# Need 80 volt 8 amp power supply

#### BReeves

Joined Nov 24, 2012
410
Trying to figure out the easiest way to come up with a cheap 80 volt ~8 amp DC power supply for the LED flash project I posted in the projects section. At my disposal are several UPS power supplies and a 1000 watt 220-110 step down transformer. The supply doesn't really need to be regulated.

If I reverse the step down transformer I will end up with about 55 volts RMS, full wave bridge and a couple caps will get me close to 8o volts but not quite. ~77 volts with no load.

Wondering if I could cannibalize a UPS power supply and come up with anything usable. The UPS supplies are mostly APC and Direct UPS with batteries that died and weren't worth replacing. Imagining schematic might be hard to come by and the transformers probably have a low voltage secondary for the inverter.

Only other thought is a switching supply from scratch with the hard part again coming up with the proper transformer.

Any other ideas for something that won't cost several hundred bucks?
Thanks

#### ISB123

Joined May 21, 2014
1,236
LM317 with bypass transistor.

#### MikeML

Joined Oct 2, 2009
5,444

#### ISB123

Joined May 21, 2014
1,236
80V at 8A? I'd like to see that...
You are right totally forgotten that he wanted 80V.Would work fine with 30V and nice heatsink.

#### bertus

Joined Apr 5, 2008
21,683
Hello,

A TL783 could do the job if a big bypass transistor is used.
The TL783 can be used upto 125 Volts.

Bertus

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#### GopherT

Joined Nov 23, 2012
8,012
LM317 with bypass transistor.
Not that I would use it that way but the LM317 can be used at higher voltages, it is specified to DROP 37 volts. A Zener to the adjust pin to ground can push the voltage up. Fine for a fraction of an amp but I've never seen a circuit that it setup to do this for 80 V.

Back to the question, why the heck is the OP designing Led strands at 80 volts. Why not make a series/parallel array to allow him to use a more available power supply. Try to stay under 30 or 40 volts for safety and, if possible, stay under 12 V if you want easy access to high amperage, ATX power supplies that are a few dollars from broken computers. Or, buy a new one with 800 watts of 12v power for under $100. #### MaxHeadRoom Joined Jul 18, 2013 23,949 Look at the surplus market Magnetek at the bottom https://www.surplussales.com/Transformers/IsolationXmers-1.html Has various secondaries, 80vdc with a bridge/capacitor combo requires 56vac. The alternative is to find an isolation transformer and strip windings off until you get a suitable secondary voltage. Usually works out at around 2.5turns/volt. Max. Last edited: Thread Starter #### BReeves Joined Nov 24, 2012 410 Back to the question, why the heck is the OP designing Led strands at 80 volts. Why not make a series/parallel array to allow him to use a more available power supply. Try to stay under 30 or 40 volts for safety and, if possible, stay under 12 V if you want easy access to high amperage, ATX power supplies that are a few dollars from broken computers. Or, buy a new one with 800 watts of 12v power for under$100.
The plan is to drive a 3 1/2 amp 35 volt 100 watt LED array with about 37 amps for ~200 microseconds and see if I can emulate a flash tube. Going to charge a couple 1000uf caps to 80 volts and use a MOSFET triggered with a 555 to fire the LED. The project is outlined in this thread.

If the basic idea works I eventually want to be able to fire it at 15 to 20 flashes in a second. I am trying to see if I can emulate a commercial unit that uses a flash tube.

I looked into the UPS idea and see nothing in them that would really help. The transformers are low voltage and useless for what I need without rewinding. For the initial tests will probably just use the step down transformer and live with whatever voltage I end up with.

#### yourownfree

Joined Jul 16, 2008
99
The plan is to drive a 3 1/2 amp 35 volt 100 watt LED array with about 37 amps for ~200 microseconds and see if I can emulate a flash tube. Going to charge a couple 1000uf caps to 80 volts and use a MOSFET triggered with a 555 to fire the LED. The project is outlined in this thread.

If the basic idea works I eventually want to be able to fire it at 15 to 20 flashes in a second. I am trying to see if I can emulate a commercial unit that uses a flash tube.

I looked into the UPS idea and see nothing in them that would really help. The transformers are low voltage and useless for what I need without rewinding. For the initial tests will probably just use the step down transformer and live with whatever voltage I end up with.
There are of course many ways to arrive at the same thing but here is an idea.
make a 555 or similar switching circuit not too hard, drive some heat sinked MOSFETs to drive the transformer in step up mode. The MOSFETs powered by an identical transformer if you have one or at least one capable of doing 1000 watts. The 555 running at 120 Hz, divided by 3 or 4 your choice with a switch, then drive a flip flop to get 50 percent. In other words 555 runs at 120 Hz, then say a cd4017 ring counter or something else if you prefer, this is where you choose to divide 3 or 4 or by choosing whatever pin you want can be anything from 1-9. that in turns runs a flip flop which drives a push-pull type final (MOSFETS) that the transformer is attached . Now I am thinking that I dont know if you are using a battery as the source voltage or mains power. Whatever voltage you have going to the mosfets, make it it adjustable to control the output of the project transformer. I also think that because you will be using 15-20 Hz as the drivng frequency that the voltage will be different for 15 and 20 Hz, or should say the output will not be the same. I want to say the transformer the transformer will be less voltage with less frequency, but I am not 100% sure. I know that transformers become more efficient with a higher frequency. I guess what I am saying is at 60 Hz the transformer will be what it is as it would be in a circuit but running it at 15 or 20 Hz not so. But if you can adjust the voltage at the MOSFETS I think it would work. I have not done any calculations but I suppose an electrolytic cap of substantial size would be nice as a cushion. I am not sure how much MFD's so to speak you can charge it to before it fires again. Obviously there is a point you can charge to in that short amount of time something like .066 seconds for 15 Hz to reach 2/3 capacity of the cap. Like I said I have not any calculations so I really dont know what size cap to use. what I am saying is you might get a cap too big, or more than you can actually charge up in a short time. I dont think hooking a cap to the circuit without a startup or inrush resistor is a good thing. Holy crud those things are like a short when they charge at an instant. So a resistor just for starting purposes then flip over to direct once it is charged the very first time you turn on the unit. maybe a 555 timer attached to a MOSFET to drive a relay would do.
Ok so this is just an idea I had. Ideas are good to help you to get to the final result. Always helps me to have many ideas then choose mix and match.

#### R!f@@

Joined Apr 2, 2009
9,751
I used to series connect UPS transformers secondary to get desired voltage

#### ronv

Joined Nov 12, 2008
3,770
The cap is the right way to go. The question is how much time in between flashes?