I need to run multiple traditional 12v MES bulbs, 2 watts each, and use old toy train transformers, which can provide an amp or two, which rather limits the number of bulbs each transformer can power. Usually around ten bulbs each.
Seen on eBay for 10 - 15 pounds LED Switching Power Supplies, which output 12v 150w, and therefore capable of powering 75 of my little bulbs. Would that work? Any reason not to use an LED supply to light incandescent bulbs?
Thanks
Probably not. Incandescent lamps have a much lower resistance when cold an therefore take a much higher startup current. The LED supply won't have been designed for this.
Cheers
Try a power supply from an old PC? Many of those can do 12V at 150W easily.
If you're buying, power supplies from old servers can do up to 3kW at
12V for a few pounds on ebay - if somewhat noisily!Random example that does 25A @ 12V:
The only snag is you have to work out which two pins on the connector you have to short to turn it on. Also you'll likely have to solder on some terminals. There are breakout boards if you don't want to do this:
- or use a power supply with an ATX power connector.
Theo
LED power supplies are mostly constant current supplies, incandescent bulbs require constant voltage. The LED PSUs voltage can vary considerably as they try to keep the current stable (ish). A constant current PSU will have a specified output current much lower than its wattage rating would indicate - as you increase the load it increases the voltage. The rated current is fixed, the rated wattage is fixed, the voltage will vary to maintain the rated current within the power maximum. If you connect bulbs in parallel to the PSU then as bulbs fail the PSU will ramp up the voltage to maintain the current at a stable level. Incandescent bulbs really dislike over voltage so they will all start failing rapidly.
For example a typical constant current LED drivers provides 700mA output in power ratings of up to18W. It will automatically set the current output and allow the voltage required vary automatically to cover voltages from 3 - 54V as the load varies..
You're looking at 8-10x initial current surge for filaments, but an LED PSU should be current limited. Whether it'll work at more than 1/8th load depends on how it behaves with overcurrent. If it can soft start that could minimise overcurrent.
You could 1/4 the current surge with a relay connecting the lamps in series, then changing to parallel after a second or 2.
75x12v >> 240v so you could also run them in series strings on 240v. If wanted a neon with R across each would indicate a dead bulb instantly.NT
If the LED supply is any good, it should have current limiting protection.
Personally I stay the hell away from ebay for mains things that are permanently live - I get mine from RS, so I can see the full data sheet and have some hope it's not a dangerous PoS.
Those usually don't start up all the power lines until a signal is seen from the PC.
You can get one of these:
for similar money.
In message , Theo writes
Thank you for all the replies. I now understand why an LED power supply will be unsuitable.
However, this is brilliant :
I must have at least 4 old desktops here, not used for years. I'm guessing they will have power output printed on the label. I confess I did not know breakout boards existed. So, a power supply from an old PC, a board like this :
The answer to a maiden's prayer. I assume two wires, connected to 12v and GND, will supply all the little 12v bulbs I need, all wired in parallel. Perfect.
From memory, the power supply will come out of the PC complete with 'kettle' mains socket, cage, fan etc. all in one.
All you need is to short pin 16 (PS-ON, green wire) of the ATX power connector to ground, and the PSU will start. Put a mechanical switch on it as the 'on' switch.
On server PSUs it's a different pin, which will take a bit of probing to find out (eg take a resistor like 470 ohms, wire one end to ground and poke pins with the other end until the fan comes on)
RS' website is currently down so I can't look at that, but the OP seems to be asking for something to drive incandescent bulbs. The previous discussion suggested an LED PSU isn't suitable.
Theo
It'll likely be made down to a price, will just about achieve its specifications and not a penny more.
Cheers
I've used lots of these for all sorts of jobs including driving electronics and motors in the same job. They are current limited and behave very well. The most recent one is powering a lift to raise a 1/4 tonne machine from its storage position to operating bench height based on the works from a motorised golf cart. A good cost effective PSU solution.
That sounds very interesting - any more info, or pics?
That should do it. However you should watch that the 24 pin 'ATX' connector has a limited current rating - I'd budget about 5A per pin[1]. There's only two 12V wires on the ATX connector so 10A available.
Most of the 12V current is intended to be supplied on the 'EPS' and 'PCI Express' connectors:
So you might use the above breakout to supply other voltages and switch the PSU on, and then snip off and solder yellow/black wires to access more
12V current.
Yes. The outputs should be on the label. Note that some PSUs are split rail (12V1, 12V2, etc) so it's best to not to join outputs from different looms if you need more current.
Theo
[1] The official Molex pins are rated at 9A, but safer to assume they've underspecced the wiring (and to avoid pins getting hot).
OK - but why would you want to? Big lumpy and probably old (fire risk?) PSU with a noisy fan verses a few quid for a designed for purpose nice safe zero noise unit.
Where do you get the notion that server PSUs are more hazardous than PC PSUs?
NT
Halogen incandescent 12V transformers are cheaper and are still available from the likes of Screwfix etc...
if those PSUs are designed for LED tapes that generally require fixed
12V and have current limiting resistors in the strip (every 3rd LED or so) could be ok ... but if they're of the type designed for a long series strip of LEDs, they'll be constant current (20-30mA) and vary the voltage depending on the length of the string.
Just strap the green wire to a black one, that'll start it.
Yes it was an interesting project and a great sense of relief when the first lash up managed to lift my own weight with quite a small proportion of the PSU current being drawn! I wrote it all up for a woodworking forum but since the decision of Photobucket to apply extortionate charges that I refuse to pay, the photo links are all broken.
I'll try and dig out a working link to a video I took of it running.
Here are the photos
and the text is here
they are sort of in order. Hope you can make something of it.
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