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voltage drop and power supply



Jan 1, 1970

First, I have been attempting to use a PC (atx) power supply to supply 12VDC
30A to a string of lights (each light 2.3V 0.3A is wired in a pair -> series
and then these pairs are wired in parallel - like christmas lights. There
are 20 pairs or 40 lights total. I followed some advice given to me on this
list a while ago - thanks :) ) This worked very well. But now it seems the
lights near the end of the line are quite dim, also the power supply begins
to make a high pitched sustained sound if I leave it on for a while).

So I am not sure what I might try next. Maybe the atx supply is not a good
choice. Also the voltage is a little higher than I need (even with the
approx. 3VDC drop in voltage for the length and gauge of wire). I would see
voltage dropping like this for each light (at 5VDC for a test):

4.18 -1.0
3.97 -0.21
3.75 -0.22
3.57 -0.18
3.44 -0.13
3.31 -0.13

Does this make sense? Any ideas on how to proceed. Any input is greatly
appreciated, I'm truly a beginner at this.



Jan 1, 1970
how long wires do you have, and how big are they ??

normal as a guideline is, 4amp in a 1mm2 wire when it's DC

but remember the lengh have someting to do with the resitance, i have a
small nice program, in danis, that can calc all that out... so if you give
me the data, i can calc it out


Jan 1, 1970
Hi Stan,

First it's a 5 volt supply not 12 volt. I Looked up your old post.

2 - 2.3 volt lamps in series is 4.6 volts. On a 12 volt circuit, they won't
last long. You would need about 5 in series to work in 12 volts.

Most, newer, Christmas tree lights are in series. (At least in the USA) Only
old 115-volt Candelabra base (same size as a night light) Christmas tree
light strings were in parallel.

In a string of 50 mini lamps, the voltage across each lamp would be 2.3
volts. They are in series. There is a small jumper in each lamp that is
designed to short it out if the lamp burns out.

If you mix .3 amp and .5 amp lamps this could cause the dim lamps you

If you're still using 12 gauge wire you should not be seeing a voltage drop
greater than about 5%. About ¼ volt at 68 feet with 10 amps. You should be
using about 6 amps.

It's hard to get a good connection splicing copper and aluminum wire, they

The most probable reason (my best guess) for the dim lights and high voltage
drop that you describe is poor connections.