Either something's not right or I'm incredibly stupid!

Brucebruce · Post by **Brucebruce** » Thu Sep 25, 2014 5:13 pm

Hi guys,

I have a problem and I have no idea what I'm doing wrong!

I'm trying to connect three white leds together and I can't get it to work.

Doing the math on paper and using a resistor calculator it says I need a 120 ohm resistor.

The ebay guy I got the leds from lists the stats at 3.1v and 20ma and my power source is 12v. It's a plug in wall adapter that you can change the voltage on.

I've got 120 ohm resistors which are red brown red gold.

I have more than triple checked that my positive power supply is going to the resitor, to the positive leg of the first led and so on.

I have checked each individual led with a little 3v battery and they all work.

They don't light up when all connected though. In a momment of desperation I tried connecting them without a resistor...and nothing.

Any idea what I'm doing wrong??

Tchail · Post by **Tchail** » Thu Sep 25, 2014 6:48 pm

Make sure that the positive leg of the second led is connected to the negative leg of the first led. The same goes between the third and second leds.

It actually doesn't matter whether the resistor is connected on the positive or negative sides.

Best of luck!

mhvink · Post by **mhvink** » Thu Sep 25, 2014 7:48 pm

Well, you have the resistor value correct at 120 ohms, but you have the wrong resistor in your hand. According to the color bands, you have a 2100 ohm (red=2, brown=1, red = 2 zeros, gold=5% tolerance).
The color bands you want is brown (1), red (2), brown (1 zero) and either gold or silver for the tolerance.

With a 2100 ohm resistor across 2.7 volts, your current is 1.28 mA. Nothing will light with that little current.

Hope this helps.

Mike

Brucebruce · Post by **Brucebruce** » Fri Sep 26, 2014 1:26 am

I thought you'd cracked it and I'd just been really stupid! Alas no, I typed the resistor color code wrong. The one I'm using is brown red brown gold.

I've checked the polarity of the legs over and over!

This makes no sense, right? It's not just me is it?

konp · Post by **konp** » Fri Sep 26, 2014 4:55 am

Brucebruce wrote:I thought you'd cracked it and I'd just been really stupid! Alas no, I typed the resistor color code wrong. The one I'm using is brown red brown gold.

I've checked the polarity of the legs over and over!

This makes no sense, right? It's not just me is it?

You could always scrap the resistor and try to run your string on a 9v battery. Also, pair 2 AA batteries together and test each LED individually. I'd wager that one of them is either dead or around the wrong way.

Brucebruce · Post by **Brucebruce** » Fri Sep 26, 2014 5:49 am

Hehe I've tried all that! Individually they work, together they don't. I'm also using about 5 different leds. I have no idea what's going on.

Brucebruce · Post by **Brucebruce** » Fri Sep 26, 2014 6:05 am

I solved the problem!

It's quite a technical issue so I don't want to bog everyone down with the detail but the basics of it are as follows. When using a 4 way extension cable and when each plug has an isolated switch, it is necessary to turn the switch on.

Jesus

Madman Lighting · Post by **Madman Lighting** » Sun Sep 28, 2014 7:58 pm

Hey... don't feel bad.

I r an engineeer and I make dumb mistakes like that plenty of time.

Maybe I should learrn to speeell sometime too.