How many LEDs can a decoder lighting function cope with?

[JST]

I am sure this has been asked before but I tried a search and failed. Basically I want to use a few spare decoders to power coach lighting that I can switch on and off. The decoder function outputs are 100mA and I want to use SMD strip LEDs and associated resistors.  How many LEDs can I have on one decoder function output? Also, can anyone recommend a source of LED strips at a sensible price?

Thanks.

[John ks]

This is a bit like the "how long is a piece of string " question

 

It depends on specification of the LED,  the series resistance value, if there are more than 1 LED in series with each resistor & how bright the LEDs are

 

this will be a bit inaccurate but will give results that should work OK

 

First ignore the LED & calculate the current draw for the series resistor 

A 1K resistor connected across 12V will draw 12mA (from the formula V=IR  "I" being current)

8 resistors across 12V will draw 96mA which is less than the 100mA rating of your decoder 

 

If you put a LED in the circuit then the current draw will be less

so in the above case you could put at least 8 LED/resistor sets for each decoder output

 

If you are referring to the LED strips that come on  rolls with each group of 3 LEDs fed by one resistor

Then you may find that with the onboard resistor they are still to bright

Cut a group of 3 off the strip & connect another resistor is series with that group of 3 LEDS

Keep increasing its resistance until the LEDs are at a suitable brightness

 

With a 1k resistor the max current draw for the group of 3 LEDs will be 12mA as shown above, Now you can have 24 LEDS per output(8 groups of 3)

 

With a 4k7 resistor the max current draw for the group of 3 LEDs will be 2.55mA. Now you can have 39 LEDS per output (13 groups of 3)

 

With a 10k7 resistor the max current draw for the group of 3 LEDs will be 1.2mA. Now you can have 96 LEDS per output (32 groups of 3)

 

Hope this helps

John

 

 

[JST]

That is very helpful. Thanks.

[SamThomas]

If you have spare decoders why not use the motor output ? It will enable you to set the lighting level to an extent.

 

You could use the function outputs for tail lights.

[JST]

I hadn't thought if that.... I will give it a try. Thanks.

 

[298]

I'd have to dig it out of the stock box to measure properly, but I ran out of functions when illuminating a HO Bombardier cab car so used the positive and 0v outputs from the keep alive through a voltage regulator to power LED strip through plugs and sockets in each coach. It saves having to switch it on and the current draw is less of an issue, the moderately sized keep alive will keep the full 120 LEDs alive for about 50 seconds. 

 

[Nigelcliffe]

On 13/07/2021 at 17:31, JST said:

I am sure this has been asked before but I tried a search and failed. Basically I want to use a few spare decoders to power coach lighting that I can switch on and off. The decoder function outputs are 100mA and I want to use SMD strip LEDs and associated resistors.  How many LEDs can I have on one decoder function output? Also, can anyone recommend a source of LED strips at a sensible price?

Thanks.

 

And, if the sums suggested above result in "too much current" for the LEDs proposed, then can use the DCC function output to switch a transistor, which in turn connects/disconnects the LEDs across the track pickups.  Requires transistor (tiny), a couple of resistors and if doing belt&braces a few diodes to stop any back current paths.    Then your limit becomes the switching rating of the transistor, or the power available from the DCC system.   

 

 

- Nigel

[melmerby]

11 minutes ago, Nigelcliffe said:

 

And, if the sums suggested above result in "too much current" for the LEDs proposed, then can use the DCC function output to switch a transistor, which in turn connects/disconnects the LEDs across the track pickups.  Requires transistor (tiny), a couple of resistors and if doing belt&braces a few diodes to stop any back current paths.    Then your limit becomes the switching rating of the transistor, or the power available from the DCC system.   

 

 

- Nigel

Some decoders (Lenz) have a lighting function for the motor output.