TECH: Powering up many bulbs with AT power supply

[Barkuti]

Well, here we are again with our next gadget.

Things are progressing, finishing the design of my standalone 4 player CP. As all of my buttons are going to be of the illuminated variety, I've been tinkering with the idea of getting bulbs for them lately. After all, all those coloured buttons should make the panel nice when powered up.

The problem is giving juice to those bulbs... 26 or 28 of it (let's say around 30 VAs). I want to go cheap this time so I'm not spending $50+ on a single output 12/24 V power supply. However I do have an old AT 200 W PSU, rated for 8 A @ 12 V, 20 A @ 5 V, and 0.5 A @ -12/-5 V each.
My question is related to the possible crossload power requirements of the PSU, as I now it has. If you plug a couple 12 V fans to it, the rail voltage will drop significantly (and the 5 V one will raise). You know those old AT PSU's were designed to provide most of its power from the 5 V rail. I was wondering if the same thing would happen when loading the 5 V rail alone (with 6.3 V bulbs). I do not have a proper 5 V load at hand for testing.

Should I get a 100 pcs pack of 6.3 V bulbs and be confident on it?
Or maybe spend another $18 on a 12/14 V bulb pack, and drive a bunch of these through the 12 V rail to stay within the PSU crossload reqs?

Another possible option. A dual secondary 48 VA (2x 24 V @ 1 A) power transformer; I have one lying around. Bulbs don't care about unrectified or alternate current. What do you think about this route? Would it be safe to wire the thing straight to the mains? Or I'm better using a filtered plug?

Your feedback is welcome.


*** EDIT ***

I'm not very inspired today. Lack of sleep I think.
I can get a 6.3 V bulb pack. Wire some of them to the 5 V output. Wire another amount in serial connected pairs to the 12 V rail. Crossload requirements met.

Still, is the power transformer thing a good idea?


Cheers

[forbiddenlyrics]

Why not just use LED's? It would be so much easier than using regular bulbs and they would pretty much last forever. I guess the only drawback I can see is their light may be too focused.

[Barkuti]

Why not just use LED's? It would be so much easier than using regular bulbs and they would pretty much last forever. I guess the only drawback I can see is their light may be too focused.

If that's the only drawback of LEDs, then for sure you have plants growing banknotes in your garden!

For the price of a 100 pcs bulb pack, I can get a whopping 5 LEDs . Game, set & match for bulbs!


Cheers

[Ken Layton]

If you're using the bulbs as general illumination and all are powered up at once then the bulbs are usually wired in parallel.

A 6 volt #44 miniature lamp draws 250 milliamps (mA) so 4 of them wired in parallel would draw 1 amp from the power supply. So total up the number of lamps and current drawn. Then add 50% to the power needed to be provided by the power supply for initial surge current drawn by the lamps each time you turn on the machine. You need that overhead on the power supply's current rating so you don't blow it.

 Six volt lamps can be run from the 5 volts of the power supply and the bulbs will last a long time.

[Barkuti]

Yeah, I know I have to run the bulbs in parallel. I was referring to team up pairs of bulbs in series, to allow them to be connected to the 12 V rail of the PSU. That way 6 V would drop through each bulb in the pair. So, the total bulb load can be shared between the 12 and 5 V PSU rails as required, meeting the PSU crossload requirements and hence keeping the two rail voltages within specifications. We're speaking of a PC switching PSU, designed to work with load on both 5 and 12 V rails.

Rated for 100 W on the 5 V rail, and 96 W over the 12 V one, it seems available power won't be an issue for around 30 or less bulbs. The only "problem" of all this trickery is that the bulbs attached to the 12 V rail will be fed with 6 V, and the ones plugged to the other rail will receive 5 V, so there will be a difference in brightness. But I don't really care; I can give 6 V to the bulbs under the darkest coloured buttons, it may look nice overall.

Going this route you can buy a very high powered el-cheapo PC PSU and feed a ton of lamps, even if the PSU is fairly overrated. Don't you think?

Thanks for the figures on the surge power draw for lamps Ken.


Cheers