Could anyone help me with creating a PWM controller?


Hello,

I would like to create a PWM controller that has three pot's:
- Duty-cycle pot
- A pot for adjusting the frequency
- a pot for adjusting the output voltage.

I'd also like to create this one just with transistors (so no timer IC's).
I have some MOSFET's that I can use for switching on and off the load.

The PWM controller should output an AC voltage, but if that is too hard to make, an alternating DC voltage is also good.

The frequency range should go from 30-50Hz up to 4-6kHz. (I don't really know if this is going to be possible to adjust with just one POT, but I can also use a Trim pot for this).

You don't need to explain every little detail. I have basic electronics knowledge. I just don't know where to start.


This is what I have until now. It's not working very well because when I change the duty-cycle the frequency also changes.
The frequency POT is also not good because the frequency decreases when I go to higher resistance, and when I'm going to the highest resistance, the frequency increases again :(


Is there a reason why you want to avoid using any kind of IC? This really is the kind of application that is best done with at least some kind of ICs, usually a dedicated PWM controller IC or a microcontroller.

Also, what do you mean when you say the controller sould output an AC voltage? The output from something like this would typically be a unipolar square wave, with variable duty cycle and frequency.
DVSduck5 years ago
After considering it for a few minutes it seems that you are trying to control two parameters with three pots. The frequency is easy to set up by using a variable frequency source. However, the voltage and duty cycle are highly inter-related. The more time the output is on, the higher the actual RMS (Root Mean Squared) voltage is applied. The only way to accomplish all three is to have a variable input voltage to the controller, of which case, why use PWM, although PWM is more efficient and reduces the heat in active current controlling devices.

I hope i shed a little light on what you are asking for and some of the issues that must be considered when designing it. Let me know if there is anything I can do to help.
If you have other ICs like op amps, comparators, and digital logic on hand you could build your own 555 timmer. The 555 timer is just a couple comparators that are used to set and reset an RS flip flop. The internal diagram of that IC is usually part of the datasheet, which you shoud be able to find easily. If you understand how to build comparator circuits and flip flops, it wouldn't be too hard to build the circuit you want that way.

I don't know your location, but it looks like the prices you quote are in Euro. I don't know how many stores there are in your area that would carry hobby type electronics, but it might be worth a seach to find a local source to get around the shipping cost. Another idea is that if you have other friends that are interested, see if they would pool an order and split up shipping, if that helps make it affordable. It might also be worthwhile to post a question or a topic in the forum section to see if there is anyone in your area to pool and order, etc.
There are ways to use 555 timer ICs to generate PWM signals, where the duty cycle can be changed while keeping the frequency the same. They differ slightly from the typical 555 timer circuits, and they need two diodes to keep the charge and discharge portions of the cycles separate. I could supply you with the circuit if you are interested.

If that sounds like it would meet your needs, I would use some thing like that. 555 timer ICs are very cheap. They may still carry them at Radio Shack..

As for making the output voltage variable, you could accomplish that by powering the 555 timer circuit using a variable voltage regulator like the LM317. The 555 timers can usually accept a range of input voltage like 5 volts to 18 volts, (depending on the exact variation of the timer you have)