Universal PSU ideas...

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Hello All!

Story; I have recently found this gigantic 300W, 230 to 110 volts Isolating transformer from a dumped soda-pop automat I found in the forest (of all places!)

This I would like to use in a powerfull universal Power supply to have in my, ahem, 'test lab' :)

I don't have much experience in large powerful PSU's so I was wondering if anyone have got any ideas / shematics or something to witch I could put the transformer to good use.

I am thinking about adjustible output voltage... 1-50 volts for example. Nothing advanced. Though, a current limiting adjustment thingy is nice to have around. maybe 0-5A or something....

Any ideas are welcome....

As far as I see, given the high secondary voltage of 110 VAC, you would get roughly 150 VDC after rectifiers. This means that you have to get rid of 100 VDC at the current you want which means something like 250 W if the maximum current is around 2.5 A (which it is for 300 VA at 110 VAC I think. That makes the PSU kinda hot. I would find another transformer and use this one for something else...it would probably be cheaper (hint: check what people think about cost for heating houses with heatsinks here).

hmmm, well, we use 230V AC here where I come from....

It is obviously possible to open the tranny by removing some screws, so if the windings is not dipped in lack or something it may be possible to wind off some turns on the secundary to give around 50-60V AC out.... this will improve current-handling too!! Will investigate this further tomorrow.

Opened up the tranny today, despite AudioFreaks advice....

Was easy, infact, the two windings (prim. and sec.) was completely sectioned, so the only hard work was to remove and re-install the E and I's... took some time.

Winded of some 60 turns I think, forgot to count :-( Added a smaller winding to be used in series if I had wound off too much.
When finished I wrapped the windings with plumbers teflon tape and put the E, I's back. (got 5 leftovers I couldnt get in...)

Anyways, I've now got a 57V AC out and a 19V AC out. Gives around 80 volts of DC... still a bit high or what?

Yeah, I'll fuse it ofcourse.

How about using f.ex. a LM 723 to control a transistor, f.ex a MJ15001 (140V, 15A max).... I can use the 19V winding to power the LM 723...

Or what about putting f.ex. three LT 350 in series, so that I don't go over the max Voltage rating?

Maybe using a LM317 HV to control a transistor?

I'm thinking like this. Anyone got any ideas / experiences?

Over 35V with an LM317

Note first, there is a higher voltage LM317 -- the HVK, but this won't get you to 80V. To regulate a voltage above the max permitted by the regulator you use a series of zener diodes in the "adjust leg" to bring the base voltage up to the level you desire. You must also protect the LM317 from "overvoltage" with a zener from the adjust leg. The description gets a little wordy, so look at this link of a 290 Volt regulator I used with a much doctor-ed up Fairchild 255a:
it's not pretty, but it works and hasn't blown up. I used a BU208 transistor as that's what I had in the junk box. For your purposes, you would use zeners to get you in the vicinity of 80V.
btw, you can get an LM317 regulator down to "0" volts by biasing the adjust arm with approximately -1.25 volts.
Oooops, rectifiers wrong way in the schematic.

To the second winding I used the wire I wounded off from the first winding. 1mm diameter + isolation.

The LM317 HV can source 0,5A, at 60V. The TL783 takes 0,7A at 125volts. Quite suitable me thinks... That means I can reference it to ground like in the schematic, yes?

One thing though. The TL783 is rated to 15-700mA. What does that mean? Does the regulator consume 15mA, or do I have to have some bleeders after it to have a constant 15mA running through it?

hv regulators

both the LM317 and the TL783 have to be derated when there is a large input to output differential. the last time I looked at the application notes at National Semi they indicated that the regulator needs a minimum base current to "regulate" at the specified level of ripple -- I haven't found this to be a problem in any of the applications where I use an LM317.

I was interested to see on TI's website the use of the TL783 as a high voltage amplifier by modulating the "adjust" pin. Do we have the kernel of an electrostatic speaker amp here? Jack
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