PIC driving Adjustable Voltage Regulator

[Charles Linquis]

Your circuit will allow you to vary the output voltage, but only over a narrow range. I see that your input voltage is 24 - which implies to me that you need an output voltage that is probably above 10V.

The LM317 series of regulators is designed such that the output voltage on pin 2 is 1.2V greater than the voltage at the "adj" pin 1. For that reason, your circuit will have a voltage output of a little over 1.2V (at "0" PWM) or to a maximum of 6.2V at PWM = 255.

If you want full-range, you should add an op-amp with a gain of about 5 at the R-C junction, and the output of that op-amp would go to the LM317 adjust pin. I would also pull up the PIC output pin to Vcc through a 1K (or so) resistor to help insure that the pin swings close to 5V when it is HIGH.

Adding an op-amp to the circuit would also let your raise the impedance of the R1/R2 circuit. That would allow for a smaller capacitor and lower your PIC loading.

[skimask]

Adding an op-amp to the circuit would also let your raise the impedance of the R1/R2 circuit. That would allow for a smaller capacitor and lower your PIC loading.

Or maybe use one of those digital potentiometers, like the Xicor series types, for your voltage divider...

[sougata]

Hi,

Look at this article here http://www.elecdesign.com/Articles/I...rticleID=13077 . I hope no explanations are necessary.

[Acetronics2]

Hi Flotul

I already saw a '317 as a couple of KHz PWM amplifier ( yes ON/OFF !!! ) ... OK ... precisely ON/1.25v ...

Seems it's also already used in classA amplifiers !!! ( see National Datasheet )

Just have a nice filtering for the PWM out of the Pic and it will work just as you want !!!

Alain

[flotulopex]

Alain,

I downloaded the datasheet and quickly read it.

Could you give me a little more info about what you mean by "a couple of kHz amplifier"?

Sougata,

Your link just gives everything I need . I thought I could make something with less components; I'm very short in space.

[Acetronics2]

Hi, flotul

I just mean the ref pin is driven by a 1kHz 12v p.p. square wave !!!

The use is a variable switching supply for model Trains ...

Alain