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Old 4th May 2009, 01:53 AM   #1
Spax is offline Spax  United Kingdom
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Default Component Identification

Being curious (others say distructive) I took apart an LED bulb I had fitted to my car. There was the expected resistor soldered on the anode but on the cathode was this little chap;

Click the image to open in full size.

The markings on it seem to be
C18
ST

At a guess I'd say it's a capacitor but I'm not too familiar with stuff like that.

Anyone have an idea what it's for and what it does?

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Old 4th May 2009, 02:46 AM   #2
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that's a diode in series. if it's connected to the cathode, then it's forward biased, just provides a 0.7V drop. it's probably there to keep inductive spikes from the starter or other motors from burning the LED out.
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Old 4th May 2009, 03:17 AM   #3
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Quote:
Originally posted by unclejed613
that's a diode in series.
A 1watt zener diode, to be precise.
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Old 4th May 2009, 12:35 PM   #4
Spax is offline Spax  United Kingdom
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Thanks for the swift replies, I suspected it may be something to protect it from surges or incorrect connection.

It also explains why a few LED's I've made myself for the same application without a diode have failed, I guess they got spiked.

I've googled 'zener diode' and now understand that they are used like a valve, allowing a certain amount of voltage through but no more untill voltages reach a much higher level.

Is there a key to decoding the figures on the diode or are they just to indicate batch numbers?

How do you go about specifying the capacitor?
e.g.
Code:
An LED used for instrument illumination;
LED  
   Fv 1.8--2.4v  
   Fc 20m/a    
Resistor  
   680 Ohm 1/2W    
Capacitor?  
   Would it be a 12v diode or one which maches the LED's forward voltage?

Thanks for your time and patience
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Old 4th May 2009, 10:49 PM   #5
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it wouldn't be a zener, since it's in series with the LED, and oriented the same way. LEDs usually don't have a very high reverse breakdown voltage, and reverse current damages LEDs, so a switching diode with a 75 to 100V reverse breakdown voltage is enough to protect the LED. a zener here would defeat the purpose. some LEDS will be destroyed by as little as 7-10V reverse voltage, and inductive spikes are usually much higher than that in a car. that's why car-fi equipment have diodes across the power terminals. which brings up an alternative way to protect the LED, with the reverse biased switching diode across the LED terminals. if the diode shorts for any reason, the LED will still never see any reverse voltage (of course, it won't work until you replace the diode).
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Old 6th May 2009, 04:53 AM   #6
jitter is offline jitter  Netherlands
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Quote:
Originally posted by unclejed613
it wouldn't be a zener{snip}.
And yet the picture does show a 18 V zener diode.

These little diodes, that look a lot like the ubiquitous 1N4148, are zener diodes that are very likely from the BZX55C-series. I see them at work about every day...
Most of the time they are stamped C+value only (e.g. "C2V4" for a 2.4 V zener or "C10" for a 10 V zener, etc.).
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