Blue power LED

michaelab

michaelab

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Right, of course my new power amp wouldn't be complete without a blue LED on the front panel to indicate it was switched on....:D

So, erm, how do I do that then? I've seen some mains voltage LEDs (of all colours) on RS and Maplin sites but most of them come in rather naff looking mountings. Also, I would of course want to ensure that the LED didn't screw up the incoming AC in any way.

Any ideas? Circuit diagrams are OK.

Michael.
 
Usually it would go on the DC end.

A fancy 5mm blue LED from RS (454-4756 for example) is specced at 20mA and 3.6vF. So if your DC rails are +-50, you need a series resistor that will pass 20mA at 96.4v if you put the LED across both rails. V=IR so that's 96.4=0.02xR, R=4820Ohms, call it 4k8 or 5k0.

You can inline a resistor into one leg of the LED and cover it with heatshrink, neat wiring back to the PSU reservoir caps will conjure nice atmospheres when you power up for the first time.

Paul
 
how about drilling a 3 or 5 mm hole in the front panel,fill the hole with clear expoxy resin and mount the led behind the window?
As for wiring the led well heres one sugestion,the resistor valuye = ( supply voltage - led forward voltage) / If. As its A.C you can half the calculated value
to work out the minimum power rating of the resistor its
( supply voltage - led forward voltage) x If.

best to use 2 resistors,so if one blows up the other will offer a bit of protection
 
Another thing to consider is the rated light output ( in micro candela's i believe) . I've just bought a load of Blue LED's for a little project of my own, simply changing the LED in my X-can from red to blue ( to denote other mods i've carried out :) ). The angle of the light is also very important , if you get a very bright one with a narrow beam it can leave a bright spot on your retina for a good few minutes :o .
So basically you will need to sort out what kind of voltage you have available, maybe look at stepping it down using resistors, then you will need to pick an led with the right colour and brightness and also consider at what angle you wish to view the LED. I went for one with something like a 60 deg angle as it was visible from a wide range. the narrow beam ones are surprisingly difficult to see in a bright room unless you are head on to them.
If your amp already has a LED fitted then it will be easy to swap it out for a Blue one, you may have to fine tune the series resistor though to get it to work howyou want.

I've got quite a few spare ones that I'll probably never use , you can have one or two to play with if you wish.

Theres a little bit of my own info here

HTH
S&C
 
you will need to sort out what kind of voltage you have available, maybe look at stepping it down using resistors
Click to expand...
The resistor isn't stepping down the voltage, it's defining the current given the voltage. Once the LED is lit the voltage across it will remain roughly constant, so in the absence of a current defining resistor huge currents can flow leading to a very short and bright life. It's a trivial application of Ohm's law once you have the 'on voltage' or vF, which varies slightly by type but is 0.6v for a silicon non-LED, 1.2ish for an ordinary red LED etc.

Paul
 
Originally posted by Paul Ranson
A fancy 5mm blue LED from RS (454-4756 for example) is specced at 20mA and 3.6vF. So if your DC rails are +-50, you need a series resistor that will pass 20mA at 96.4v if you put the LED across both rails. V=IR so that's 96.4=0.02xR, R=4820Ohms, call it 4k8 or 5k0.
Click to expand...
Thanks Paul. My DC rails are +-60 so, in order to get 20mA at 116.4v I'd need a R of 5820 Ohms - call it 5k8 or 6k0.

Probably best to wire up to the DC side rather than AC as you suggested.

I'll see what the local electronics shop has. Hmm, I might even have to remember how resistor colour banding works...it's been a VERY long time :)

Michael.
 
Thanks for the office Penance. I'll see what I can get here locally and then I'll let you know...the postage would probably cost more than the bits!

Michael.
 
Originally posted by Paul Ranson
The resistor isn't stepping down the voltage, it's defining the current given the voltage. Once the LED is lit the voltage across it will remain roughly constant, so in the absence of a current defining resistor huge currents can flow leading to a very short and bright life. It's a trivial application of Ohm's law once you have the 'on voltage' or vF, which varies slightly by type but is 0.6v for a silicon non-LED, 1.2ish for an ordinary red LED etc.

Paul
Click to expand...

Paul,
Agreed, ( edited: I think ), brain was a bit addled at 5am when i posted. I found that when i used the bench power supply at work to test my selection of LED's, the Blue LEDs that i wanted to work didn't light up until they had about 2.7v ( more usually about 3.5v ) where the Red ones lit up at about 1.3v. Once i'd got them to light up i used a variable resistor to fine tune the brightness and finally substituted the variable for a fixed once i'd got the brightness i wanted. Anyway it's working and looking a very cool blue so imust have done something right :) .
 
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might even have to remember how resistor colour banding works
Click to expand...
Bald Bastards Rape Our Young Girls But Violet Go's Willingly
3 bands-first 2 colours the value 3rd nuber of zeros,
4 bands first 3 colours the value 4th band number of zeros
5k8=5800=green,grey,red
 

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