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!