Author Topic: Diode test and forward voltage drop  (Read 1115 times)

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Offline fixit7Topic starter

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Diode test and forward voltage drop
« on: June 10, 2019, 02:37:09 am »
I am trying to identify this led. It comes from a laser printer.

I did a diode test and 1.43 v is the approx forward voltage drop.

Does that mean the meter is supplying 1.43 volts as part of the test?

I could see no light coming from the led and the camera showed nothing as well, so I think it may be an infrared led.

http://tinypic.com/view.php?pic=29w0bwi&s=9

When i use my phone to make a movie of my tv remote, it shows a purplish light blinking.

The circuit that contains the "IR" led, has some 178 resistors and two 34 ohm resistors in series that go to the IR led.





 

Offline MosherIV

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Re: Diode test and forward voltage drop
« Reply #1 on: June 10, 2019, 06:44:29 am »
Yes, the dmm supplied 1.43V (and more) but the Voltage drop across the diode is 1.43V

DO NOT LOOK at the laser diode. Yes the laser diode will be in the infra red and not visible.
The laser diode will burn out your retina and you will not see or feel anything.
 

Offline fixit7Topic starter

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Re: Diode test and forward voltage drop
« Reply #2 on: June 10, 2019, 02:30:07 pm »
Ok.

I wonder what it was used for?

It had some kind of lens over it.

 

Offline golden_labels

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Re: Diode test and forward voltage drop
« Reply #3 on: June 10, 2019, 03:05:01 pm »
While I fully agree with MosherIV — you should never look at a laser diode to see if it works(1) — that doesn’t look like a laser diode.

It seems to be a normal LED with collimating lens on top of it. The board itself is sold as “power distribution board”, so I expect it is either an indicator or the emitter part of a detector (e.g. of the case being open). Seeing the other side of the board would help.

Also, please consider adding attachments directly to your posts, instead of using 3rd party services.
____
(1) You can see laser only twice in your life. Once with your left and once with your right eye.
« Last Edit: June 10, 2019, 03:07:22 pm by golden_labels »
People imagine AI as T1000. What we got so far is glorified T9.
 

Offline Vtile

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Re: Diode test and forward voltage drop
« Reply #4 on: June 11, 2019, 10:01:19 pm »


Does that mean the meter is supplying 1.43 volts as part of the test?


Actually a bit falsely stated above, the meter is not delivering the voltage/tension for the diode in this type of test. It is rather delivering constant current for the device and measures the tension/voltage generated across the diode. The voltage/tension over measured diode is limited by the sourcing capacity of the meter, but constant current is the input for test and tension/voltage is the response.
 
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Offline rfengg

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Re: Diode test and forward voltage drop
« Reply #5 on: June 12, 2019, 12:29:52 am »
Most Fluke and other multimeters source a test current of 1mA in the diode mode check.
So the voltage you read on the DMM is the voltage subtended across the diode at a test current of 1mA.
 
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Offline ogden

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Re: Diode test and forward voltage drop
« Reply #6 on: June 12, 2019, 12:47:42 am »
I wonder what it was used for?
It had some kind of lens over it.

Could be light source part of paper (position) sensor.
 


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