I Think I've blown my Diodes ??

GrowingLad

GrowingLad

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I Think I\'ve blown my Diodes ??

I decided to clean up the high powered light on top of the wheel house as it was looking a bit worse for ware.

Turns out there were two pairs of cables. One going to the high powered lamp in the centre and another pair going to what I think are diodes (is that correct) which lit up the bevelled glass around the edge of the lamp. The way is was wired was they both come on at the same time. But I'm thinking that they should be on separate switches so that you can have the lower powered lamp on if you don't need to light up the night sky.

Now begins the silly part. I took the diodes out and connected them to a 12v battery BRIGHT light and then nothing...oopps.

I thought in my ultimate wisdom that it didn't matter which connection went to what terminal but in hindsight I may have been a little wrong...

So my question is, if I can source some new diodes will I be okay putting 12v through them and connecting them as you would a normal lamp (all be it, making sure I get +ve and -ve terminals the right way around).

Thanks Simon.

DSC03747.jpg
 
Re: I Think I\'ve blown my Diodes ??

Can't see the detail, but that looks like two LEDs -I assume in series, as I don't see a dropper resistor.
The polarity is important. neg side may have a flat to identify.
Your question... I think you will blow individual diodes if you apply 12v (14.8v if charging!), that's why I think they are in series (pos to neg , like putting batteries in a torch) unless there is a component hidden in the shrink tubing.
Actually, why bother with them if you don't have the facility to use them separately?
 
Re: I Think I\'ve blown my Diodes ??

all you have is two led lights soldered with the pos lead to the negative and negative to posative on the other side, its then switched elsewhere for the relevant light and also the dropper is elsewhere i think. (yes you have blown them)

ultraleds.co.uk is one good place but even ebay if you search, you dont need the highest mcd output, just a difussed angle.

steve
 
Re: I Think I\'ve blown my Diodes ??

I did wonder if the photo would show enough detail.

One leg goes to a large section of the inside of the diode, lets call this the +ve side ( I think that is correct, I suppose the colour of the cables was a bit of a give away..wish I realised that before I got them the wrong way around) The other leg of the diode goes to the smaller terminal of the inside of the diode (lets say the -ve). +ve is soldered to +ve and -ve to -ve. The black cable is connected to the +ve side, blue to -ve. No other components apart from that. I think you've answer my question...as long as I get the polarity correct I should be okay with 12V.

To answer your other question, I've pretty much replace/repaired/fixed every part of the boat and I'm still in the process so to pull another cable in wouldn't be a problem....she's how would you say..hmmmm...a project /forums/images/graemlins/smile.gif
 
Re: I Think I\'ve blown my Diodes ??

Your LED's blew because you passed 12v through them with no current limiting resistor. ( bright light then nothing )

Replacing the LEDs and doing the same again will blow them again. You need about 400 ohms to limit the current to around 25ma placed in series with the LEDs.

You can buy 'raw' leds from somewhere like Maplin or RS.
 
Re: I Think I\'ve blown my Diodes ??

This is what I couldn't work out. I thought it strange to put 12V straight through the LEDs, but there wasn't any resistor in the fiting itself, and they must have been working before..

I shall have a read up on LEDs....mmmm, might just be easier to forget about them but I do like to find myself little jobs during the winter.
 
Re: I Think I\'ve blown my Diodes ??

2 in series is OK across 12v but 3 is better IMHO.
AND, I add an ordinary diode in series, to protect against polarity errors. See my low-power anchor light below.
anchorlite2600x450.jpg
 
Re: I Think I\'ve blown my Diodes ??

nice led mod, i did this with the white led lights when they were still prototypes in 1999, i used two rows of four with the spacing staggered between the rows. at the time the output was very low mcd by it improved every few months as they perfected the technology. Funny in a way as they boasted 10 years plus life and my research didnt give much more that 3 before the light level had decreased.
 
Re: I Think I\'ve blown my Diodes ??

It depends entirely on the type of LED's you are using. The forward drop across the semiconductor junction and the current rating are what you are interested in.

A typical 5mm red, green or amber led will have a typical forward drop of between 2 and 2.5 volts and a current rating of 20 mA. Putting 2 of these across 12v will 'let the smoke out'

White and Blue LEDS have much higher forward drops, A 10mm white led typically 3.2 to 3.8v and 80mA operating current.

The high power leds will run at 3.8 to 4.2v forward at 700mA or more.

Exceeding these perameters will shorten the lifetime or blow them immediately. LEDS are semiconductor devices, much like a transistor, you cant just 'slap 'em on a battery'!

Billjratt has forgotten to mention the current limiting resistor plainly visible in his photo.

The big challenge with LED driving circuits is that using a simple limiting resistor as in most cheap setups means that the LED brightness varys with the battery voltage and spikes from alternators or other equipment are allowed through the device, severely shortening its life.

One step up linear current regulators work very well but you stand to dump as much energy in heat as a normal incandescent bulb would use and...

Using the ideal, a switching current limiting circuit is complicated, more expensive and in some cases leads to radio interference.
 
Re: I Think I\'ve blown my Diodes ??

[ QUOTE ]
2 in series is OK across 12v but 3 is better IMHO.

[/ QUOTE ]

No, No, No, No!

Normal LEDs need something to limit the current through them, because they can't do it themselves [1]

Indicator LEDS need about 15ma, high brightness ones maybe 25-30 ma, and ultrabright ones (like in your light) maybe 75ma or more, but check the spec!

The OP won't have blown the LED's with reverse polarity, but with excessive current.

The LED will drop a certain voltage when it's working (depending on colour: red is about 1.2v, blue might be 4v, or more; white commonly about 3v) - again it will be in the spec - usually phrased "Vf = 3v @ If = 25ma" - 3volts drop at 25ma operating current.

Armed with this info, you can calculate what the limiting resistor needs to be:

1 diode 3v @ 25ma from 13v ("12volt" battery) leaves 10v to drop across the resistor @ 25ma = 10/0.025 = 400 ohm (390 ohm is the nearest common value)

2 diodes in series = 2 x 3v from 13v = 7v to drop across the resistor @25 ma = 7/0.025 = 280 ohm (270 ohm nearest common value)

3 diodes in series = 3 x 3v from 13v = 4v to drop @ 25ma = 160 ohm (150 or 180 ohm nearest common value)

etc.

Check the power rating for the resistor by multiplying voltage drop by current, e.g. for the first example 10v x 25ma = 10 x 0.025 A = 0.25W (so a bog standard 1/4 watt resistor will do fine).

If the anchor light is still working, it is because your LEDs will tolerate an over-current and/or that the connecting lead is long enough, and thin enough to limit the current to a tolerable value. If it works, good luck to you. /forums/images/graemlins/smile.gif

Andy

[1] Some LEDs have built in current limiters, but these are pretty rare (and could be destroyed by a reverse polarity).
 
Re: I Think I\'ve blown my Diodes ??

[ QUOTE ]
LED-1.jpg


Hopefully the above diagram clears things up a little.

Am I right in thinking that these are in parrallel??

[/ QUOTE ]

Yep, parallel, but I think you're still looking for a resistor (somewhere) in the black or blue wires.

Andy
 
Re: I Think I\'ve blown my Diodes ??

sorry, i read it wrong, i was thinking you meant the two led lights were switched indivually. as everyone else has said there must be a resistor somewhere, check voltage at wires.
steve
 
Re: I Think I\'ve blown my Diodes ??

Alternatively, go to Maplins and buy a couple of LED's that are internally resistored and suitable for direct connection to 12 volts, no chance of blowing them up then
 
Re: I Think I\'ve blown my Diodes ??

Just to reiterate in case there is any doubt. LEDs must have a current limiting resistor. They have a non linear current curve such that when the forward voltage drop requirement is met then there is no limit to current flow (until the smoke escapes). Reverse voltage on LEDs if it is only 12v will not normally hurt them just won't work until it is correct. olewill
 
Re: I Think I\'ve blown my Diodes ??

[ QUOTE ]
Reverse voltage on LEDs if it is only 12v will not normally hurt them just won't work until it is correct. olewill

[/ QUOTE ]

Sorry but that is just not true. A lot of LEDs have maximum reverse bias voltages around 4v.

LED's are not simple plug and play replacements for incandescent bulbs. They are semiconductor devices that need the correct conditions to operate, last the predicted lifetimes and deliver the expected light.
 
Re: I Think I\'ve blown my Diodes ??

Chaps....thinking aloud now...and being as the little bit of electronics that I studied was way back in the past I could be barking up the wrong tree....

For sure there wasn't a resistor...whether this was correct or not I don't know....but what if the diodes where in series after the lamp...could this be possible? Obviously they would be on the same switch...could the high powered lamp replace a resistor?

I must admit at the beginning I thought it would be just a case of getting replacement diodes and making sure I got the polarity correct...after reading the above I'm getting very close to ahhh...just leave it....the more complex things are the more chance of em braking.
 
Re: I Think I\'ve blown my Diodes ??

I told you it had been a long time since my electronics lessons /forums/images/graemlins/smile.gif

Thanks for the info, I shall revert to plan B.
 
Re: I Think I\'ve blown my Diodes ??

Yes sorry Nibusgb I went and checked the LEDs I have on the data sheet 5v max reverse volts. I thought it was higher. olewill
 
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