Hi,
I'm thinking about repairing/modifying/making some mods for my pinball. But I want to understand how to deal with the LED, and the electrical issues. In the case of a mod that is plugged on a WPC controlled lamp.

I learned that the lamp inserts of a WPC are powered from the lamp matrix with a 18V DC input switching on/off very quickly. To emulate a kind of 6,3V.
It is ok from an incandescent lamp point of view. But for a LED, why is it working ?
As soon as the LED receive 18V, even if it is some milliseconds only, the LED should blow.

On my Twilight Zone, I have some mods with Led. The slot machine for example.
There is 2 Led on the left/right of the slot machine (3mm Led, ~2,2V, 20mA), and they are linked to the "slot machine controlled lamp" of the door panel. And there is no resistor to protect them.
So 18V for 2 LED 3mm, it is far too much. They should blow immediately.
Why is it working anyway ? Where am I wrong ?
Thanks for your help & technical explanations

Yes it is the current that blow the led.
But during the 4-5ms when the current is on, the voltage is 18V and then the current is far more than 20mA. And the led should blow.
Then during the following 20-30ms, the current is off, but it should be too late.

I agree. That was exactly what I was thinking, and the subject of my question.
My twilight zone was not designed to work with led. I have some well known mods (the slot machine, the robby robot). They include 3mm/20mA led without resistors, and they work.
Why? How? Where the current is limited?

Yes of course, I will do that if I design/adjust a mod by myself.

But now my understanding is :
- I need to calculate the resistor value based on a 18V input. (even if it is 18V during 5ms every 30ms)
- the mods that are sold every day on the market are designed to fail on a short term !! Because of the power sent into the led.
And it is annoying...

Thanks for the links. It is useful and easier to calculate the resistor need.
But as you can see, it confirm the essential need of resistors in our mods, that are powered with a 18V input !!
So a lot of the expensive mods that are for sale here in the market are designed to fail !

For a circuit with 1 led, a resistor of ~820 ohms is needed.
For a circuit with 2 led, a resistor of ~680 ohms is needed.

Led resistor need 1 led (resized).jpg

Led resistor need 2 led (resized).jpg