LED SMPS

[Mark Hennessy]

Guys
I dont think a computer power supply is what I need. Unless you know of a way to adjust the 12 volt supply up.
I want a supply that can give 14 volts at 10 15 or 20 amps

Well, as mentioned, a lot of PC PSUs do incorporate a trimmer. Most, in my experience. If not, it's easy to add the means if you understand how the TL431 regulator circuit works (and if not, I can explain it).

As to current, you can easily get 20A from them. Picking an example at random:

https://uk.rs-online.com/web/p/pc-power-...s/1366052/

I recently bought one of these for my NAS box - as such, it's much smaller than a regular PC PSU. But, electrically, it's a standard ATX power supply, rated at 250 watts, which is pretty modest by modern standards.

This one gives 24 amps on the 12V rails. Comfortably more than 20A.

You can find some internal pictures of this supply here:

https://www.flickr.com/photos/66071596@N...2767148918

As you'll see, there is a trimmer. Whether this gets the voltage up to 14V I can't say, but as I mentioned earlier, it's just a resistor change if not.

Note that I'm not specifically recommending this unit for your application - it's a compact type (Flex-ATX form factor) - a conventional unit will be easier to come by (and modify if needed). It's just an example that happened to be at the front of my mind right now.

I want a supply that can give 14 volts

To be fair, you didn't state the 14V requirement until now.

Notwithstanding the fact a PC PSU can be made to give 14V, I'd like to know how much different those 2V really makes. I have no working knowledge of class C RF stages, but for a class B audio stage, it's just over 1dB, which is totally inaudible in all practical circumstances. Also, I wonder how well a transceiver will work with the output noise of a SMPSU?

Away from PC power supplies, do watch out for LED PSUs. Some of them are current sources rather than voltage sources. In my experience, they're not very good at either - they're doing a specific job which is a bit different to a regular power supply.

Turning to the example you gave, I took a close look at the 300W unit, and reckon it's a cheap clone of a decent Meanwell PSU:

https://www.meanwell.com/webapp/product/...od=SPV-300

Meanwell make good stuff. Compare this datasheet with the eBay listing - whoever "ELENKER" are, they are not Meanwell. Will the ELENKER be safe? Will it have genuine certifications? Note how much more information the Meanwell datasheet gives - things like output ripple voltage, output voltage adjustment range, MTBF, etc. This unit has active PFC - does the ELENKER? At 300W, it ought to! The fact I couldn't find a proper datasheet tells me everything I need to know...

However, I did find this page, which details a 360W version:

http://www.imajeenyus.com/electronics/20...ndex.shtml

He modified the feedback network to produce an adjustable output voltage (4.8 to 15V). There are lots of photos, a video, and even a schematic!

As suspected, there is no PFC. This is not allowed (EN61000-3-2 states that all switched-mode PSUs above 75W must have PFC in some form), so this unit has no approvals for use in Europe - hence their use of the CE mark is illegal, and I don't see any other safety marks like UL or TUV (what a surprise!). There are some dubious areas of PCB design (inadequate clearance, etc). Very limited input filtering, and no output filtering at all. No over-voltage protection crowbar. Basically, not a patch on the Meanwell. But it is quarter the price...

In short, avoid. But if you wanted to risk one of these cheap horrors, at least it's been comprehensively documented and you know what you're getting into

Personally, I'd still prefer an ATX PSU... Perhaps because I have a load in the attic