Thread: Quick LED query
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18 Dec 2012  

Desk1 7 Home Prem / Desk2 10 Pro / Main lap Asus ROG 10 Pro 2 laptop Toshiba 7 Pro Asus P2520 7 & 10

Quote   Quote: Originally Posted by Dwarf View Post
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
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