Electronics > Beginners

LED Blows Up - Simple Circuit FAIL!

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EccentricRuss:
n00b to the forum, let's get that detail out of the way.  My name is EccentricRuss and I am an electroholic...

So the wife were baby stuff shopping (one on the way) and we bought a consumer electronics product that came with a wall wart, but charged $30.00 for the car lighter adapter accessory.  No way I was going to pay $30.00 for a simple circuit like that.  So, I decided to build it and it was all fairly straight forward until I decided I wanted a pretty blue LED to show that there was power at the lighter plug.

The original design was straight off the datasheet.  I chose a Rohm LM7809, placed a 0.33uF cap before the input pin and a 0.1 uF cap on the out pin which provided me with the exact voltage and current needed, 9v and 1A.  Under breadboard design review and testing, I received consistent and anticipated results.  I found a sacrificial cigarette lighter plug attachment at Ollie's (a discount clearance item store), gutted it and modded my board to fit in the enclosure. For under $5.00 in parts I had my adapter...and then I had to screw with it.

I saw on one of Dave's blogs (live blog probably) that you could soak up some voltage with an LED.  Since my enclosure was limited on room I wanted to avoid a heat sink.  Under no load, the LM7809 is cool as the other side of the pillow, but under load it warmed up.  So I wanted to suck up some of the front end voltage and provide just enough to allow the LM7809 to function and get my 9v.  In hindsight, I should have considered a voltage divider, but instead chose to place the LED in series and see what happened.  I could hear Dave Jones's voice in my head, "I hope your next...project...fails!"  But hell with it, what was the worst that could happen.

It worked, at least for a while.  In reality the cigarette lighters put out over 13V and at that voltage the blue LED looked "purty" and I have 9V output.  The next part is fuzzy, and now that I have laid out the background I need help figuring out what may have gone wrong in the order of likelihood.  See there was a cat...my cat and she has an incontinence issue.  Or she just hates me.  So I heard her scratching and I got up to see what she was doing.  When I came back, the smoke had been let out.  So the test leads were off the table and in my memory I remember the sound of two alligator leads touching.  The multimeter read "0.00" and the LED was no longer illuminated.  My heart sank and all I could think about was that I fried the whole thing, but especially my Rohm LM7809 which was a Digi-Key purchase (and with their shipping rates you have to order a butt-ton of stuff $$$ before you come out ahead).  After desolder, the damaged component ended up being the LED; I bypassed it in circuit and the board came back alive and 9V, 1A output.

I hypothesize that short circuit is not only the obvious choice but also the only reason with no other underlying cause. However after looking for a datasheet in vain, I found that this LED burns up often under normal usage.  Let me dissect that last sentence, I mean schmucks like me put in a circuit and overdrive it until it fails and then we blame the store where we got it and chalk it up to poor product design.  I bought the LED at Radio Shack.  Please spare me the lecture, its the only show in town and they have improved upon their selection of brands in stocking their parts drawers.  Don't talk to the employees, they know cell phones and batteries only.  The stock number is 276-0316.  5mm Blue LED. 3.7V 20mA 2600mcd.  So after the blow up, I read the back of the package which was all I had since Radio Shack won't put up datasheets!!!  Here are the details that put my hypothesis in question: "Foward [sic] (supply) voltage 3.7 typ. 4.5 max", "Foward [sic] (supply) current 20mA typ.  30mA max."  I had "ass-u-me"d that since they misspelled forward, that they intended forward voltage drop, but I may just be stupid.

So looking at it, was the short circuit alone the reason for failure? did I exceed the limits of the LED by subjecting it to higher voltage? should i place a fuse or another type of diode in the circuit to limit damage to components?  its a simple circuit, but the voltage regulator and blue LED are the expensive elements.  In my defense, the LED was powered at 13.8V for 20 minutes before I shorted it, so I still have a modicum of belief that without the injustice of a short circuit, it would have survived.

I have attached a DaveCad of the circuit.  The DaveCad has been hacked off its native OS of PostIt and ported to BackOfEnvelope.  At least I hope the file stays attached.  first post on this forum, you know...

cheers,
Russ

jahonen:
The problem is that the led would have to last 1 A current when the output is fully loaded. Input and output current of the linear regulator are just about the same, the input current is a bit higher due to regulator internal quiescent current. So 20 mA spec will be exceeded by quite a bit when you load the output, so it is not a wonder if the led burns out.

Furthermore, the voltage drop might be too much if you want the regulator work with 12 V input. So just put the LED in parallel to the input, with an appropriate series resistor and it will work just fine :)

Regards,
Janne

Mechatrommer:
its not how much voltage across it, but how much current.

EccentricRuss:
Ok, so I have learned something about forward voltage and forward current.  I really wish I had been able to locate a datasheet.  The voltage regulator drop out is moot; though originally intended for 12V, both cars this adapter were used in put out between 13.7 and 13.8 volts.

Jahonen:  so following your explanation, if the LED will die after allowing the flow of 1A through a 20mA pipe, is the difference in voltage not a limiting factor as well?  i understand that voltage and current are separate realms, but if the max voltage is expressed as 4.5v, does not the potential difference ~9V excess voltage across the diode negatively impact or break it down?

I will put the LED in parallel though.  When I bought it, it was more wishful thinking (absorb extra voltage and save on the heat sink space) than logical thought.

Thank you Jahonen!

Cheers,
Russ

EccentricRuss:
Mechatrommer, I did not think the voltage was an issue either.  but why then does the manufacturer have a max voltage?  I think i have read into Jahonen's response and I should go back to the bible (art of electronics) and read up on diodes and focuse on LEDs.  Although at this point, I may need to revisit Ch 1 to get my head straight on V vs A.

thanks a bunch though.

cheers!

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