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I am considering building a train that uses a 12v motor from the 1980s and Power Functions LED lighting. For this to work it would be necessary to run the Power Functions LED set from one of the 12v sockets on the motor.

Obviously the physical connection is not compatible, but it should be straightforward to remove the connector brick from a Power Functions cable and fit 12v-style plugs instead. Where I'd welcome information is the electrical aspect. Since the LEDs are designed for 9v, exposing them to 12v may damage them, and they will need a resistor in series to reduce the voltage across the LED set to 9v.

Has anyone determined the correct value of the resistor? Or the current that the LED set draws at 9v, which would allow the necessary resistance to be calculated using Ohm's Law.

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2 Answers 2

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https://www.philohome.com/pf/pflights.htm

So the leds draw 2mA, a voltage drop of 12-9 = 3 V is required. So Ohms law says you need about a 1.5 kOhm resistor in series to have the leds shine at the same brightness...

On the other hand, it could very well be that the only consequence of omitting the resistor is that the leds shine 30% brighter... you could test and monitor if the leds heat up. If you cut off power fast enough as soon as you feel them heating up, it's unlikely that they would be damaged.

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    It is very easy to burn out LEDs when you leave out the resister in my experience. Even AA batteries can fry an LED without a resistor in seconds.
    – chicks
    Commented Mar 16, 2023 at 15:33
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    Note that there IS already a resistor internally of 4.7 kOHm, so you are not exactly short-curcuiting the LEDs. Knowing Lego, the resistor in place is probably on the safe side. What one could do is use a potentio-meter and regulate from the 1.5 kOhm slowly to 0 Ohm and see what is appropriate...
    – Michael Verschaeve
    Commented Mar 16, 2023 at 17:31
  • @user253751, perhaps you should expand your comment to an answer
    – Michael Verschaeve
    Commented Mar 23, 2023 at 9:39
  • @MichaelVerschaeve done
    – user20574
    Commented Mar 23, 2023 at 9:50
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using the numbers on the page that Michael linked to, I calculate 1mA per LED which is very low. When increasing the voltage to 12V I calculate 1.7mA per LED which is of course 70% brighter but still very low*. Therefore I think it will work just fine with 12V.

* Random LED circuits are often designed for 20mA and this is fine. 20mA is often used due to outdated design advice for the less-efficient LEDs from 30 years ago. With a modern LED, 20mA is annoyingly bright and that's why the power light on your TV keeps you awake.

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