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I want to power a very large light fixture: it has 273 G4 socket light bulbs. The bulbs I found for the thing are 12 V 5 W ones, incandescent. So the question is how to power the beast (as it is 1365 W of power), and how can I dim the whole thing? If it were just the power I could buy some rando cheap 12 V 1.5 kW PSU, with the dimmer it's a bit more challenging. Do I have to make some custom circuit for the dimmer, or does something proper already exist for this purpose whose name I don't know?

psmears
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A. Daty
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    It depends on how they are wired up. Do you have a schematic you can post? – Aaron Jun 15 '22 at 16:18
  • @Aaron They all go in parallel, so one power source and one switch/dimmer most basic scenario, only complicated with amount of the bulbs – A. Daty Jun 15 '22 at 16:23
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    1365 W at 12 V is about 114 Amps. Just to be sure you know what you are dealing with, so you can seek switches and dimmers, and calculate how thick wires you need so they don't vaporize when you first try it. – Justme Jun 15 '22 at 16:32
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    use AC line voltage ... connect the bulbs same as a christmas light string ... use a household light dimmer – jsotola Jun 15 '22 at 16:38
  • @jsotola so rewire them in series? Would be fine, but 273 doesnt really divides by 20, but with some fiddling could work, thanks for the advice! – A. Daty Jun 15 '22 at 16:47
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    Also note that incandescent bulbs are not fixed resistance. They start off lower resistance and increase as they warm up. I've seen this trip breakers when too many are used and the person is saying, "But it should only be 12Amps." The 12Amps would be the hot current, it's higher when the bulbs are cold. – Aaron Jun 15 '22 at 16:47
  • @Aaron thanks for that, already forgot about it, i mostly work with led's these days – A. Daty Jun 15 '22 at 16:51
  • Do you really want that many lights at full power? You could put more than 20 in series , the Lightbulbs act like a constant power load , slightly reducing voltage will decrease resistance which will mean higher current draw than just a linear load would be. – Password Jun 15 '22 at 16:51
  • I would think you'd need more than one dimmer to accomplish this. – Voltage Spike Jun 15 '22 at 16:52
  • @jsotola It still needs to deliver 1.3kW. I don't think your typical dimmer can handle this. – Eugene Sh. Jun 15 '22 at 16:53
  • @Password not at all times, thats why whould i need dimmer, it's 20 in series only for a possibility to actually have full power on all of them if it is needed for whatever reason – A. Daty Jun 15 '22 at 16:55
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    It is about 9 kW inrush power into cold filaments. You need a controlled power up sequence. Do you really want to build a heater like this with light as a side product? – Jens Jun 15 '22 at 17:02
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    120 V outlet? 10 in series and 27 such strings in parallel. 230 V outlet? 20 in series, 13 such in parallel and some spare bulbs. I made such a contraption once with 50 W ones and not so many in parallel. Made for one hell of a Christmas tree ornament for some tens of seconds before the tree started smoking. – winny Jun 15 '22 at 17:05
  • Based on lots of helpful comments to my original question, there are few options. I think i will go with switching to the 220V 5W led bulbs with diffuser and some powerful thyristor SCR voltage regulator for the dimming – A. Daty Jun 15 '22 at 17:18
  • G4 LED bulbs are tiny. Not much room to fit a capacitor. Therefore many of them flicker a lot. For the same reason, very few are dimmable. – bobflux Jun 16 '22 at 17:37
  • What should you do when one bulb is burned? One scheme detection or bypass? – Antonio51 Jun 16 '22 at 21:11
  • @A.Daty How aware of HC safety standards and EMC standards are you? Can you quote the actual specification numbers, or do you at least have a link bookmarked which you've been referring to for the last few years? If the answer to any of these is "not so much", ***STOP RIGHT NOW***. You're juggling lit matches over a fuel tanker, and you don't get to claim you couldn't foresee what happens when it goes wrong. Also all insurance policies (property, liability and life) will be invalid when it goes wrong. Basically, if you have to ask the question here, you shouldn't be doing it! – Graham Jun 17 '22 at 14:07

4 Answers4

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DO NOT drive it AC (mains) power!

AC mains installation/implementation expert here.

  • AC mains power is much more dangerous and requires great care.
  • It will create a huge mess when a bulb burns out, as you now must hunt down a bad bulb in a series string. I've done this much too much, and remember, it must be done with the power on.
  • 4mm is awfully close for 230V.
  • The series string will violate Code and the UL White Book/EU equivalents.

This will be a code violation and a safety train wreck from the day it is built, and you have a fair chance of sitting in a deposition room being asked "What were you thinking???"

Drive it PWM

The #1 reason to do this is inrush current. It may be 1350W steady-state, but on startup it will be closer to 10,000 watts. So drive it PWM at a low duty cycle (high dimming), and ease it up, watching your input current so that you aren't drawing more than 1500W.

Think like "drive the incandescents constant-current during startup, to suppress inrush current".

Of course the PWM also provides the means for dimming.

Make it LED

Get a large nerf bat, and beat the customer around the head and shoulders until they remove the "replaceable bulbs" part of the specification. Replace immediately with NON-replaceable LED emitters. Now, if you want to run series strings at 300 volts, go for it. Your supply can be 12 volts boosted, because now your whole-fixture power draw will be in the 100-watt range, depending on how efficiently you choose to under-drive your LEDs.

Harper - Reinstate Monica
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    _"AC power is much more dangerous and requires great care."_ - AC isn't more dangerous than DC at all. Did you mean "mains" instead of AC? Also, I agree running this installation on mains is far from ideal. But running the entire thing on 12VDC means OP needs about 115A. 115A @ 12VDC is basically a welder; shorts have the potential for serious arcing, so at this point it's choosing between evils... – marcelm Jun 16 '22 at 08:26
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    Hunting down a failed LED in a series string is at least as tedious as doing the same with G4 bulbs. (Which isn't all that tedious to begin with: grab a DMM, pick a bulb in the middle of the chain, measure voltages to either end, the half that appears to have full supply voltage across itself is the bad one, bisect it again). Also, (replaceable) G4 LED bulbs are a thing, so you don't have to sacrifice easy servicing just to get LEDs (which should be a no-brainer these days). – TooTea Jun 16 '22 at 09:16
  • _"4mm is awfully close for 230V"_ OP isn't suggesting to run 230 V across each socket. – winny Jun 16 '22 at 11:41
  • What's the issue with "replaceable bulbs" when going for LED? There are G4 LED bulbs that are dimable. – asdfex Jun 16 '22 at 12:57
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    @winny No, but if a single bulb fails in a string, it will see 230V across its socket, so Harper makes a good point. – marcelm Jun 16 '22 at 14:03
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    Note that LV bulbs with closely-spaced contacts, in series, run off 230V, are common. They're the only things I can think of (in the UK/EU, anyway the OP hasn't specified a location) that have only a single layer of insulation between mains-carrying copper and the user. They're called Christmas lights, and of course are an order of magnitude lower power than we're talking about here. It's still not what I'd do, but it's not totally ruled out – Chris H Jun 16 '22 at 14:21
  • @marcelm Ah, now I see your point. Not ideal, but the current would be limited by the rest of the bulbs. Creepage across it could take months to develop, if ever but if it does and strike across it, it would be bad. Fill in the horizontal connections in the bridge and this would never be a problem. – winny Jun 16 '22 at 14:53
  • My old fashioned incandescent Christmas lights have 230 V across the whole string and only a few mm between terminals – D Duck Jun 16 '22 at 18:02
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    @asdfex because if you eliminate the socket you remove dangers associated with that socket. LED emitters (properly heatsinked and driven) are essentially indestructible so no need to socket them. Since the invention of the transistor, people have been socketing semiconductors, and have found it to be pointless and counterproductive, and have stopped. Diodes, transistors, ICs were once socketed. – Harper - Reinstate Monica Jun 16 '22 at 20:30
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    @Winny yes, this has been the death of many heritage streetcars, which use five 120V bulbs in series. A transit-grade bulb burns out so the operator runs into the dime store to buy a common AC rated bulb, and when *it* burns out, it lacks the arc suppression a transit bulb has. So a 600V series arc strikes, and burns right down through the bulb, the socket, the wiring etc. – Harper - Reinstate Monica Jun 16 '22 at 20:33
  • @marcelm agreed. – Harper - Reinstate Monica Jun 16 '22 at 20:35
  • @Harper-ReinstateUkraine Why 600 V supply? – winny Jun 16 '22 at 20:43
  • @winny because that is the usual tram/trolley voltage. Pre-war cars didn't have low voltage systems at all. Even when they used a lower control voltage, they derived it from a resistor ladder. That was good enough to multiple-unit 2-3 cars. – Harper - Reinstate Monica Jun 16 '22 at 20:58
  • @Harper-ReinstateUkraine AC or DC? – winny Jun 16 '22 at 21:09
  • @winny sorry yeah, DC. Reason is streetcars started when DC was still winning, and (series-wound DC) traction motor field pieces are not laminated. They could have changed later (replace all motor fields) but by then, substations were fed from the national 3-phase grid, and the power companies wouldn't sell them single-phase AC. Plus sync challenges. – Harper - Reinstate Monica Jun 16 '22 at 21:17
  • So not applicable here. AC is order of magnitude harder to sustain any arc, and voltage is lower. – winny Jun 16 '22 at 21:25
  • @winny yeah I just mentioned the streetcars as an aside. – Harper - Reinstate Monica Jun 17 '22 at 00:28
  • by the way PWM with a big series inductor (and catch diode) is a buck converter. Adding the inductor may reduce EMI. – user253751 Jun 17 '22 at 12:12
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No need for 12V - totally impractical. If the bulbs are already wired in parallel then someone messed it up and it has to be all redone.

You’ll be making series strings of 10 bulbs, and those strings have to be connected in parallel to 120VAC. That’s how it’s done. You can then use any suitably rated dimmer designed for lighting.

Also pay attention to the wire ampacity (current carrying ability). The concealed wires in incandescent light fixtures have a hard time shedding heat so should be oversized, by 2-4 AWG numbers, but check it first. Also make sure to pay attention to wire temperature ratings.

Another thing: running this chandelier will be almost like opening a hot kitchen oven. Make sure the HVAC system can handle it without it getting unbearably hot upstairs.

Kuba hasn't forgotten Monica
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I'd consider 48V strings - 1/4 the current of 12V but still plenty of choice of PSUs and components (with MOSFET PWM dimming) and without the safety hassle of mains.

272 bulbs would divide by 4, so the odd one may need to be run off 12V; the PSU for that could be regulated down to power your controller. The controller would provide soft start but also doesn't need to switch all strings on at the same time - that would be 68 strings and you could run 17 strings off each of 4 PWM outputs, staggering the soft start

But the wiring is going to be a pain almost whatever you do

Chris H
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Firstly, why 5W LED as replacement for 5W halogen? They will be many times as bright, probably blinding! Do you really want a light output similar to 10kW of filament lamp?

Using mains voltage LED bulbs raises a whole other can of worms around trying to dim LED bulbs which have built-in electronic drivers. See https://www.blue-room.org.uk/topic/76287-led-practicals-and-dimming/ for an example of the kind of issues which show up. Briefly, only some mains-voltage LEDs are designed to be dimmmed at all. Those that are often need a specific type of SCR dimmer to work correctly, and even then will only dim down to a certain level before cutting off suddenly. (don't have enough reputation to comment on OP's answer).

This is quite different to using LED light sources directly as was suggested. If you only have the LEDs junctions (and no driver electronics) on the light fixture, then they can be dimmed by controlling the DC current through them (either a constant-current regulator with variable set-point or via PWM), this gives much better control. On the other hand, you have to take responsibility for maintaining the correct LED operation conditions, i.e. a (regulated) constant current supply to the LEDs, not a constant voltage.

RichardAshAudacity
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