Make a LED tester

[star882]

Make your own LED tester.
http://forums.gideontech.com/vps/showphoto.php?photo=576
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9v batt LED socket
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[Empath]

Moved to the Electronics forum

 

[luxO]

wow, nobody here would have thought of that...

 

[Albany Tom]

You probably want to add a 4.7v zener diode and a momentary switch to that. The max reverse voltage of some LED's is 5v.

 

[Streak]

I just hold the LED across a CR2032 battery. More than enough voltage to see if the LED is working and that the polarity is correct.

www.worst4x4.freeservers.com

 

[star882]

"You probably want to add a 4.7v zener diode and a momentary switch to that. The max reverse voltage of some LED's is 5v."
I accidentally put in one of my blue LEDs(the radioshack one Craig says "tends to fail at a drop of a hat") backwards, and it is just as bright as the other radioshack LED that has not been treated in this way.
"I just hold the LED across a CR2032 battery. More than enough voltage to see if the LED is working and that the polarity is correct."
It doesn't work for blue LEDs(I have tried it).

 

[MrAl]

Hello there,

I had a similar experience starr.
I put a red led in backwards across a
potential of 12 volts. To my surprise,
the led didnt get bothered at all.

In general though, i cant recommend this.
The manufacturers specs are clear about the
5 volt limit, so i can only recommend using a
supply of less then 5 volts.

Three 1.5 volt batteries in series work fine,
but if you want the ease of using a 9v type,
use a run down 9v battery.
A 9v battery run down to 5 volts works fine and
wont damage the led no matter how it's connected
or how lucky you are that day :-)

Another idea that works great is to connect
two red led's and a diode in series and use that
for the 'zener'. This way you get an indication
that the battery is charged enough also before
you test your LED.

If you like, you can use three red led's, a
470ohm resistor, and
a 100k resistor all in series, across the battery
terminals. Then, connect the led to test across
the 100k resistor. The three led's in series
limit the voltage that can get to the test led.

Take care,
Al

 

[evan9162]

Or use an LM317 in current-regulating mode (20mA supply) with 4AA batteries as the power supply

1) will never be above the 5V reverse voltage for an LED, since the dropout voltage for the 317 is rather large (>1.2V)
2) You will always get the correct current for any LED.

-Darin

 

[Albany Tom]

Originally posted by evan9162:
Or use an LM317 in current-regulating mode (20mA supply) with 4AA batteries as the power supply

-Darin

<font size="2" face="Verdana, Arial">I built one of these, but used a 9v battery and a 4.7v zener, not trusting what the dropout voltage of the 317 was. It works great, and lets me judge intensity of different LED's, too. I think if I built another I'd use the 2x red LED trick above, though. It's cheaper, and gives a power good indicator at the same time...good trick.

 

[MrAl]

Hello evan,

The LM317 would certainly make a nice regulator
for testing the LED's, but i would be careful
about the assumed dropout voltage too like
Albany was saying.
When an led is connected 'backwards', it draws
negligible current, meaning the LM317 has little
or no current flowing through it. This means it
might not drop any voltage at all.
It would be wise to check the data sheet for
the minimum current output when used in the
regulating mode--i think its something like
10ma or so. Perhaps there is a version
that has less?

Good luck with your LED circuits,
Al