ESU decoders and LED control

Dog Star

Western Thunderer
I am trying to work out how to control LEDs from the auxilary outputs of a LokPilot decoder and sorting the resistors for the LED circuits is of concern. Model Railway suppliers of LEDs often state that a LED must be protected by a resistor - good thinking except there is no information about the resistance value.

For those LEDs which might be used for lighting in rolling stock or signals, what is a typical maximum voltage which can be applied to the LED? Similarly, what is the typical current draw of such LEDs?

I have read through the ESU user manual for the Lokpilot V4 and, so far, I have not found details of the maximum / typical voltage which is available from an auxilary output - anyone know?

thanks, Graham
 

Pugsley

Western Thunderer
Whilst I can't help with the LokPilot specific info, in order to calculate the resistance required, you need to know the specs of the LED concerned, something like this:
http://www.vishay.com/leds/leds-0402/

It's the max current rating you need (If for Iv in the case of that link) usually given in milliamps. That, with the function output voltage, will allow you to calculate the resistance using Ohm's law. I suspect that the function output will be track voltage, in the absence of any other information, so 16-18 volts.
 

Dog Star

Western Thunderer
Thank you Martin,

So if I go with 18V max for the AUX output and a (mythical) LED of Vf = 2V and If = 20mA... then I need at least 800 ohm to protect the LED with a rating of 0.5W. I accept that 18V might be more than provided, at least using that figure is on the safe side. LED values of Vf=2V and If=20mA seem typical for the white / red / green discrete components that I can get from DCC Supplies.

regards, Graham
 

Dog Star

Western Thunderer
Thank you Richard, I have ordered resistors of several values and current rating so I shall be able to start safely and experiment within the limits expressed in this topic.
 

Pugsley

Western Thunderer
1000 ohms will give you a forward current of 16-18 milliamps (assuming track voltage), so they should be quite bright, whilst being within the rating of the device.
 

Steph Dale

Western Thunderer
Pugs,

Yeah - I keep a stock of 1k .25W resistors for this purpose - the dimming can then be adjusted on the decoder to whatever brightness threshhold(s) required.

Steph
 
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