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I am using led’s with resistors. What is a common power supply? The accessory side of a transformer or something more sophisticated..? Starting out I will have approximately 20 lights.
 

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The voltage of your P/S will depend upon the value of the resistors you are using. If the LED's came with resistors pre-wired, check the packaging for the rating.

If you wired them, you will have to tell us the value of the resistors you used.
 

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Your power supply needs to be DC (not AC) for LEDs. Many older transformers provided AC on their accessory terminals, so check that out first. You could build a diode rectifier bridge to create sudo DC power from your AC terminals. However buying a 5, 9, or 12 volt DC wall wart power supply is really cheap. Any of these will work fine. LEDs draw so little power that running 20 of them should be no problem for any cheap wall wart power supply. Which ever voltage you decide, use one of the online LED calculators to determine what resistor value you need. You may find you want to run a higher resistance so that your LEDs aren't too bright.
 

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I am using led’s with resistors. What is a common power supply? The accessory side of a transformer or something more sophisticated..? Starting out I will have approximately 20 lights.
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I am using led’s with resistors. What is a common power supply? The accessory side of a transformer or something more sophisticated..? Starting out I will have approximately 20 lights.
[/QUOT
 

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The voltage of your P/S will depend upon the value of the resistors you are using. If the LED's came with resistors pre-wired, check the packaging for the rating.

If you wired them, you will have to tell us the value of the resistors you used.
I am using 12V 4mA. Thanks
 

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As Mark has said, you must have DC voltage for your LEDs, though very
little current. Unless you build a simple rectifier using 4 diodes, you should
do as he says also...use an old WALL WART. The label on each states
whether it is AC our DC output and it's output voltage. You could use
anything from 2 to 12 volts with appropriate resistor.

Don
 

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I would not suggest a 2 volt wall wart. You should have a resistor connected to each LED that is connected in parallel to the power supply. A typical LED driver output 20 ma thus at full power 10 LEDs would only consume .2 amps and usually those wall warts are 1 amp. As the current driving the LEDs becomes too small the LED shuts off so I'm not sure what you mean by 12v 4ma? 20 ma driving the LED is most likely going to be way to bright. If the resistors are already fixed then you only easy option is changing the PS voltage to reduce the brightness. You may have to experiment with 12v, 9v or 5v wall warts to see what works best. This all has been discussed recently so you might search for "LED's are driving me crazy" topic in the "General Model Train Discussion" and pay heed to what GTR has to say
 
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