control this with a potentiometer turned by a servo.
Are there any other ways to do this? I wouldn't mind getting into micros because I want to learn that at some point but I need to learn how to code. I can write computer code so the concepts are sort of there but language is an issue.
Nothing impossible please, I'm only 16, I have most of the knowledge but not all of what a fully aged person would.
Thanks in advance.
 
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well there's any number of paths to success here.

Dont give up on anyone until you've pushed it a bit.
Let me ask

what happens to a 25% duty cycle if you invert it?
does it not become 75% ?

now look at your 555
if you tie TRIG to THRESH and use them for input leaving RES high, , will it not act as an inverter?

Is a 556 two 555's in one package?

Probably it's pretty easy with PIC controllers too, but i never learned them.
 
Humm I guess that makes sense (hardly), but how do I determine which circuit I'm using as the output?
I'm no technical geniusm so it might require a little teaching to get me started. Remember, I'm still in high school
 
This circuit is commonly recommended for getting full PWM control from a 555.

I used it once for a model train controller and it worked OK.

http://forum.allaboutcircuits.com/cache.php?url=http://www.sentex.ca/~mec1995/circ/pwm555.gif

You might have to fiddle with the size of C1 to get the right frequency if this matters.
 
Last edited by a moderator:
Thanks, I'll play with it a little bit to see if I can get it working.
Is there any way (without giving me a schematic, I want to try it on my own) to get a PWM input and output but different freq/dr?
 
Packocrayons said:
Thanks, I'll play with it a little bit to see if I can get it working.
Is there any way (without giving me a schematic, I want to try it on my own) to get a PWM input and output but different freq/dr?
No, I can't think of any way.

The servo command you are using is only meant for servo motors. Does your micro have any other PWM commands?
 
I think the frequency you want for your fan is very higher than the frequency generated by radio receiver. Here's some ideas. You might need to up-convert the frequency by mixing with some local high frequency. Or you can use a local oscillator and control the width using an SCR.
 
I think I'll just use the 555 and use a servo to control a pot. The frequency was functioning before it just didn't go to a high enough duty rate to get the fan to full throttle. I can not change the output unless I go into the firmware of the receiver. Since this receiver is being used in multiple applications, I don't want to do that.
Thanks for all the help.
 
I had an old servo controller lying around. Works like any brushed motor controller, using DC pwm current. I locked the potentiometer (used to determine servo location) with a little glue and now I have full throttle control of the fan. Now I might want to consider a way to shut it off when I give zero throttle, right now it stops slowing at 30% or so.
 
  • #10
Packocrayons said:
I'm trying to control the speed of a computer fan using the radio input from an RC transmitter/ reciever.
The PWM output of the reciever is 1ms to 2ms high and 20ms low, so 10%-20% duty cycle.
So its period varies from 22ms to 21ms?
The computer fan requires a 30% to 100% duty cycle, at 25khz (about a 40uS pulse)
How precise must this frequency be? For example, could it be 30% duty cycle at 20kHz to 100% duty cycle at 50kHz?
 
  • #11
I believe the receiver outputs at 22mS and changes the duty cycle. Servos can take anything up to 50mS so accuracy is not an issue, duty rate is what matters.
Same with the computer fan, I'm using a low frequency (~50hz) PWM input and it's still working just fine, again duty rate is being measured, but I am surprised it worked at this low of a frequency.
Yeah this is the easy way out, maybe I'll try the other way some other time.
 

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