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Confusion with LED current and Voltage drop

Last response: in Other Consumer Electronics
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December 25, 2013 4:23:55 AM

Well from what I have read when you hook some led's up in a series you get an overall voltage drop per each led forward voltage x how many led's you have. That's why you need a decent amount of volts if you want to hook enough in series.

Then in parallel there is no voltage drop per led number but they individually use current potential from the source so again if the source can't put enough amps or milliamps out then again you can't really rig a whole lot of led's up in parallel since they wont get enough current.

But now with my multimeter I just tested something with some led's that didn't even make sense.
I thought in series that the current stayed the same but from what I have tested it didn't.

I hooked one up to a 9V battery and it was pulling 45 mA. Then I put another in series so there were 2 led's and the current went from 45 mA to 19 mA.

So I am confused now. Since I didn't think current worked that way. I know that in series the voltage drops but the current. WTF. If that's true then everything I have read is wrong.
December 25, 2013 5:05:18 AM

The problem might be your multimeter. You need a very good one for low currents like this, because in fact you are measuring LED + LED + multimeter in your scenario.
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December 25, 2013 5:46:52 AM

noidea_77 said:
The problem might be your multimeter. You need a very good one for low currents like this, because in fact you are measuring LED + LED + multimeter in your scenario.

No I don't think it's my multimeter. It gets everything else right and is good. It's pretty new also.

Something is up though. A way to prove it without the multimeter was this.

Each Led pulls 20mA at 3.5V I am sure. Well the 9V batter is not exactly new and according to my multimeter it can put out 85 mA.

Well I grabbed some old button cells that are around 1.2V each and connected them to the 9V battery which made it like pretty much 11 volts. So then I thought hey if this is 11 volts and I wire 3 LEDs up they will want 3.5V each which is 10.5V so I have enought volts and nicely enough the half dead button cells actually reduced the mA from 85 mA to Around 40 mA.

So in theory we have enough volts and current isn't meant to reduce after each LED so they should all be getting 40 mA which is more than enough to light them up. But no what actually happened is they were all dull and barely lit at all. Well the voltage isn't the problem obviously, The damn current is somehow being reduced after each LED which is annoying me since I am sure this isn't meant to happen.

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December 25, 2013 6:34:29 AM

Just did it again with another set of batteries that make 12 volts but I have limited it with a resistor to only put out 30mA.
One led lights, But the more leds I add the less bright they all get. Only way to make sure they don't go dull is to remove the resistor. WTF. Nothing works for me.

There are totally enough volts for 3 of these LED's yet even when I put just 2 in they get duller significantly like somehow they have to share the current between them making it only 15mA each, But damn current isn't meant to work like that. What the actual fk is going on here.

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Best solution

December 25, 2013 8:41:23 AM

You may want to start reading here: http://en.wikipedia.org/wiki/Ohm%27s_law
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