I wish to remove the 3mm LED supplied on the circuit board and wire the board with two small 1.6mm LED’s in parallel. (on a SDH104K1A + FN04K1 Combo decoder, Digitrax)I believe the 3MM LED supplied should run at approx. If = 30ma and have a Vf = 4.0V. The two 1.6mm LED’s in parallel will have a combined If = 50ma (25ma each) at a Vf = 2.0V .Will the 2 smaller LED’s work with this circuit board and decoder without any problems when wired directly to the circuit board?The voltage looks to be Ok but will the decoder handle the added 20ma extra? I am estimating on your 3mm LED ratings!
The digitrax guys say it should be Ok! but if they are wrong it could be an expensive , should be!
First these are LEDs and one cannot apply the same calculations to them that one would for light bulbs. Remember LEDs are current controlled devices, not voltage controlled devices like lamps.
If I understand what is being said, the current limiting resistors are already built into the decoder circuit. If so, those resistors are what determines the amount of current that gets through to the LED(s). That current will not necessarily change by changing the LEDs. The new LEDs would get the same 30ma that the existing ones get today. As the prior poster noted this could result in dim glowing LEDs because they are current starved. Why not just put them in series?
The other scenario is that the LEDs (either the old or the new ones) have their own own current limiting resistors built in. In that case the calculations would have to be made to adjust for that. The Decoder manual should state what the maximum current draw on an auxiliary output can be. I can’t imagine it being lower that 50ma.
Texas has a very good point. The current limiting resistor is onboard the decoder and is probably sized in order to handle a 2.0V drop 30mA load. If you put two LEDs in series, they would half the current and get only 15mA each. If you put them in series, then you are doubling the voltage drop of the load (from 2.0V to 4.0V)
…interesting problem. Perhaps if you use the Decoder’s output to drive something simple like a Switching NPN transistor. Add in a FWB to take track power and make constant DC, switch the power with the Switching transistor, and then you would have a circuit all to your own and you can add as many LED, Lights and subsequent resistors that you want!