LED's burning?

Basic electrical and electronics, such as DC/Analog control.
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ste234
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LED's burning?

Postby ste234 » Sat Apr 17, 2010 9:42 pm

Hi,
Recently bought 3 yard lights from layouts 4 u, and they are wired in series, but everytime i apply the voltage, each gets very hot and a burning smell emits from them, now, IMO, this isn't good is it! Any ideas why this may be happening? There are two LED's on each lamp, so there are 6 overall.
Cheers,
Ste :shock:
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Son-1
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Re: LED's burning?

Postby Son-1 » Sat Apr 17, 2010 9:50 pm

What voltage you running them on? If you push an LED and resistor to hard, the extra has to go somewhere and it gets hot. I built some light once, and had the resistors running so hot the solder would melt. :shock:
Stephen T

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ste234
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Re: LED's burning?

Postby ste234 » Sat Apr 17, 2010 9:58 pm

12v, the lamps are 6v each and the instructions say wire 3 in series for best results... :shock: I did have a resistor in the circuit when i was using one light, but it burnt out :shock:
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Brianetta
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Re: LED's burning?

Postby Brianetta » Sat Apr 17, 2010 10:19 pm

Is your 12V supply actually supplying 12V? I have three. Two are switch-mode supplies, the other is unregulated. The unregulated one spits out 18-20V unless you really load it. Just a couple of LEDs isn't enough of a load on that one.

If you have a multimeter, measure the voltage across each lamp. Three in series, if there's nothing else, should have 4V across them. If they're in parallel by mistake they'll be getting 12V, which is twice what they want.

Also, measure the current running through them. Any more than 20mA is bad for a coloured LED, more than 30mA is bad for a white one. If it seems to be a bit high, try putting a resistor in series with them.

Here's a page about LEDs for you.

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ste234
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Re: LED's burning?

Postby ste234 » Sat Apr 17, 2010 10:34 pm

Hi Brianetta, its an unregulated supply coming from an old H&M controller, it measures at around 14v.
I've just measured the voltage across the lamps, and it comes out at around 1-2 volts, unfortunatley i only have a voltmeter so can't find the current.
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D0260
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Re: LED's burning?

Postby D0260 » Sat Apr 17, 2010 10:59 pm

Is the power supply output AC, as I suspect .
You need to add a rectifier diode , something like a 1N4001 in series.
What is happening is that on one half cycle you are generating light from the LEDS , and then the voltage reverses and is -18VPeak or more if lightly loaded , and LEDs dont like reverse voltage . reverse voltage more than 5V will lead to the LED becoming an avalance diode , conducting in the reverse polarity direction and is potentially terminal , and /or damaging to the semiconductor junction. Silicon migration has almost certainly taken place on the chip, and its non reversible, probably best to dispose of those lamps if the problem persists.
As a quick check , ditch the mains power supply, and use a PP3 square 9Volt battery for a short time- get the polarity right and measure the true DC volatge across the resistor ( and check the value of it) . If its a 1K resistor , and its got 7V across it, then its 7mA flowing thru it and the LED. Post the results back here , and check if the lamps seem hot when used with a battery.
Sorry for the bad news.
It is very possible that the rectifier(s) in the controller are 'leaky' which would not be noticed on a loco , or short/open circuit.

EDITED to include the bit about a PP3

EDIT2 about the faulty rectifier
Last edited by D0260 on Sat Apr 17, 2010 11:09 pm, edited 4 times in total.

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ste234
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Re: LED's burning?

Postby ste234 » Sat Apr 17, 2010 11:04 pm

On controller it says uncontrolled 12v DC output,and i have tried them on my gaugemaster controller (on the controlled output which controls the trains) at 12v and exactly the same happens?
I've emailed the company i bought them from, and am awaiting a reply with hopefully some good news :roll: :lol:

*EDIT* and funnly enough no matter which way i connect the two wires into the controller, the LED's still work :shock:
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Re: LED's burning?

Postby Brianetta » Sat Apr 17, 2010 11:17 pm

Works both ways on DC, gets hot? Looking at Layouts4U.net's page full of lighting, I'm now fairly certain that you have 6V filament lamps there, not LEDs.

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ste234
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Re: LED's burning?

Postby ste234 » Sat Apr 17, 2010 11:21 pm

So, if they are filaments (i can't see inside as they are painted black) what explains the smell of burning?
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Re: LED's burning?

Postby Brianetta » Sat Apr 17, 2010 11:36 pm

Well, filaments get hot. Very hot. It might be that after you've run them for a while the smell will stop, just like it does on a brand new toaster.

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ste234
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Re: LED's burning?

Postby ste234 » Sun Apr 18, 2010 11:38 am

Hmm, i think i might leave them turned off until Layouts 4 U reply to my e-mail, im not very sure about them especially the smell, and if it's burning the paint thats not good!

Straying off point here a little, but that website you linked me to, it says not to wire LED's in paralell, but ive got my 3v LED streetlights wired in paralell with a resistor at the start of the circuit, is this OK to do?
Ste
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Daniel S
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Re: LED's burning?

Postby Daniel S » Sun Apr 18, 2010 11:54 am

Hi

Have you got a resistor on each led that is connected in parallel? Because that website says only to avoid connecting LEDs in parallel with only one resistor.

On my layout I have loads of LEDs all connected in parallel but with a resistor on every LED. You should be fine, if it is connected like this.
Hope this helps.

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Re: LED's burning?

Postby D0260 » Sun Apr 18, 2010 11:56 am

LEDs behave like a 1.7Volt ( typical- more for white LEDs) zener diode, not a linear resistor. The 1.7V figure can vary throughought a production batch, batch to batch, and manufacturer to manufacturer.
Paralleing LEDs is NOT good practice, one could be over- running , whilst another will be starved of current and dim.
You really want to wire a resistor in series with each lamp if the LEDS are in paralell across a supply. Make sure the resistor is the right value , use R=V/I where R is the resistor value in ohms, V is the voltage across the resistor ONLY ( not the supply voltage) , and I is the current you want to run the lamp at in AMPS ( not mA).
If you sit and think about it though and change R to kOHM and I to mA then the status quo is restored. Try somewhere above 100 Ohms resistor or more PER LAMP . It doesn't matter how many streetlights you connect like this as long as you don't exceed the transformer or power supply rating , which typically is enough for 50 to 100 LED street lights.

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ste234
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Re: LED's burning?

Postby ste234 » Sun Apr 18, 2010 12:05 pm

Hi, i will have to put resistors on each then, i just thought wiring them in paralell would be better 'cos if one goes out i can tell which one.
Looks like i could have to spend some time putting resistors in :lol: :wink:
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Re: LED's burning?

Postby Flashbang » Sun Apr 18, 2010 3:57 pm

Hi
LEDs are IMO best operated with each LED having its own dedicated series resistor.
For 12 volt operation most 2.2/2.5volt LEDs will be bright enough with a 1k0 (1000) OHMs 1/4watt resistor in series with either the Anode (Positive) or Cathode (Negative) lead of the LED. It doesn't matter which side the resistor is fitted into, but try to keep them all the same to aid later fault finding and keep things standard.

I know LEDs can be wired together in series (positive leg of one to the negative leg of the previous one and so on) and one series resistor used for the whole bank of LEDs, but IMO its better to use one resistor per LED.

To prove they are LEDs once the 1K0 resistor is fitted in series with either leg of the LED, temporarily swap around the dc power supply - the LEDs won't illuminate with the incorrect dc polarity applied. Filament lamps would still illuminate.
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Broken? It was working correctly when I left it.


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