LED current related death?

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1-3-2-4

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SO I was adding LED's in a radio that used to have non LEDs.. anyways long story short I used White LED's for testing because I ran out of the color I wanted (Orange)

So the circut voltage is 12.9V DC and the white LEDs I used first are rated: 3-3.2V/ 24 mA Max/ 6,000 MCD (3mm LED) I used a 470 Ohm 1/2 Resistor in the circut and it also seems like a 70 Ohm was upstream of the 470 I put in..

So the Orange LEDs arrived today and the ratings on this on is: 1.8-2.2V / 24mA Max / 3,000 mcd.

So I connected them up like I did the white Leds and for about 30 sec it was good.. then one of the LED's failed so I open the radio and I smell the smell of hot parts it turns out one of the 70 ohm resistors got really hot along what seems to be the LED burning out.

I was also using a 470 Ohm 1/2W with the orange leds as well.

**broken link removed**
 
That shouldn't happen. Even running them at max current, though it will reduce their expected life over running them at say 10ma, shouldn't cause them to burn out that fast.
What was the color of the bands on the 470Ω resistor? Also, double check their resistance with a meter. I have, in over 30yrs of electronics, come across mislabeled resistors that were out by a factor of 10, but only once.
 
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If the resistors are correct, then maybe the supply is outputting much more than you think or perhaps very large AC ripple?
 
Mikebits said:
If the resistors are correct, then maybe the supply is outputting much more than you think or perhaps very large AC ripple?
Click to expand...


I don't know.. why is it when I had white Leds in I ran it for over 30 min without anything getting hot or a led burning out..

Now I just replaced the orange one that burned out but I haven't left it on any longer then about 30 seconds.. I just wanted to get some views before I tried again.. This thing has to stay on hours at a time when it's finally up and running.
 
Hmmm. Since you say you are replacing the original light bulbs that were in the radio, I would double check that the voltage really is 12.9Vdc and not low voltage AC or AC mains!
 
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If it was 12.9 Vac, the leds could have burned as well:

12.9 Vac (rms) means a 18.2 Vpk, the LED is 24 mA maximum: 18.2 - 2.2 = 16V/470 = 34mA

P = R.I²

P'/P = 34²mA / 24² mA = 2 => 100% more than maximum power allowed.

And if it was really AC, then your LED endered in the breakdown region, which means that it were conducting 34 mApk on both cycles.
 
the whole radio is ran off a 13.8VDC rated power supply And if you want to know the details it's a Kenwood TR-7950
 
Well, then there is really no reason for anything to burn out because the max current through a 470Ω resistor at 13.8V is 29ma making it dissipate 0.4W. The current and power will be less due to voltage drop across the LED and other circuitry.
According to your data ( 12.9V - 1.8V / 470Ω = 23.6ma) the current should be within limits for the orange LEDs. It could be that when you added the LEDs that you caused a solder bridge or short elsewhere in the radio which burned out the 70Ω resistor (Odd value there).
 
Hayato said:
If it was 12.9 Vac, the leds could have burned as well:

12.9 Vac (rms) means a 18.2 Vpk, the LED is 24 mA maximum: 18.2 - 2.2 = 16V/470 = 34mA

P = R.I²

P'/P = 34²mA / 24² mA = 2 => 100% more than maximum power allowed.
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I doubt that would blow the resistor because it's RMS current and voltage and average power dissipation that count, not peak.

Anyway, I make the peak power (13√2)²/470 = 720mW, so the average power will be 360mW, half the peak.

And if it was really AC, then your LED endered in the breakdown region, which means that it were conducting 34 mApk on both cycles.
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That's true but I'd expect that to destroy the LED, not the resistor as well.
 
It is not clear if the resistor has burned or not, he just mentions that it got hot. (The resistor is dissipating 71% of the maximum ratings, so I'd expect it getting hot to the touch).

One interesting thing is that he mentions a 70 ohms resistor. I think he might have connected the 470 ohms and 70 ohms in parallel, if they were in series the 70 ohms would not have gotten hot.
 
No one makes 70R resistors so perhaps it's 68R?

Something is wrong here.

Have you measured the supply voltage to ensure it really is 13.8V?
 
All right, I stand corrected, you can buy 70R resistors, but they are expensive, much more so than a standard 68R resistor.
 
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hmm well when I replaced it I put the same leads on the same way as the one above that blew out..

It could of been a short.. it's sitting between a back of a LCD with traces but I would expect the LCD to show funny stuff..

Last night I ran the radio for about 5 min and no sign of anything getting hot or any hot smells.

Also the LED did not burn out.
 
1-3-2-4 said:
This radio was from the mid 80's around 1984 so maybe 70ohm was more common then..
Click to expand...

Possibly or it could be 68R measure 70R which is well within tolerance?

This is why values like 70R aren't very common, a typical resistor has a tolerance of 5% so there's no need to use non-standard values. Only more accurate resistors (<1% tolerance) are made in non-standard values and even then they normally stick to the E96 series.

See Wikipedia for more information about standard component values:
Preferred number - Wikipedia, the free encyclopedia

If you're servicing something and a resistor colour code reads a non-standard or overly precise value i.e 340R or 9k for a 5% tolerance resistor then it's a good idea to question it because the colour bands tend to change colour when heated or exposed to certain chemicals. If in doubt, desolder one of the resistor leads and measure it with a DVM, if the value matches the code then you know it's right.
 
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