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Thanks for your time, and I apologize for being so green. I am looking to run a series of bulbs to emulate a "marquee" style of lights on lettering. I found the bulbs, but want to make sure I order the correct power supply, as well as anything I might overlook. I'll be mounting the fixtures into the letters mounted to a panel, and wire everything behind it.

If there are 80 bulbs in total, at 12V 0.3A, what all do I need to power it correctly? My brain hurts atm. My first guess was a 25W 12V but now I've confused myself thoroughly. Any help would be greatly appreciated!!

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  • \$\begingroup\$ oh. letters with lights inside, not times square. \$\endgroup\$
    – Jasen
    Jan 27 at 23:36

4 Answers 4

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It's just a simple multiple of Pd= 12V x 0.3A x 80 pc. .

But the hard part is the PTC positive temperature coefficient for tungsten bulb resistance means that power drops to this level only when they are hot. When cold it is about 11 or 12 times this power level but only for the 1st cycle of AC.

This means 80 tungsten bulbs are a real big problem for DC power supplies because they will trip with overcurrent protection on startup unless you sequence them on and have excess capacity.

Coincidentally it is not much different to driving a 288 Watt motor as DC motors also have a surge power and current rating of 10x the full load rated power (+/-20%).

A far better solution is to use LEDs.

There is an alternative but it requires some electronics with a 30 Amp choke and low ESR cap with a 50 A FET switch in PWM. This is more of a challenge.

Anecdotal

Back in the late '70's, a music-award-winning composer-for-film friend I grew up with, http://peterallenassociates.com/site/ was in a rock group called RAINTREE and he wanted me to make exactly the same thing in "Fat Albert Lamps." So I made each letter a 15A Triac Controlled 120Vac channel with 8 channels. Then I went further and made it into a "Chaser" with bi-directional scanning and speed control. That was just a weekend project and I still had time, so I add phase control with a zero-crossing switch using a CMOS XOR gate offline then an electret mic input for the drums so the rhythm of the drums would sequence the lights. That worked famously with tape over the mic because it was too sensitive. But I heard years later it failed when the console technician put on 1kW flood lamps on each channel and turned the sequence speed up too fast allowing the lamps to cool down enough so they drew 20 to 40 Amps per channel and blew out all the Triacs. So even the Technician did not know about PTC effects. But then "Black Pete" moved from Winterpeg to Vancouver and the rest is history with over 185 film scores with famous actors and directors for Sillywood.

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  • \$\begingroup\$ Thank you, LED sounds like the best route. Can you advise me on my 2nd attempt? I'm unsure of the draw, but found some cool 12V LED e10 bulbs, would a 36W 12V wall supply do the trick? \$\endgroup\$
    – Jason M
    Jan 28 at 13:44
  • \$\begingroup\$ Yes better. Choose to have extra power capacity to prevent max temperature rise for reliability. Any old PC ATX supply can be molded to work too. \$\endgroup\$ Jan 28 at 14:30
  • \$\begingroup\$ Another approach is to wire in a series resistor that will drop about 10% of the voltage when the bulbs are hot, but a much bigger fraction when the bulbs are cold. This would cause the bulbs to heat up much more slowly, but the slow startup may have the advantage of improving bulb life. \$\endgroup\$
    – supercat
    Jan 28 at 15:43
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80 bulbs × 0.3 A = 24 A.

12 V × 24 A = 288 W.

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    \$\begingroup\$ Since it's a marquee, if they can guarantee that they will never have all lights on at once, a lower power PSU might be usable as well. Though you should be able to get 288 watts at 12 volts relatively easily, with an ATX PSU if nothing else. \$\endgroup\$
    – Hearth
    Jan 27 at 21:19
  • \$\begingroup\$ Powering 1 bulb could use 32ga wire.... powering all 80 would take 14ga. If long distances are involved, even thicker wire would be better. \$\endgroup\$
    – rdtsc
    Jan 27 at 21:32
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    \$\begingroup\$ @Hearth Given the amount of people asking why their ATX power supplies do not work well as bench/lab power supplies and shut down when driving arbitrary loads, I'd suggest trying something else than an ATX supply. \$\endgroup\$
    – Justme
    Jan 27 at 21:38
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Wire the lights in parallel (which it sounds like you're planning on doing) to easily accommodate different lengths and to make replacing burnt out lights easy. I would try to find some lamp holders with 6" or so pre-wired and stripped leads and then use some wire nuts to attach short jumpers of 16 or 18 gauge lamp cord to make strings of, say, 10 bulbs (the connection to the first bulb has to carry the current for all the bulbs in the string). The strings would then attach to your power supply in parallel.

You might be able to find a pre-made string designed to run off of 120V too, just cut it up into sections instead of trying to run the whole thing off of 24V since bulbs running off of 120V will draw far less current and can be run off of thinner wire.

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Connect them in series until you have enough to connect to the mains directly. (strings of 10 for US 120VAC)

this will be a pain if one fails, but a non-contact voltage detector can be used to find which one needs to be replaced.

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  • \$\begingroup\$ Are old-fashioned incandescent light bulbs legal and available anymore? Why not use modern LEDs instead? \$\endgroup\$
    – Audioguru
    Jan 28 at 15:58
  • \$\begingroup\$ some places have banned the sale of some types for room illumination, they are still allowed for many other purposes. \$\endgroup\$
    – Jasen
    Jan 28 at 22:52

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