basic electronics question - help

[HIB]

Hi everyone,

I'm new to this forum. Could really use a little help on a simple question.

OK...
I have four LEDs in series. I connected them to a 12V power supply and they draw 500 mA.
This is too much current for them, so I am afraid they will break.

To reduce the current, I put a 10 ohm load resistor in series with the circuit. Now, with 12.5 V, I get 150 mA. This will work perfectly for me, but I don't understand the maths of this.

Here's how I would work it out - based on easy high school physics:

4 leds in series; 12V; 500 mA -> LEDs have a combined resistance of 24 ohms
Adding the load resistor: R = 34 ohm (10 + 24); 12.5 V; I should have 12.5/34 ~ 300 mA.
I only have 150 mA.

Why?

Any help is much appreciated. I can't figure this out.

Thanks

 

[BobK]

Hi and welcome to the forum. This should tell you everything you want to know about using LEDs.

https://www.electronicspoint.com/got-question-driving-leds-another-work-progress-t228474.html

Bob

 

[HIB]

That explains it brilliantly.

Thanks!