LEDs

If these are the cylindrical body with dome end type, they probably have a small flat at the base of the cylinder (usually indicating cathode (more negative lead). But I have seen exceptions. But which ever version they are, they should be consistent. You can also look through the epoxy and see which lead is the reflector base the die is mounted on. That is also usually the cathode. The anode lead generally has a wire bond to the top of the die.

the

Thank you. I needed a magnifying lens to see the flat! Looking through the side I see two small triangles with the hypotenuses facing each other. The upper one is associated with the flat on the body.

Roger

I suggest you pull one LED and find out by test, whether the flat is the cathode or the anode.

For decades, I've always had a 9v battery lying around with about a 1K resistor soldered to one terminal, and sometimes a piece of wire soldered to the other. Since you know the polarity of the battery, when the LED lights up when you connect it, you know which terminal of the LED is which. If it doesn't light up, you reverse the LED. If it doesn't light up then, the LED is a dud (or your battery has finally died.

This not only gives you polarity when it may not be clear, but if the flat of the package isn't visible (like when you've installed it with one of those LED holders) or whatever reasons, you can easily check before soldering. I got into the habit of checking LEDs this way before soldering because when I first started buying them, LEDs were relatively new and it was easy to get some that were not particularly bright, or for that matter I think some that were cheap because they had the terminals reversed.

Michael

Most LEDs have a max. reverse voltage rating of less than 7V and might be damaged or degraded by the full 9V battery, so a 4.7V Zener across the O/P should limit the reverse voltage to safe values. But beware of the continuous current draw.

As a side-bar, I've noted this site:

Which appears to offer a nicely featured tester. I haven't purchased it (and I am definitely not affiliated in any way), but I was recently considering the idea of buying one.

Jon