Electronics Overly bright power LED's.. Resistor size?

I want to dim the power and HDD LED's in my new Coolermaster case. They are blinding.

I believe they run off of 5 volts from the mobo.

Anyone have a tried and true resistor size that would make a good fit?

100-150 ohm would get me about half the voltage assuming 20-25ma

Any other dimming suggestions? Guess a resistor would be the easiest..

 

Think bigger. Try 1k, maybe 4.7k if that doesn't calm them down.

 

or if you lazy like me some bluetack works great

 

I guess this bugger is going to be some trial and error.. Guess I could try not to be lazy and try a few different ones before soldering them in..

 

Glad I grabbed a handful of sizes. 4.7k was the highest I got. Ended up needing to put two of them in series to get the dang LED's dim enough.

These are nice bright lights, I hate to not steal them for something else and put some dimmer LED's in there..

 

I would have though that 4.7k is too high. Cpemma, can you please explain the high number values?

Figuring that .02 A, I'd think a 4.7k resistor would drop close to 100 volts.

As technical as you can get, I can handle it, so please lay it on thick!

 

The power and HDD led header supply voltage is only 2 to 2.2Volts.

 

No, that's the forward voltage of a typical case LED you're measuring. There's a current limit resistor dropping a few more volts at about 10mA current.

Go back to the start, a 5V supply, 2V LED, typical motherboard resistor 220 ohms

Voltage left for the resistor to drop = 5 - 2 = 3V

Current through the resistor will be 3/220 amps = 13.6mA

Now add a 4.7k resistor. Same 3V to be dropped.

Current through the resistors will be 3/4920 amps = 0.61mA

Series circuit, so the same current goes through the LED in each case.

Basic Ohm's Law V = IR or R = V/I or I = V/R

In this case, voltage and resistance are the known quantities, current is the unknown to be calculated.

 

Got it, thanks!

Ok, so with the LED, there is now 0.61 mA as opposed to 20.0 mA, right? My problem was that I was assuming that the current was held constant at 20 mA and that the voltage needed to be dropped further my an additional resistor. I was considering the LED as a lightbulb.

Thanks for the clarification!

 

cpemma = guru


It still kinda behooves me too.

If I'm understanding this right, there's really a resistor on the board before the LED headers. Reading the header pins on the mobo show 5 volts on a multimeter because there's not much current flowing really. Add about 20ma, and it drops to 2ish volts.

Right?

 

Right. The average digital multimeter has an input resistance of a few megohms on the voltage setting. On the 5V supply, put just 1M of meter resistance in series with the 330R board resistor and the meter will read 5 x 1,000,000/1,000,330 = 4.998V. Or near as dammit, 5V.

With the drive activity LED it gets slightly more complicated as there's an ordinary diode and a transistor switch in the circuit so if either IDE channel is active the one LED lights up, but let's not go there.

 

Cool

That would explain why the HDD LED is slightly dimmer then the power LED. Not enough so that it bothers me though.

Thanks for all the help!