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Splinter · Thu Jul 08, 2004 3:26 pm

I removed the LEDs from my fanbus because they drive me nuts at night, only to find they must have been tied into the circuitry somewhere because now it'll only give me voltages down to 10v instead of 7v. I want to replace them with resistors so the voltage goes back to where it was, only my multimeter won't tell me, probably because they are semi conductors and such.

Any ideas, other than reconnecting them, measing the current they draw and the voltage drop across them?

mattsteg · Thu Jul 08, 2004 3:33 pm

You can probably just short them. Remember they're diodes - did you check the resistance in both directions? It'll be really small in any case.

Splinter · Thu Jul 08, 2004 3:39 pm

Yea I checked both ways and it came up as infinite. Weird.

Yea, I'll try shorting them

Thu Jul 08, 2004 3:39 pm

Semiconductors have a non-linear relationship between voltage and current. The apparent resistance will vary depending on how much current is passing through them.

How about blackening the LEDs with permanent marker, and re-installing them?

Splinter · Thu Jul 08, 2004 3:41 pm

jbi, that's frigging brilliant.

Although I'll use a combination of marker and electrical tape.

Thu Jul 08, 2004 3:42 pm

Splinter wrote:
Yea I checked both ways and it came up as infinite. Weird.

Yea, I'll try shorting them

If the test voltage used by your ohmmeter is below the threshold voltage of the LED (typically a couple of volts or so), then the resistance will appear to be nearly infinite.

Also keep in mind that polarity matters on LEDs... if you decide to put 'em back in, make sure you've got the polarity right. (If they don't light up, then you've got 'em installed backwards.)

Splinter · Thu Jul 08, 2004 3:48 pm

I didnt actually take them out, I just snipped one leg to break the circuit.

A little unrelated, but do you think it would be difficult to change the range of this (it's a nexus nxp-205) so it'd go down to 5v instead of 7v? And how would I even go about doing so?

mattsteg · Thu Jul 08, 2004 3:53 pm

The standard way of getting down to 7 volts would be to use the +12 and +5 rails for a difference of 7 volts. I would imagine that your bus has a pot between ground and 5 volts along with a bunch of resistors to accomplish its task. There's not really any convenient way of going down to 5 while still being able to hit the high end. It's be a completely different circuit.

AMM · Thu Jul 08, 2004 5:20 pm

I use bluetack for unwanted protuding LED's with the added bonus of being completely reversable.

Thu Jul 08, 2004 6:48 pm

Most LEDs have a specifed voltage and current rating. When the given voltage is applied to to the LED it will produce light at it's rated output and in doing so will draw the specied current. 2.5V is a comman voltage and 20milliamps a fairly comman currect draw though the brighter the LED the high the current and blue and white LEDs generally have a high current draw too.

Using Ohm's law: V=I*R

2.5=0.02*R
R=125 Ohms

All other things being equal, you could probably replace the LED with a 120Ohm resistor and get approxately the desired effects. Assuming of course that the LED really does play into the voltage control circuit.

As others have noted, LEDs resistance does vary with voltage. Below 0.5v-0.7v (the P-N junction voltage of a silicon semiconductor) a meter will read infinite because the LED won't conduct at all.

--SS

Splinter · Sun Jul 11, 2004 10:56 pm

I fried the Fan Adapter somehow.

Ah well.

Usacomp2k3 · Sun Jul 11, 2004 11:21 pm

haha..that was a humorous read

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