12V Regulator problem

[ifrythings]

Thanks mods for giving this another try

I see your point there dhaen that this maybe a bit high of a start for me. I know (what I think is) quite a bit about smps but I have never made one so what would you guys recomend for a first time smps for me?

P.S thanks eva for beening the one of the few people who saw what I was asking.

Oh, and also I do have a 100MHz tektronics 465B scope and 2 0-30V, 0-3A power supplys(reged of course).

Thanks for any Ideas!

 

[Eva]

The next concern about your startup circuit is that there is going to be a lot of permanent dissipation in the 5K resistor. This may be fixed by biasing the zener with a much lower current, let's say with a 100K 0.5W resistor that would produce 1mA or more, but then the voltage drop across the zener would be lower than expected. The common and obvious approach is to use a zener rated at a higher voltage so that you get again 12V in the emitter of the pass transistor.

Also, use a 1.2K or 1.5K resistor as a load, dont use 100 ohm!! (note that the pass transistor still requires a heatsink with only 1.5K load).

 

[EWorkshop1708]

If he's wanting to get 12V from 120V AC (160V DC after bridge), couldn't he use a PWM driving a large toroid filter and capacitors at a low duty cycle to get 12V similar to the way computer motherboards get 1.5V or less from 5 or 12V?

 

[Eva]

Such a complex approach has no point when only 8mA are required.