Resistor value needed please [DCC DIY coach lighting]

[Chrissaf]

Anything from about 150 to 250 ohms should work, possibly higher, depending on how bright you want the LEDs to be and how much current they take on 12 volts.  They may work perfectly well without adding any extra resistors.  Ideally, I would measure how much current they take on 12 volts dc first and then decide what resistor (if any) is needed.

[Chrissaf]

It depends what value resistors were supplied with the LEDs you have purchased. The resistors have coloured bands on them. The colour and order of the band denotes the value. Here is an online tool where you enter the band colours and it will tell you the value:

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https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-4-band

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(Note that the page that opens is for a 4 colour band resistor, there is a navigation tab above the tool for resistors with 5 and 6 colour bands if you have one of those instead).

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Look to the table on the right of the tool page to see which end of the resistor has the Band 1 colour so that you then know which order to select the band colours in.

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The resistors that have been supplied with your LEDs are likely to be in the region of 470 Ohms. If so, then this value is in my opinion too low. A value of 470 Ohms will drive a current on a 12 volt supply for a white LED of about 20mA per LED. If you add all these 20mA together for the number of LEDs you intend to use then that is drawing quite a lot of current from your DCC track. A 20mA current is driving the LED quite hard into its operating range. As an analagy it is a bit like setting your car engine tickover rate to 3,000 RPM so the engine speed can never drop below 3,000 RPM, thus putting more 'wear n tear' on it.

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LEDs will work perfectly OK (and in my opinion better, because of the longer operating lifespan at the lower current) on currents of 5mA.

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A standard white LED (I assume that your lighting LEDs will be white) drops a forward bias voltage of typically 3 volts give or take a bit across it when illuminated. Your rectified DCC track voltage will be in the region of 15 volts. Thus you want a resistor value that drops 12 volts across it at a current of 5mA.

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The value can be calculated using Ohms Law R=V / I

where I = current in Amps

where R = resistance in Ohms

where V = voltage in Volts

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Thus for your calculation R = 12 volts / 0.005 Amps = 2,400 Ohms

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Resistor values go up in a standard value progression, thus the most appropriate standard value is 2,200 Ohms.

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In reality, any value between 1,000 Ohms (for 10mA) and 2,200 Ohms (for 5mA) will probably do, but in my opinion do not go below 1,000 Ohms. 1/4 Watt resistors will be more than adequate, and at these resistance values you could use 1/8 Watt value resistors as well.

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Technically, it is much better to correct the value of the resistor for each individual LED resistor than it is to add one single shared resistor as your original question proposed.

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It is possible, but maybe unlikely, that your LED supplier has provided a 1,000 Ohm resistor with your LEDs. If so, then you could safely use those 'as is' without further modification. Use the online calculator I have given the link for to check the value of the resistors provided by your supplier.

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Of course if you have a multi-meter, then just use that to measure the value of the supplied resistors directly.

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TIP: I write long posts. If you intend to write a reply, it would be appreciated if you didn't use the 'White Arrow in Blue Box' button. This is not a 'Reply to this post button. It is best to write any reply you want to make in the 'Reply Text Box' at the very bottom of the page and click the Green 'Reply' button.

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[Correl]

I have asked the person that I bought the LEDs from what resistance he would recommend and what resistance he has put with the LEDs as the resistor is covered with heat shrink on all the LEDs I can't without damaging the protection work out what he's put on the LED

[Chrissaf]

If you have a multi-meter, measure the current drawn on a 12 volt supply (as suggested by Rog in the previous reply) and I can tell you the resistor value. If you don't have a multi-meter then when you get a reply from the seller, tell me what value resistor the seller says he has inserted under the heat shrink and I will give you a more detailed solution to your question.

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One question in the meantime. Is your intention to only put one single 5mm LED in each loco & carriage or more than one. If so how many?

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The number of LEDs affects the Ohms Law calculations I would need to perform.

[morairamike]

Early carriages had quite dim lights, not bright white like led will give . Experiment with lower voltage. On my control box, of my design, I use an adjustable 12v regulator which is giving out 4.5 v driving two bi light LEDs. These are in parallel one red one green via 2.2k. these are my front panel point direction indicators, so one leg is red one green. Operating the mom switch changes them over. My point (excuse pun) is that making them dim is easier in this case on the eye. 

So select the right resistor for the max voltage then use a pot down to set the brightness you want.

[Chrissaf]

I have a DCC track and have several locos and coaches I want to light up when on the track, I don't want to have to chip them all as it will get expensive. What I need is a resister to take the voltage to 12v for a 5mm LED I have built some rectifiers  and a small capacitor to go with this and the LEDs came with a resister to take them from 12v down to the LEDs operating voltage. 

Has anyone done this I'm sure I'm not the first please help