[skyskraper]

ok so i'm switching the led's on something from white to blue. currently the circuit that has the white led's supplies 5.2v. my blue led needs 3.5v so i go to drop in a 1k resistor. solder it in, test voltage. still 5.2v. take the resistor out, clip it in series in to the circuit. still 5.2v. test resistance, yup 998ohm, close enough. test again. still 5.2v. thinking What is going on here lets try a 10k resistor. test resistor, 10k no probs. put in circuit, still 5.2v. im going out of my mind here, what am i doing wrong. i tried the resistors on the positive lead, negative lead. can't figure it out. i thought my multimeter was screwed, but nope seems to be measuring different voltages ok. what am i doing wrong? im going insane....

 

[setmenu]

To calculate led resistors, subtract the led voltage drop[found in data sheet] from the supply voltage and divide the remaining voltage by the desired or recommended
led supply current.

It sounds like you are measuring your voltage from the + - supply terminals.
To check the voltage across the led ,do just that, measure across the led
terminals when it is in circuit with your resistor.
If you are measuring across the total series circuit you will only get the total voltage drop across it.


Setmenu

 

[skyskraper]

well with the led in, and the resistor in im still getting the same reading. whacky. ive stopped worrying about it as the blue one seems to be ok with 5.2v.....

 

[setmenu]

So with:

V+ ------Led-----Resistor------V-

So with supply V =5.2 plus a led and resistor in series you still get 5.2v across only the the led?

My experience is that led v drops are from about 1.7- 4.5 v depending on the
type.
Driving a led beyond it's rating can severely shorten it's life!



Setmenu

 

[Megaptera]

Have you checked for shorts?

 

[robzy]

Quote:

Originally Posted by Megaptera Have you checked for shorts?

As above, its got to be something like that. Try it with a battery and a multimeter quickly to see if the resistor works then.

If it does, then your either measuring something wrong or there is a short.

 

[gsferrari]

LOL Its funny how you can build sensational amps and then something as whacked as an LED can make your life miserable by refusing to comply with the ohms law

HA HA - I love this thread

 

[setmenu]

Quote:

Originally Posted by gsferrari LOL Its funny how you can build sensational amps and then something as whacked as an LED can make your life miserable by refusing to comply with the ohms law HA HA - I love this thread

Yeah, don't you just hate when the laws of physics just keep messing you around..

Setmenu

 

[ECM]

to measure a voltage drop you need current across a resistor. if you put the resistor in series and measured the circuit *without* a load, then there would be no voltage drop. remember that e=ir. so the voltage drop is equal to the current times resistance. if current is zero, so is the voltage drop.

i agree with setmenu when he recommends you calculate the led voltage drop and compute the resistance needed for the desired current.

then again, if the LED is working properly, why worry?

 

[skyskraper]

i think i was measuring wrong...... the led seems to work so my stress is gone. ill leave this one for the moment

the other led when i put the resistor in seemed to adjust the voltage. no short in the system, the leds connected to a pair of wires that terminated 40cm away, no problems with the power supply since. i have a feeling my $5 multimeter had a ghost in it. ahh itll be replaced soon anyway

 

[setmenu]

This crazy lamp was the most fun I have had wiring leds.....

Setmenu

 

[tangent]

Have you measured the voltage drop across just the resistor, while the LED is running? I think you'll see 1.7V = 5.2V - 3.5V.