2.5V to power LED

Trzalica · 26th July 2011, 06:14 PM

Hi gyus,

I have a PSU (for pre amp) that have +/- 24V and +/- 12V. I need to get a power for two LEDs (stand by and power indicators). What would be the easiest way to do it?

Thank you!

Mooly · 26th July 2011, 06:19 PM

Led and resistor... doesn't come any easier. The series resistor is calculated using ohms law.

e.g. 12 volt supply. LED current needed say 5milliamps. Volt drop across LED say 2 v nominal. That varies with colour of LED but with highish supplies it's effect on the calculation is negligable in practive.

So we (12-2)/0.005 which is 2000 ohms. So 2K2 would be nearest...

Trzalica · 26th July 2011, 06:59 PM

I was thinking along that (ohms law) line but wasn't quite sure how to apply it B-(

Thank you Mooly!

Mooly · 26th July 2011, 07:04 PM

Your welcome...

Also some LED's are super efficient and may be bright enough at even lower currents. It's trial and error really to get the brightness that looks acceptable to you.

Trzalica · 26th July 2011, 07:17 PM

Actually, I will feed several different LEDs, the yellowish one as a stand by, green as power and 6 blue (input selection indicators). Each of them needs a slightly different voltage so, as you said, it is a "try until you happy" process.

Since now I am on good track it would be much easier to play.

luka · 27th July 2011, 05:03 AM

just note, that leds are more happy with constnt current source then constant voltage source

Trzalica · 27th July 2011, 03:47 PM

Hvala Luka B-)

So what is your suggestion, should I use resistors only or something more sofisticated?

sawreyrw · 27th July 2011, 04:08 PM

Resistors are fine given that the supply voltage dominates what the current will be.

luka · 27th July 2011, 05:30 PM

with constant voltage supply, resistor is ok
if a bit more advance, transistor and one or two more components will be needed
if even more, led driver would be the best

martin clark · 27th July 2011, 07:45 PM

No need for such elaboration at all. LEDs have very low dynamic resistance and therefore also low noise (Zd well below 50ohms at 1mA, depending slightly on colour and operating current) so follow Mooly's example - pick an operating current and calculate and appropriate resistor.

Even easier - 10K works for almost anything. If it's not bright enough, tack another 10k in parallel. Repeat until happy.

26th July 2011, 06:14 PM	#1
Trzalica diyAudio Member Join Date: Oct 2008 Location: Vancouver - Canada and Belgrade - Serbia	2.5V to power LED Hi gyus, I have a PSU (for pre amp) that have +/- 24V and +/- 12V. I need to get a power for two LEDs (stand by and power indicators). What would be the easiest way to do it? Thank you! __________________ Cheers, Marin

26th July 2011, 06:19 PM	#2
Mooly diyAudio Member Join Date: Sep 2007	Led and resistor... doesn't come any easier. The series resistor is calculated using ohms law. e.g. 12 volt supply. LED current needed say 5milliamps. Volt drop across LED say 2 v nominal. That varies with colour of LED but with highish supplies it's effect on the calculation is negligable in practive. So we (12-2)/0.005 which is 2000 ohms. So 2K2 would be nearest... __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

26th July 2011, 06:59 PM	#3
Trzalica diyAudio Member Join Date: Oct 2008 Location: Vancouver - Canada and Belgrade - Serbia	I was thinking along that (ohms law) line but wasn't quite sure how to apply it B-( Thank you Mooly! __________________ Cheers, Marin

26th July 2011, 07:04 PM	#4
Mooly diyAudio Member Join Date: Sep 2007	Your welcome... Also some LED's are super efficient and may be bright enough at even lower currents. It's trial and error really to get the brightness that looks acceptable to you. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

26th July 2011, 07:17 PM	#5
Trzalica diyAudio Member Join Date: Oct 2008 Location: Vancouver - Canada and Belgrade - Serbia	Actually, I will feed several different LEDs, the yellowish one as a stand by, green as power and 6 blue (input selection indicators). Each of them needs a slightly different voltage so, as you said, it is a "try until you happy" process. Since now I am on good track it would be much easier to play. __________________ Cheers, Marin


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27th July 2011, 05:03 AM	#6
luka diyAudio Member Join Date: Sep 2005 Location: in Slovenia :)	just note, that leds are more happy with constnt current source then constant voltage source __________________ home page @ http://www.classdaudio.co.cc @ 24/7 all year long I FEEL SLOVENIA

27th July 2011, 03:47 PM	#7
Trzalica diyAudio Member Join Date: Oct 2008 Location: Vancouver - Canada and Belgrade - Serbia	Hvala Luka B-) So what is your suggestion, should I use resistors only or something more sofisticated? __________________ Cheers, Marin

27th July 2011, 04:08 PM	#8
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	Resistors are fine given that the supply voltage dominates what the current will be.

27th July 2011, 05:30 PM	#9
luka diyAudio Member Join Date: Sep 2005 Location: in Slovenia :)	with constant voltage supply, resistor is ok if a bit more advance, transistor and one or two more components will be needed if even more, led driver would be the best __________________ home page @ http://www.classdaudio.co.cc @ 24/7 all year long I FEEL SLOVENIA

27th July 2011, 07:45 PM	#10
martin clark diyAudio Member Join Date: Jul 2003 Location: Bath, UK	No need for such elaboration at all. LEDs have very low dynamic resistance and therefore also low noise (Zd well below 50ohms at 1mA, depending slightly on colour and operating current) so follow Mooly's example - pick an operating current and calculate and appropriate resistor. Even easier - 10K works for almost anything. If it's not bright enough, tack another 10k in parallel. Repeat until happy.

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