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I'm just about ready to work out my streetscape lighting and it looks like I'll have anything from 10 - 20 lights initially, all controlled from 1 switch either on 12v or 9v DC (not decided yet). I have drawn up a resistor list so that I only have 1 resistor to control all the lights. Example - working on 9v -: all lights being in parallel from a lighting bus: 10 lights would require a 33ohm resistor, 20 lights requires 15ohm resistor, and, in an extreme case 30 lights would require 10ohm resistor.

I'm hoping that is going to work out ok! These figures have been calculated from www.hebeilted.com.cn/. I will stand to be corrected if anyone thinks it won't work.
 

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I'm just about ready to work out my streetscape lighting and it looks like I'll have anything from 10 - 20 lights initially, all controlled from 1 switch either on 12v or 9v DC (not decided yet). I have drawn up a resistor list so that I only have 1 resistor to control all the lights. Example - working on 9v -: all lights being in parallel from a lighting bus: 10 lights would require a 33ohm resistor, 20 lights requires 15ohm resistor, and, in an extreme case 30 lights would require 10ohm resistor.

I'm hoping that is going to work out ok! These figures have been calculated from www.hebeilted.com.cn/. I will stand to be corrected if anyone thinks it won't work.
Don't forget to calculate the wattage rating for the resistor
 

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The problem you run into is that the resistor choice is dependent upon how many LEDs you wire in parallel and if there are failure or disconnects you can easily destroy the remaining lamps. By keeping the resistor with the lamppost you can connect more or less without any such effect. little SMD or even 1/4 watt resistors are cheap, I don't think the risk of using a single resistor is worth it.
 

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I bought about 100 3 volt lamps and was not about to add a resistor to each and every one so I also bought a couple of 3 volt power supplies to power them. I think they were $5.00 each free shipping.
Are you referring to LEDs or lamps? Because if you're referring to LEDs and you hooked them up without any resistors, then you got REALLY lucky! The resistors aren't there to control the voltage going in to them, the resistors are there to control the current. You probably just lucked out with a low-current power supply that happened to not exceed the total current of all the LEDs combined. An easy example of this... when I was a kid I tried hooking up an LED to a pair of AA batteries, because hey that's only 3.0V and the LED was rated for 3.3V. But no, batteries put out vastly more current that an LED can handle, and they were instantly destroyed.
 
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