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I'm having an issue in a simple (n00b) LED circuite - ended up burning out a couple of LEDs due to some mistake I'm making, but can't figure out what the error is.

I've got a 9v battery, trying to drive two blue LEDs (3.4-3.8v rated, 20mA) in parallel. I calculated necessary resistor as R = (9 - 3.4) / 0.04 = 140 ohms. To be on the safe side, I used a single 180 ohm resistor (brown/gray/brown/gold) in series (battery + to resistor; resistor - to + pole/anode of both LEDs; - pole/cathode of both LEDs back to battery) on a breadboard. Upon briefly connecting the 9V battery the first time, circuit lit; on 2nd connect, it stopped and I smelled distinct LED burnout smell - after which both LEDs would no longer light, even when connected individually to a 3v (2x AA) source.

Would appreciate any advise re: what I'm doing wrong. Thank you!


Thank you for the assistance - so in the circuit I'm considering with 9V battery powering three LEDs (details in my comment above), would the following be the way to go?

Proposed Circuit for 9v battery with 3 LEDs

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  • 2
    \$\begingroup\$ Did you by accident connect them the wrong way? You may have reverse-voltage killed them \$\endgroup\$ – KyranF Oct 20 '14 at 9:38
  • \$\begingroup\$ One resistor for 2 LEDs? Could it perchance be a cascade failure? You need one resistor per LED when in parallel. \$\endgroup\$ – Majenko Oct 20 '14 at 9:39
  • \$\begingroup\$ I confirmed the LEDs were connected the right way (after checking each LED indivdually with a 3v power source to see which way it'd light). \$\endgroup\$ – netarc Oct 20 '14 at 9:49
  • \$\begingroup\$ sorry, what's a cascade failure? So I'd want to replace the single resistor with one in each LED's parallel circuit, so that each LED has a resistor wired in series? Would I still use a ~140 ohm resistor for each LED? \$\endgroup\$ – netarc Oct 20 '14 at 9:50
  • \$\begingroup\$ fwiw, if I recall correctly - upon teh 2nd connection attempt one LED went out almost immediately, followed direclty by the other one. I'm guessing this is a cascade failure, in that when the first LED burnt out, the 2nd suddenly experienced a V increase, causing it to burn out? \$\endgroup\$ – netarc Oct 20 '14 at 9:52
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What you have suffered from is what I term (I don't know if it's the real name for it) a cascade failure. From your description your circuit is like this:

schematic

simulate this circuit – Schematic created using CircuitLab

You have sized your resistor assuming a total of 40mA through a pair of 20mA LEDs. You have also assumed a forward voltage of precisely 3.4V.

If both your LEDs were absolutely exactly 3.4V forward voltage, then you would have a chance of that working, since the current would split evenly between them. However, that will most probably not be the case. Imagine what would happen in that circuit if one LED had just a 0.1V difference in the forward voltage drop? How much current would flow through each one?

Well, most of your 40mA would go through the one with the lower forward voltage. That would get far more than the 20mA limit it's designed for, and the other one would get next to nothing. Yes, they may both light up, but one would be much brighter, at least for a moment, before it burned out.

Now, LEDs normally initially burn out in a dead short. But that short soon overheats and fuses, so becomes an open circuit. So it's like not having that LED there at all. So now all your 40mA is getting pumped through the second LED. That's way too much for it to handle, so it then blows as well.

A cascade: one blowing causes the next to blow. If you had lots of LEDs in parallel like this and could slow down time (maybe with a very high speed camera) you'd see a distinct sequence of them blowing one by one in the order of their forward voltages (at least for the first few - as the currents got too high they'd just all go at once).

So what do you do? Simple - you treat each individual LED as a separate entity - calculate a resistor for each by itself. For this it'd simply be double the resistance, but twice over:

schematic

simulate this circuit

So now each branch of the circuit gets its own share of the current, and each branch decides for itself what current it needs - ~20mA each in this case. If one LED should blow the other branch is still, as an individual circuit, getting just the 20mA it needs.

To better illustrate what happens, I have drawn a pretty graph of LED current (note - this isn't a real LED diode graph, just some numbers I made up for illustrative purposes. A real LED graph would have much sharper curves, but it serves to demonstrate my point):

enter image description here

When you have a single resistor limiting to 40mA you're limiting the yellow line (I(tot)). At the point that's at 40mA, ~3.3V, the two LED currents I1 and I2 are very imbalanced - you can see one gets ~18mA, and the other ~24mA. The one with 24mA then blows. Now there's no blue and red lines, only the yellow line. I1 becomes 0, and I2 becomes I(tot).

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  • \$\begingroup\$ ah, that correlates with what I saw - one LED 'blew,' followed quickly by the other one, after the 2nd time I connected the battery! Too, I can confirm that the two LEDs in question - though they had similar specs - were NOT identical, so your explnaation makes sense. Thank you! \$\endgroup\$ – netarc Oct 20 '14 at 9:59
  • \$\begingroup\$ BTW, my ultimate goal is to have the 9V battery power three LEDs: two are microtivity IL604 (5mm RGB Slow-Rotating LED; forward voltage is 3.0-3.6 volts) to be wired in parallel, the third will be a blue LED (3.4-3.8v, 20mA) in series with the parallel circuit of the IL604s. So if my calcs are right, the resistor for each of the IL604s (parallel with each other) should be ~50-80ohms, while the resistor for the 3rd LED (in series) should be 100 ohm? \$\endgroup\$ – netarc Oct 20 '14 at 10:00
  • \$\begingroup\$ each 'parallel' chain of LEDs (even if the chain is just one) should have a current limiting resistor. @netarc \$\endgroup\$ – KyranF Oct 20 '14 at 10:01
  • \$\begingroup\$ Treat each LED as a completely separate circuit. Calculate the resistor for that LED as the current just that LED needs with its own forward voltage, so a 3.6V Vf and 20mA would be (9-3.6)/0.02 = 270Ω. A 3.8 Vf and 20mA would be 260mA. Of course there will be little visual difference between 260Ω/270Ω and say 330Ω, which gives you extra protection and is a common value. \$\endgroup\$ – Majenko Oct 20 '14 at 10:09
  • \$\begingroup\$ Hey Majenko, just a side question, what do you do if you have a 3V supply but the LED forward voltage is 2.8V? Does the resistor trick still work? \$\endgroup\$ – KyranF Oct 20 '14 at 10:29

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