Well, I took out the old magnifying goggles and took a good look at Kato's Golden White LEDs. Sure enough, one side has a flattened spot on the base! I compared it to some Sunny Whites and found the same flat spot on the "-" lead! Thanks everyone for helping me to come up with this really simple and cheap way of determining polarity!
Frank Eva
Not all LEDs have the flat spot in the same place, not all LEDs have a flat spot. LTG :)
Look at the LED from the side. One wire leads to the base of the substrate which emits the light, the other is connected to a very fine wire leading to the top of the substrate. The one leading to the base is the cathode (-), the other is the anode (+). LTG :)
That is not always true either ! I have LEDs which are reverse from what you stated (ie. The bond wire is on the negative side). Same problem could show up if you were using the shape of the leads inside of the case. The fatter end of the lead which holds the LED die itself is usually negative and the smaller end to which the wire is bonded is usually positive (but not always).
Also, many new Blue, True Green and White LEDs have 2 wires bonded to the chip.
And some cases are opaque enough to make determination difficult...
Do yourself a favor and build a very simple LED tester. I have build one over
20 years ago and I still use it quite often !Take apart old 9 volt battery and salvage the end which contains the terminals. Snap it on top of a new 9 volt battery. On the positive side solder a
1/4 W 1K ohm (1000 ohm) resistor. Take 2 pieces of wire (stranded 26 or 28 gauge so it is flexible) and cut two 5 inch pieces. Strip and tin the ends and solder one wire to the other end of that resistor and the other wire to the negative terminal.Voila ! You now have an LED tester ! It drives LEDs with low current and it will light up all LEDs.
Since you know which of the tester leads is positive, you now can test LEDs. And since the current is small, you can even try to power up the LED while it is still in the circuit. Or, note how the LED was placed in the circuit, unsolder it, test it and then note the polarity. You can then use that info to determine the circuit polarity.
I made my tester little fancier by also wiring in a small rectangular piece of PC board (with the copper split into 2 pads by cutting it in the middle). That way I can test SMD LEDs by placing them on that board.
Simple and very useful ! :-)
Also, I know that Blue and White LEDs are quite sensitive to reverse voltage but I never had any burn out on my yet (using my 9 volt tester).
Peteski
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.