FreeTrack Forum

So I'm wiring a 3 point clip, I have my 3 SFH485P LEDS and I'm hooking them to a 9v battery.  I had some E24 1/8th watt 220ohm resistors lying around - calculators around the web seem to generally indicate that one of those resistors in series with the 3 LEDS will see them running with 1.5 forward volts and about 22 forward ma's, which sounds good.

OK but the SFH485P's are well used LEDS around here right, and they probably obey the positive/negative standards, with the negative leg being the short one on the flat side of the LED and the positive being the longer one on the rounded side....

So i wired a simple circuit to test things out, with just one LED.  I twist the resistor around the LED's positive leg, and twist the 9v clip's red positive wire to the other end of the resistor.  Then I twist the battery clip's black negative wire to the LED's negative leg.  When i attach the battery to the clip the LED doesn't light.  When I reverse the battery however, and just touch the contacts to the clip (since it doesn't fit in backwards obviously) the LED springs to life.  I can see it glow a bit with my eyes, and its superbright on my webcam.

So what gives.  Is my battery clip or LED wired backwards?  I honestly can't tell anode/cathode from looking inside the LED (bad eyes)  I mean, if the LED lights that means its ok right, if polarity was incorrect the thing just wouldn't light at all right.  So should I go ahead and wire the up the circuit 'backwards' with the clip's negative wire attached in series to the LEDs' positive legs?

if I were you, I'd test each Led and make what the politary is, just to be sure.

I don't know why companies don't follow standards anymore.  Rediculous.

I recently ordered these LEDs as well and my local hardware store sells them, there I found this datasheet, maybe it helps you?

http://www.datasheetcatalog.org/datasheets/50/486602_DS.pdf

if I interpret this correctly, the long leg is the cathode which ought to be positive (+), right? I am absolutely new to electricity

Well in a diode the cathode is usually negative right....so wait wait, in these LED's the negative leg *is*the one on the flat side...but its also the longer leg.  so the longer leg is negative....I wasn't expecting that, and therefore couldn't see it.  weird.

ah ok, so I learned something today and the datasheet did actually help