LED wiring. Which side is common?

[Deleted]

It has been almost a year since I wored up a decoder. For an LED, which side is connected to common?

I am being lazy.  I really do nto want to crack open one of my units that is already done.

[lyled1117]

If you mean the blue lead common of a decoder, there are several things that can indicate the lead. The longer of the two leads is nearly always the more positive lead. If it's a T1 or a T1-3/4 (3mm and 5mm respectively) there is commonly, but not always, a flat spot on the lower lip of the plastic that indicates the NEGATIVE lead.

Lyle

[Deleted]

Lyle,

No, just off the LED itself. There is a positive and a negative contact on the LED. One of them goes to the common I just can't remember which one. I know the blue wire from the decoder is the common.

[lyled1117]

Brian, my answer is for the LED. I wanted to be sure that the 'common' you refer to is the blue lead of the decoder which is a positive voltage. 'Common' is a relative word, just wanted to be sure of the obvious, that you meant the one on a decoder. The long lead of the LED is the one that goes toward that common. Either directly or through a resistor (sequence of the two components does not matter, just the polarity of the diode).

[Deleted]

OK, I am soldering the wires to the LED. It is one of those super small LED's that doesnto have wires already on it. So are you saying the positive (+) side is the common?

[Mark R.]

All decoders use positive voltage as the "common" blue connection. Individual function output connections are the negative voltage. As for the surface mount LED markings ....



The term "common" is irrelevant when referencing an individual component. It is positive and negative. The term "common" is only referenced to a given circuit. Some circuits use negative as common and others may use positive as common. In the case of decoder design, the blue common wire just happens to be positive.

Mark.

[emd16645]

According to the Digitrax guides (which should be in accordance with NMRA guidelines for wiring), the blue wire (also labelled "common") is the higher voltage source. This is also known as a common anode arraignment. Therefore, you would want to connect the positive (+) side of the LED to the blue (common) wire. I would double check the polarity with a battery prior to soldering the completed LED to the decoder.

[Deleted]

Thanks Mark and Chris. In the diagram above, my LEDs have the middle label on the back. So it is the positive side that goes to common. That is all I needed to know. Many thanks. So I do not forget again, I will mark this on my LED package.