IRFP240 PSU... for dac

[lykkedk]

I know this thread is slightly offtopic, but i am making experiement, with an idea of using an irfp240 based psu, for my DAC. The idea was inspired from the BoSoZ PSU.

I have tried to setup this circuit. And here is the issue:

When i connect it, the output voltage is 11,6 Vdc.
But when i connect a 20watt lightbulb, the voltage drops down to 8,8Vdc ?... The idea was, that the voltage should be regulated with the mosfet, just as in the pass psu, but something is not working correct.

I calculated the resistor, to be about 4Kohm, for having about 2mA in each zener like this...
14,7 / 4000 * 1000 / 2 = ~2mA
The reference 14,7 (look at the circuit) is correct, when meassuring, and is dropping down to 13,6Vdc when loaded.

My guess is that the mosfet dosn't regulate right ? maybe the 221ohm resistor is wrong ?

Any help would be nice.

[rtirion]

Your transformer is 18Vac at 0,5A ie something like 9VA.?

Let's assume 15VA. This would be good for a load of 10W.

I suspect your load is a 20w 12v car lamp?

If this is the case, you are simply overloading your PSU.

Put 3 or 4 of these lampes in series, and try again.

Edit:
Did you measure the no load voltage of your transformer?

18Vac should give you something like 25DC after the rectifier.

[lykkedk]

Ofcause ! Youre right... The voltage drops from ~22Vac till 17Vac, meassured on the ac output from the transformer

[lykkedk]

Well, i just tried to put in a 2,5A 24vac transformer instead.
The voltage now drops from 12,4Vdc to 9,9Vdc.
The voltage on the ac-out on the transformer drops from 27 to 24volts.

So the thing dosent work right i guess - and the mosfet, is quickly getting hot now. (I expected that through)

[Babowana]

Originally posted by lykkedk


like this...
14,7 / 4000 * 1000 / 2 = ~2mA

Should be like this:

[(unregulated V) - total zener V]/R*1000 = mA

I think you need to consider enough amount of zener current.
Look at the attached zener breakdown chracteristics, and
try to choose the zeners and proper zener current.

Good luck.