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# How many lights in a series?

#1
12-05-13, 10:39 PM
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How many lights in a series?

I'm new to this whole electrical thing but I've been doing a little reading so I can try and understand a little.

I want to make a lighted sign which would consist of between 25-50 lights. I want to do a series of miniature bulbs or LED's overlaid on top of letters like a vintage sign.

Tell me if this example is correct...

Lights - Miniature E10 base bulb
4.9v .3 Amps

So if I understand this correctly I can safely run 4 lights (4x4.9=19.6v) which will leave me with 4.4v to resist. If I use ohms law I would need a 15ohm resistor (4.4v/.3a=14.6 or 15ohms). If this is correct even if I used the entire 110v from the wall I would still only be able to use about 22 bulbs?

#2
12-06-13, 08:52 AM
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No. Voltage is a constant. It is not additive. Amperage is additive, and total wattage is the limiting factor on any circuit.

A 24V output transformer is too powerful for your 4.9V bulbs. 3.8A will supply up to 12 0.3A bulbs at 100% and up to 10 using 80% as a safety factor.

#3
12-06-13, 09:48 AM
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Geo

#4
12-07-13, 06:02 AM
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Nashkat, you may be thinking of LEDs wired in parallel. Series-wired LEDs will behave the way
richs2k6 described.

#5
12-07-13, 08:52 AM
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You could probably run 5 of those on 24v. Many lamps and LED are bright enough on lower voltage. One plus is incandescent will last longer.

#6
12-09-13, 05:03 AM
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Don't use series resistors for incandescent lamps; that will produce unnecessary heat and also represent a waste of energy. Arranging incandescent lights as you described but in series groups of five will work with excellent results in this project.

Since all of the lamps are the same, the computations are easy. Within a given series string, the sum of the lamp voltage ratings should be equal to or a little greater than the supply voltage.

By the way, dissimilar lamps can be sometimes used in series circuits or subcircuits with several limitations that are a little too complex to describe here. A hundred years ago, streetlights of different wattages were often in series circuits.

Also, by the way, all of the components in a series circuit have to be rated for the supply voltage. For example, if you ran the 120 volt power directly to the lamp sockets (24 or so in each series substring) with no transformer, then all the sockets and the wiring has to be rated for 120 volts. Using Ohm's Law, you can show that if one lamp burned out, you will have close to 120 volts between its socket shell and socket center tab if you tried to draw a few milliamperes such as when measuring with an ordinary voltmeter. A small arc or flashover could occur in a socket not rated for 120 volts. Some (not all) of the lamps you describe are rated for 120 volts because they were used in older radios and electronics with vacuum tube filaments and the lamps all in 120 volt series strings.

Last edited by AllanJ; 12-09-13 at 05:41 AM.
#7
12-10-13, 01:35 PM
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Nashkat, you may be thinking of LEDs wired in parallel. Series-wired LEDs will behave the way
richs2k6 described.
Yep. I didn't think of it the other way.

It is as you say. Thanks, Rick.

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