Confuse Between Zener And Normal Signal Diode

Yesterday my student have a LCD Monitor with no power symptom. After scanning through the power section, found the main fuse, power fet, power IC and a current sense resistor blown (0.22ohm). There was a small diode and the outer layer look like a zener diode with one additional brown stripe . The marking on board was “D” which mean a diode. For your information, in some electronic board, zener diode also can be marked as “D”.

Removing the diode and test it with analog meter showed that the diode was shorted but the problem is it is not a hard short circuit. That means when you set your analog meter to X 10 K (I’m using Sunwa Meter) and place the red probe to  cathode and black probe to anode the pointer moved to almost zero. And when you reverse the probe and test it, the pointer showed about few ohm. This characteristic is just like a normal few volts zener diode. If it is a hard short circuit then definitely there will be two same reading!

I need to confirm whether is it really a diode or zener diode. What i did was to use the zener diode tester that i have built as shown from the ERG member website. The moment i place my zener diode tester probe onto the diode (or zener diode), the pointer moved and showing 5.1 volt.

Now i compare the resistance of a good 5.1 volt zener diode with the resistance of the diode (or zener diode) and found that the reading was way out. If the diode is a zener diode then it should show the same resistance. From this test i can confirm that the diode is indeed a signal diode and not a zener diode. After the replacement of the small diode (1N4148) together with the parts that i have mention above the LCD Monitor worked like a charm!

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7 Comments

1. 

   chen

   April 30, 2010 at 11:10 am

   Hi sir,
   what do u mean by hard short circuit?how do i differentiate between a normal circuit and a hard short circuit?

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2. 

   admin

   May 1, 2010 at 10:30 am

   HI Chen,

   Hard short circuit means if you test the connections with analog meter x1 ohm, the meter shows full scale. Sometimes repair tech refer hard short circuit as normal short circuit too. It depends on your preference on which terms you want to use.

   Jestine

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3. 

   admin

   July 1, 2011 at 4:43 am

   HI Muhammad,

   Could you elaborate because i don't get your meaning?

   Jestine

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4. 

   kamalakkannan

   October 6, 2011 at 5:27 pm

   Hi sir ,
   how to check the linear voltage regulator IC like 78xx 0r 79xx by multimerter?

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5. 

   admin

   October 8, 2011 at 1:34 pm

   HI Kamalakanan,

   Place the black probe to center pin and the red probe on pin 3 output. The voltage is depends on what type of part number you use.

   Jestine

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6. 

   Amendar

   January 18, 2014 at 7:39 pm

   Thanks Mr. Jestine,

   Nice hints and experience.

   Can board and nearby components help to find out a diode or zener one.?

   How would be informed about the answer or ... on review pages, if any?

   Thanks

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   • 

     Jestine Yong

     January 18, 2014 at 9:41 pm

     Hi Amender,

     Hard to find out those components just by referring to the board and nearby components. Sorry I don't get your meaning about answer or... on review pages. Could you elaborate?

     Jestine

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