A dumb question on LEDs

curtis · 4 May 2023

I have, what I think is a stupid question, so give me a bit of rope to hang myself.

I've been adding lights to buildings across our layout. I've got a 12v DC supply and a number of small 12v LEDs (the ones with the E5.5 base). These are fed from the 12v bus and then wired in parallel from certain junction points. It's been working fine for a couple of weeks but something is niggling at me that I've overlooked something - and that actually I need a current limiting resistor because my (basic understanding) is LEDs are triggered by current, not voltage.

For transparency, there are also some of the standard LGB light incandescent bulbs in the circuit too.

I wanted to get the thoughts of this community because I fear I'm overlooking something.

(the LEDs, btw are listed as 12V to 14V DC voltage and 20mA)

PhilP · 4 May 2023

If you buy LEDs where a supply voltage range is specified, then they already have a resistor fitted.

This could be a small bulge in the waiting (covered in heatshrink, possibly) or actually built into the LED itself.

If all your lights are wired in parallel, then each will taken the current it wants, and a single 'lamp' failing will not affect the rest.

You do need to be aware that all these small currents do add up, but I doubt this will be a problem for your power-supply or wiring.

PhilP

Diesel2000 · 4 May 2023

There's a forward voltage range for each LED. If you have 12V LEDs then they will have range of operability from about 11.5-12.5V depending. If you give them 12V then there is nothing to worry about. Its when you have a higher voltage power supply that you need to current limit the LED with a resistor, like a 12V power supply and 3V LEDs.

Greg Elmassian · 4 May 2023

you can actually calculate the current from a higher voltage supply, and see if it radically exceeds the LED current rating.

As Diesel said, for your case "don't worry, be happy!" (well he really did not say that, but he is right)

3 minutes of fame · 4 May 2023

If you applied 12v to an LED without a resistor, you would have found out by now...

curtis · 4 May 2023

Thanks, all. Appreciate the quick insights and learnings.

I actually added a small meter to the circuit as the 12v leaves the garage for observability. I think we're okay for a while in terms of current draw!

Paul M · 5 May 2023

A quick tip, Curtis, I'd cover those bare connections up with something. It may only be 24V, but a short is still a short

curtis · 5 May 2023

Paul M said:
A quick tip, Curtis, I'd cover those bare connections up with something. It may only be 24V, but a short is still a short

For the distribution block above? There are plastic covers in normal usage but I had them off as I was moving things around when I snapped the picture

beavercreek · 7 May 2023

I have a similar system to you but I use 16-19v led bulbs (the ones with E5,5 bases) and a few incandescent bulbs but these are all being replaced by leds.
I have over 150 individual bulbs and a variable power supply with enough current headroom to cater for all of the bulbs and the extra resistance from connections and the 'extra' led 12v led strips for things like the movie theatre hoarding and the diner.
I have used step down 'buck' voltage boards for the strips.
The power supply is set at 18- 19v.