anyone use a radio shack LED w/built in resistor?

[janus]

Hello,
wanted to put a led on my little gem and I have a couple of these:

The power dissipation is 100mW
the foward voltage is 12v
the foward current is 15mA
there is a 680ohm resistor buit in the case..it's labeled for use with 12vdc. But I'm running the little gem on a 9v battery..I hooked it up last night and it lights up bright enough. I was wondering if adding it would make my little gem become a battery eater? Not sure how to read the voltgages and currents on such things..
any help appreciated,
Brian

[calpolyengineer]

If you use it at 9V the current draw drops to 13.2mA (a "hungry" effect draws around 10-20mA). An average 9V battery has 625mAh, that means just running that LED alone would drain the battery in 47.3 hours. That isn't very long, so we need to add more resistance to lower the current draw. Now if you add a series resistor of say 4.7k, the current draw becomes 1.7mA, which is better, and a 10k resistor will drop the current to .8mA. You can keep trying higher value resistors until the LED is too dim for your preference.

-Joe

[SteveB]

I have one in my Screamer pedal, & you just explained why I have to change batteries so often. I'll add a resistor & see what happens. Thanks for the info.

Steve

[waldo041]

i use the smaller version of the rat shack L.E.D's(like above). I believe that they have similiar specs only brian's is rated for up to 16 volt's, but my question is that the 680 ohm resistor is on the negative lead. should this be switched around or is this fine?

thanks in advance.


peace,
waldo041

[SteveB]

I think THAT one is the one I used. I'm looking for the answer, too.

Steve

[calpolyengineer]

To answer your question: no, it doesn't matter which side the resistor is on, just because of the conservative nature of electricity.

Basically, think of it as two pre-filled hoses connected together with a flow meter in between them. The flow meter would be the LED in this case and a resistor would be represented by a kink in the hose. Now if one side is connected to the faucet, it doesn't matter which side of the flow meter that the kink is on, it still slows the rate of flow through the whole thing.

-Joe