RE: OT - LED's

[Keith Doxey <ukha@xxxxxxx>]

Title: RE: [ukha_d] OT - LED's

So you use multiple series chains in parallel each with their own resistor or use a higher supply voltage.

 

eg Red LED typically 1.8V so you can put 10 in series on a 24V supply and use a resistor t drop 6V.

 

In the last 26 years I have had one LED fail ( 26 years ago). I used it without a series resistor t see how bright it would be. I was amazed that beyond a certain current it got dimmer, then the leads got VERY HOT, then there was an awful smell and it was an Ex-LED :-((

 

LED's do not blow ( well not in a short time, most have a quoted life  > 100,000 hours). The only thing that kills them is overdriving them.

 

Keith 

-----Original Message-----
From: John Hill [mailto:jhill1@xxxxxxx]
Sent: 25 July 2001 10:30
To: ukha_d@xxxxxxxSubject: RE: [ukha_d] OT - LED's

and when one of them fails, the rest go dark...

remember trying t find the blown bulb in the Christmas tree lights

But then from the figures for the LED's, only 2 could be in series so still wouldn't be very bright

 

John

 

-----Original Message-----
From: Keith Doxey [mailto:ukha@xxxxxxx]
Sent: 25 July 2001 10:14
To: ukha_d
Subject: RE: [ukha_d] OT - LED's

Put the LED's in series NOT parallel.

 

LED's do not share power equally, one will always hog the power, also is you are trying t drive 2 Parallel LED's at 20mA then total current is 40mA. Put them in Series and the total current is still only 20mA but the resistor has t drop less voltage, therefore lower resistor value, lower power loss in the resistor, less heat.

 

Keith

-----Original Message-----
From: John Hill [mailto:jhill1@xxxxxxx]
Sent: 25 July 2001 10:00
To: ukha_d@xxxxxxxSubject: RE: [ukha_d] OT - LED's

Pedro,
If I remember correctly
The viewing angle should be stated as the point where the light output is half (they usually quote full angle rather than half angle).

If you look directly (head on) at the LED, you can move 30 degrees in any direction before the light output drops t half the level compared t looking head on at it.

You WILL need to limit the current to about 20mA (approx2/3 max) through each diode by using resistors
ie
12v supply- 5.5v across LED = 6.5v dropped across resistor
R= 6.5v/20mA = 325ohms (330 is close enough and an available value)1/4 watt resistors will be OK

You could arrange to feed all LED's in a parallel array through a single higher wattage resistor, but if(as) any LED fails, the current through the rest will increase. This would not be a problem if you have a large array and only one or two fail. But if you only had say 2 in the array and one failed the current would blow the remaining one also.

One final point. The light output from these are quite low so you would need quite a few to illuminate something,
http://uk1.farnell.com/Search/search4-frame.jhtml?_DARGS=%2Fcommon%2Fprodsearchform.jhtml
list some much brighter ones, narrower angle, but if your using an array...

John

-----Original Message-----
From: Pedro de Oliveira [mailto:oliveirp@xxxxxxx]
Sent: 25 July 2001 08:53
To: HA (E-mail)
Subject: [ukha_d] OT - LED's

I thought I would try this question here first since someone is always
willing to help with their expert advice... Anyway....

I am looking at buying some LED's to brighten up a small area and have been
stumped by their specifications and their meaning. I understand most of it
but what does viewing angle mean ? I though an LED emitted light all around
but then I found specs at Maplins and these have a viewing angle mentioned
http://www.maplin.co.uk/products/frameres.tpl?partcode=JA24B:JA25C:&SKU=12703&cart=943432253&d=f&max=1&Group1=LED%205mm%20blue.

I would like to use a 12v supply but does this mean because they are rated
at 5.5v max that I would need two parallel rows of LED's with a resistor and
if so how would I wire them so they don't "pop" every week ?

Cheers in advance

pedro

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