# PSU question

Discussion in 'General Electronics Chat' started by Kinoton, May 11, 2014.

1. ### Kinoton Thread Starter New Member

Jan 20, 2014
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Hi, I would appreciate some general advice on a power supply question. I have been for some time looking for a power supply that runs off single phase 230vac and is capable of producing a variable output of 30-40Amps at 24vdc. The power supply will power a xenon lamp in a projector. I know such supplies are available but are quite expensive. My question is how difficult would such a supply be to build ? My understanding is that the supply would consist of a transformer , rectification stage and smoothing cap at the output. If the transformer reduces the 230 voltage to 24v , how does it create 30 amps from the 230 13 amp input ? Sorry for the naievity of my questions ! I d be grateful of any help or advice, even if its just to say that if I have to ask the question in the first place then its probably not something I should even consider ! I'm really just interested in how this type of PSU works in practice, is it AC in then transformed then rectifier etc ? Thanks.

2. ### paulktreg Distinguished Member

Jun 2, 2008
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Simpilify things 24V @ 30A = 720W required on the output.

Not taking into account losses you need the same power in so 720W/230V = 3A approximately.

So 230V @ 3A on the primary will give 24V @ 30A on the secondary.

3. ### profbuxton Member

Feb 21, 2014
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General advice!. Secondary 24v & 40A output is approx. 1KW. Current X voltage. This needs to be supplied by the primary of your transformer. 1KW/230v. Approx 5A.
So you would need a transformer capable of supplying such a load( assuming full load). Rather simply,the transformer will have two separate windings on an iron core and the "primary input " when energised will "induce" a voltage in the "secondary output'. The output volts is determined by the number of turns on the secondary. The wire size is determined by the current needed.
A bridge rectifier rated at at least 50v & about 70A would do the rectification. Smoothing or filtering of the output could be by a inductor but would be rather large and expensive for the current rating, also some fairly large capacitors would be needed depending on how much filtering is required.
Projectors are usually steady loads but I see you want a variable output. That would need some regulation circuits, probably some MOSFETS or such, heavily heat sinked to keep them cool, and design of appropriate control circuitry.
Another option would be to use a Switched Mode Power Supply. These rectify the 230v AC then use high frequency switching of the DC to control the output. They allow use of much small components because of the high frequency of operation. Not easily built by beginners though.

4. ### Alec_t AAC Fanatic!

Sep 17, 2013
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They are expensive because they are relatively complex. It's my understanding that you can't simply apply 24V directly to a xenon lamp, because of its start-up requirements.

5. ### kubeek AAC Fanatic!

Sep 20, 2005
4,629
785
Have you thought about using a welding inverter? You can set current on them and the voltage sounds about right. Also some have high voltage arc start, so it might be all you need in one box.

6. ### Kinoton Thread Starter New Member

Jan 20, 2014
20
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Thanks for the replies guys, much appreciated.

Alec - yes you need an ignition circuit as well, around 40KV , this is incorporated into the lamp house though so wouldn't need to be part of the design.

I have considered a welding supply but Im not sure the output would be steady enough as the 24v needs to be extremely smooth to avoid problems with the lamp, also aren't they designed for short bursts rather than a steady supply ?

It sounds like its something that will be tricky to build but out of interest does anyone have a schematic ? It needs to provide 1-1.6KW power 24VDC/around 20-45 amps (so that the light level can be attenuated)

Thanks Jon

7. ### AnalogKid Distinguished Member

Aug 1, 2013
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I think "extremely smooth" will be a problem. For a traditional linear power supply design (transformer, rectifier, filter), smooth doesn't happen without either very large capacitors or a very large filter inductor. 1 V ripple = 330,000 uF. Even with a linear regulator, a capacitor bank for a 40 A output will be big.

Your requirements sound like a job for a switching power supply rather than linear, and there are several vendors for 24 V / 40 A switchers, but I'd be surprised if you can find one for less than \$1 per watt.

ak

8. ### kubeek AAC Fanatic!

Sep 20, 2005
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You cannot have variable output current and constant output voltage at the same time. I suggest you decide which of those you actually need and go from there.

9. ### AnalogKid Distinguished Member

Aug 1, 2013
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I think what he means is a 24 V output with a 30 to 40 A load.

ak

10. ### ronv AAC Fanatic!

Nov 12, 2008
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You can get big 12 volt server power supplies for the price of shipping. Do you think you could "stack" two of those for 24 volts and switch to one (12v) as your dimmed version?