Windows 7: Quick LED query

I have an Asus p8Z77-v board and I rigged a blue LED for the HDD in the front of a HAF XM because I couldn't see it as I like to have my machines on the desk.

I found an old one that already had a 47 ohm resistor in line and it worked up to yesterday and I think form my calcs that it needs a 250 (or standard voltage) ohm resistor for a 12 volt supply and the 47 ohm should have been more than adequate for a 5 volt supply - but I cannot find out from Asus or a Googles what the voltage is for the HDD LED.

Does anyone know what the voltage is to save me taking the leads off the pins and measuring with a meter as I don't want to short something out as it is in a very awkward spot to get to with the power block in situ.

It all depends on what the voltage drop across the LED is. The following assumes a current of 50mA.

Supply: 12V
LED drop: 3V
Resistor: 180Ω

Supply: 12V
LED drop: 1.5V
Resistor: 210Ω

Supply: 12V
LED drop: 0.7V
Resistor: 226Ω

Or nearest preferred value above.

See LED Resistor Calculator

Thanks Dwarf I was using a calc that was giving me a 25R (ohm) for a blue LED on a 5 v supply the LED has a drop of 4.5 v so one is just playing with 0.5 v! That is if I am using the calc I am familiar with -
Vs-Vf/ 0.03 (30 ma)


Now I have found a way of measuring that HHD LED supply safely - I just attached some old connectors to the HDD LED supply bared the wires and clipped them to the meter and powered up ran a scan and watched the meter flicker around 3 v after an initial start up of almost 5 v.

Using the calc you sent that leaves me with approx 17R. Problem was I used a Vs of 12 v to calc the original LED which gave me 250R or the standard value of 240R or 270R. I stuck in a 100R to allow some leeway - plus the LED already had a 47R wired in series originally making a grand total of 147R and taking into account (now I know it's around Vs = 5 v) - that value should have been more than enough to prevent the LED from failing - do you agree?

However after saying all that the LED I used was an old opaque one from a recycle machine and maybe have been stressed in it's earlier life. So I am going to use a brand new clear one with a 240R in series which should be more than enough. I have already tested this set up with a 4.5 v supply and it seemed to work ok.

However if you disagree with my theory please let me know as I don't profess to know everything re this issue and maybe you could throw some light (nice pun) on why the original LED failed. I am all ears

LEDs, as with all semiconductor devices, are designed to work at specific currents. Exceed that current and the PN junction fails (burns out). We'll assume an LED with a voltage drop of 3V and a 12V for these examples (I've also included the required power rating of the resistor in Watts). Note that unless directly connected across the supply, the voltage across the LED will be 3V but not necessarily at a safe working current. The other 9V is dropped across the series resistor, yielding the current and power dissipation figures as shown.

1Ω = 9000mA (9A) = 81W
10Ω = 900mA (0.9A) = 8.1W
100Ω = 90mA (0.09A) = 0.81W
250Ω = 36mA (0.036A) = 0.324W

As I mentioned above, you should use the nearest value resistor to that which is returned from the calculation. If the calculation returns a figure which happens to be a preferred resistance rating, use that. On no account should you use a resistor of less value than the figure returned by the calculation, as this means that the current through the LED will exceed its rated value, and thus shorten its life (if it doesn't cause it to burn out immediately).

If you're running the LED off the front panel block, I believe that it's intended to be a naked LED; the resistor is already in place on the motherboard.

I haven't been able to find a reference for that, though.

Yes Dwarf I have been involved with electronics for many years and yes I have blown many a LED. My problem was find what the voltage at the front block terminals was to calculate the R for a blue LED.

As you know I found that to be around 3-5 v depending on the HDD actuating.

I rigged the LED this morning with a 240R and it works fine but I am still not sure of what this is doing as the machine is now freezing after say running a MBAM scan. I cannot think it is the LED that is causing this as the current would be really quite small now and if as Bob says the board has an inbuilt resistor then that R would be that much higher.

My assumption being that the LED draw is somehow affecting the operation of the SSD.
I had it working ok last night with a red LED and if this is going to be a problem I will have to go back to that

I have run a scannow and a scan with my main security and I don't think this is due to any malware.

A strange one this. I don't see how changing an LED can have this sort of effect, particularly when the current involved is very small. My current case has an orange HDD LED and my previous one had a blue one, and the motherboards (admittedly different ones) were able to drive them with no problems. You could try putting the red one back to see if that makes any difference. Have you got the leads in the correct place on the motherboard connector, and have you made sure that there is no way for unintentional shortcuts.

When you say freezing, do you mean that the cursor stops on the screen and that you can't move it or get a response from the keyboard? That sometimes happens to me when I try overclocking my graphics cards and take it a bit too far. Are you overclocking? If so, return to stock speeds and see if the problem persists.

No mate it beats me too cos after three reboots the machine is now running as should be.

The freezing was just that - everything at standstill and the cursor just being able to be moved as usual. What actually caused it to happen is a bit of a mystery to me as you say the small current (especially as I am using a 240R) is way too small one would have thought to cause any sort of problems.