I'd like to build a driver circuit that, as simply and economically as possible, takes 120VAC input and drives a variable DC load at a relatively constant .85 to 1.0 Amps peak. The output voltage would have to vary from 20 to 90 volts to deal the load changes and maintain the ~1 amp current flow. Ripple and a little noise is not a problem. Load is resistive (not inductive) and changes very slowly. I'm picturing a basic bridge-capacitor supply feeding a power MOSFET or IGBT circuit that is biased using a shunt (sensing?) resistor. I'm an industrial electrician and haven't done this kinda thing in about 15 years. I was hoping to find some example schematics that might help, but I'm coming up empty handed after about 2 hours of googling. Can anyone help?

 

Simple and cheap, use an LM317 and one resistor to regulate a constant current as shown Here:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page7.htm#charge.gif

Here is a whole page of links to constant current supplies :

http://www.discovercircuits.com/C/constant.htm

 

I don't think an LM317 would be up to the task, since max Vi is only 40v.

MOSFETs and IGBTs are swithces - can't really bias them the way we can with BJTs.

Might look into a power BJT, something like the MJL1302A.

Good refresher material can be found here: http://www.national.com/appinfo/power/files/f4.pdf

 

Got it! Thanks guys. Good information - After reading through some of this stuff and pulling my old TopSpice program out of the closet (on 5-1/2" disks, so you know how long it's been). I've got a working circuit using a BJT (on paper, anyway).

 

Hi,

MOSFETs and IGBTs are swithces - can't really bias them the way we can with BJTs.

Ahemmm... Linear audio amplifiers with MOSFET output stages is a myth then ?
And MOSFET (resistive) dimmers can't be done... Hmmm, one of us needs to read up on the subject I'm afraid

 

Hi,

Ahemmm... Linear audio amplifiers with MOSFET output stages is a myth then ?
And MOSFET (resistive) dimmers can't be done... Hmmm, one of us needs to read up on the subject I'm afraid

As I have often said, the universe is full of things I've not heard of.

That's what makes it such a fun place to live in!