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Old 25th January 2012, 01:28   #1
RW James
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Default Powering a string of LEDs

I bought a string of 18 small white LEDs at an after-Christmas sale. They are powered by 4 AA cells in series. I measured the voltage at 6.5 volts with fresh batteries.

I'm going to use them to light a station building and platforms. I don't want to use batteries and would prefer to use a 6 volt transformer - probably a wall wart (after observing the proper polarity). There already is a limiting resistor in the circuit.

Does anyone see a problem with my plan? Should I be concerned with the rated amperage?
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Old 25th January 2012, 12:07   #2
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Default Re: Powering a string of LEDs

That would work fine. I recon the string will probably draw around 100mA.

If you want to save a transformer, you could power it from the DCC rails if you build a small rectifier circuit (you'd need either 2 or 5 diodes , a couple of capacitors, and a 7805 regulator). All of which is readily available for a couple of $. I can give you a circuit if you want to go for that method.
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Old 25th January 2012, 15:35   #3
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Default Re: Powering a string of LEDs

Thanks Tom - but this is a DC layout. I have a bunch of old DC wall warts that I've saved from old discarded electronic devices.
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Old 25th January 2012, 19:14   #4
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Default Re: Powering a string of LEDs

Ahh right. Then yeah, a 6v DC transformer will do.
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Old 1st February 2012, 01:01   #5
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Default Re: Powering a string of LEDs

I found a transformer that will be perfect. But a bit of confusion on my part concerning the resistor. It looks like the color bands are orange-orange-black-silver. I'm not 100% sure of the orange - before I got it in a better light, I thought it was brown-red-black (it's a light blue case). However, I am 100% sure of the black. That would make this resistor at the most 33 ohms - right? That seems really low. Is that because the power source is batteries? Will I have problems using a transformer with it so low. Should I try a 1000 ohm resistor just in case?
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Old 1st February 2012, 14:25   #6
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Default Re: Powering a string of LEDs

Yeah, 33 ohms sounds about right. If the chain is rated at 6v they will be arranged as 6 branches of 3, so they won't need a large resistor. You will be fine just replacing the battery pack with the 6v DC supply without the need for anything else.
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Old 1st February 2012, 16:24   #7
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Default Re: Powering a string of LEDs

Thanks Tom. I have so much to learn...
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