I'm working on a Christmas tree lot.
The plan is pretty basic with a few poles and lights strung from the poles.
I have a 10 pack of clear Miniatronics 1.5 volt bulbs, the 30mA version (18-C03-10).
I'd like to wire these in series. With a total of six bulbs in the series.
Normally I wire things in parallel. However, since these bulbs will hang from the posts, if I wire them in series the wire will look like scale wire. Yes this way if one burns out the whole thing goes dark. But these are easy to find bulbs and I'm wiring them so they should be easy to replace.
But if I wire these in series, I think they will dim considerably.
So I'd like to know what resistor, if any, I should use for Six 1.5 volt bulbs rated at 30mA that are wired in series with a 12 volt DC supply?
I found an LED calculator online for wiring LEDs in series. But I'm not sure if this correlates to bulbs as well. Here's what my calculation came up with (a 1/4 watt 100 ohm resistor is suggested):
Matt from Anaheim, CA and Bayfield, COClick Here for my model train photo website
For starters a light bulb is not an LED so one can immediately throw away all those LED calculations, even though in this case it looks like they came up with an ok number.
It is much easier than that. In series, the same current passes through the whole system. Since the 1.5V bulbs need 30 mA that number is the same.
1. Quantity six bulbs at 1.5V = 9 V.
2. 12V power supply - 9V light bulbs = 3V left for the resistor.
3. Ohms law 3V / 30 mA = 3V / 0.03 A = 100 ohms.
Now if it were me I would burn the bulbs at less voltage (like 1.2V or 1.3V) just to make certain they last longer. If that is your desire the numbers can be recalculated with the lower voltage.
6 * 1.2V = 7.2V
12V - 7.2V = 4.8V
4.8V / 0.03 = 160 ohms.
Thanks so much for the calculations.
Yes I forgot to mention I'd like to run these a little less then 1.5 volts. But you caught that nicely.
Here are the bulbs lit:
Matt,
That is very cool.
Thanks for sharing.
Rich
Alton Junction
Why not just run 8 bulbs in series on your 12 volts and forget the resistor. Or run 9 in series, if you want them a little dimmer. If thats too many bulbs, just put a small piece of black shrink tubing over a bulb or two.
Or, pickup an old transformer at a Train Show, and dial up whatever voltage you need. Make it a dedicated power supply for lighting only.
I just noticed your photo shows 8 bulbs. You said you wanted 6 in the series.
Yep there are 8 now. I added two as I thought it looked better this way.
The 8 bulbs are running on roughly 9 volts. Didn't want full 1.5 volts for these so the bulbs last a little longer.