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Author Topic: LED confusion  (Read 2156 times)

nivapilot

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LED confusion
« on: September 06, 2015, 01:01:40 PM »

Hi all, wifey was tossing out a set of LED lights. Ideal I thought for interior lights and mastheads etc. :-)) small enough for most uses, don't think I would run out of them for a long time. :-)

Plug states that the output is 24volts AC......when I measure across the terminals of  a single LED, I get a reading of 6v AC? not DC?

I thought that LEDS were DC voltage? How does that work then? {:-{

or are they flashing so fast I can't see them flashing?
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Martin [Admin]

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Re: LED confusion
« Reply #1 on: September 06, 2015, 01:09:37 PM »


Some LED type light are dual colour, i.e. one colour +tv, -tv & a different one -tv, +tve.
Also, LED's are DC (D = diode)  so you may only be reading the DC component of the voltage.

Cut one free and stick 1.5v battery on it!

(edit)
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slowcoach

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Re: LED confusion
« Reply #2 on: September 06, 2015, 01:16:11 PM »

Hi
If it is a standard LED then 6v will blow it start with 1.5v or less unless it has a resistor in series with it.

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Brian60

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Re: LED confusion
« Reply #3 on: September 06, 2015, 02:06:14 PM »

As they are going to be thrown out forget what the plug says, you won't be using them all together!

Snip one off the wiring loom then attach a 10k resistor to one of the legs preferably the anode (+) this can be noted by a flat spot on the edge of the led itself, the legs will have been trimmed so are of no use in telling which is which. A 10k resistor is brown black orange then connect across a 12v battery, this will light it up, if it doesn't reverse the connectors on the battery it shoud light up. It will be bright, very bright or dim, the worse case scenario is it blows the led. If its too bright either stick another led in series with it or use a higher resistor.

This of course is if the led has two legs, if it has three or more than it is a multi colour led, ie red, green, yellow all in one 'bulb' and has to be handled differently.
 

nivapilot

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Re: LED confusion
« Reply #4 on: September 06, 2015, 03:04:20 PM »

Cheers lads, 3volts does the trick, so now I have nearly 40 x 4mm LEDS.... :-))

I really need to get on and order some components to make a variable power supply.
I reckon on a LTC3780, which is a powerful 130W Step Up/Step Down converter coupled up from my old CB 13.8v 8amp power supply....should be good for up to 30volts at 8amps..hopefully
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Umi_Ryuzuki

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Re: LED confusion
« Reply #5 on: September 06, 2015, 08:34:52 PM »

LEDs are DC current, but will only pass current in one direction.
The AC current is at a cycle that is so fast, you do not see the flicker in the LED.
The string will have been wired in series, so that the number of LED in any
string will equal the AC voltage of the house current.
So when you pull of any one LED, depending on its color, it will run on 2.2-3.4 volts.




 :-)

nivapilot

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Re: LED confusion
« Reply #6 on: September 06, 2015, 09:57:23 PM »

Thanks for the replies lads. Just what I needed to know.
But a special thanks to Stan who PM,d me with some special info....cheers mate  :-))
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malcolmfrary

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Re: LED confusion
« Reply #7 on: September 07, 2015, 08:11:34 AM »

A LED has a forward volt drop, which is what it shows when passing current and glowing.  It also has a peak inverse voltage, which is voltage applied in the other direction.  Too much and it becomes an ex-diode.  Two of them wired nose to tail in parallel show only the forward voltage drop in both directions.  They don't like too much current forward so you need to be careful when testing - what is good on a 3v dry battery will have a short life on a proper power supply.  A 3v dry battery has its own internal resistance, a real power supply doesn't. 
Unless the LEDs have their own current limiting built in, they will need a ballast resistor in circuit to keep current below that which will finish them off rapidly.
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