Red & White won't go.....at the same time

[mrpig]

Some of you read my problem with red and white led's not working at the same time on a TCS FL4 with the default programming.
Here is why


Photo 1.
This is the schematic for a typical Kato end car light board. As drawn, the white led lights up. Flip the polarity when running dc and the red led lights up. Simple.

Now lets throw in a decoder. Don't forget the light board has now been modded to work with dcc.


Photo 2.
In this schematic the white led has been flipped and the decoder functions are represented by switches F1 & F2.
Led's have a turn on voltage dependent on their colour.
Red is 1.63V to 2.03V.
White is 3.5V.

Values from wikipedia. When the led is on, this turn on voltage becomes a fixed voltage drop across the led due to the P-N junction in the device.
So if we turn on F1, we get a fixed 3.5V at point "A" and the white led lights up.
Turn off F1.
Turn on F2, we get 1.63V-2.03V at point "A" and the red led lights up.
Turn off F2.

Here is the tricky part. Remember that the default programming on the FL4 allows F1 & F2 to be turned on at the same time.
Turn on F1....white lights up..... 3.5V at "A"........ all well and good.
Now leave F1 on and turn on F2.  Red lights up but remember it turns on at, say, 2.03V. Point "A" now becomes 2.03V due to the fixed voltage drop across red. This is no longer enough for the white led to fwd conduct and so it turns off.

This all goes away when the FL4 is programmed to suit our needs as red & white will never be on at the same time.  This is also why led's normally have their own limiting resistors when they are wired in parallel.

[CaptOblivious]

Ah, this is very helpful, mrpig. It nicely explains why, with the Kato boards, turning both lights on at once only actually lights one of the lights. Thank you!