the difference between good/open and short/open in measuring diodes - Page 1
Forum section: electronics / beginners

author: madeeb
author_rank: Newbie
timestamp: December 10, 2020, 11:28:35 am
content: Dear AllWhat is the difference between good/open and short/open in measuring diodes

author: Zero999
author_rank: Super Contributor
timestamp: December 10, 2020, 12:29:15 pm
content: A good silicon diode will measure around 0.6V, when forward biased and be open circuit, when reverse biased. A bad diode will most likely read short circuit, in both directions, but could fail open circuit in both directions.Shotkky diodes have a much lower voltage drop, normally around 0.3V and a higher leakage current. High current, low voltage devices might read a low resistance/voltage in both directions, using a multimeter, because the leakage current is relatively high. A more reliable test is to use a small fillament lamp, say 100mA, 6V and a 6V power supply, or battery. The lamp should only light, when the diode is forward biased.

author: madeeb
author_rank: Newbie
timestamp: December 10, 2020, 12:45:05 pm
content: Thank you for considerationMy question is what is the difference between Good and Short in measuring diodes?regards

author: Renate
author_rank: Super Contributor
timestamp: December 10, 2020, 01:10:49 pm
content: You are using a multimeter to check?You have it on the special "diode" range?You are checking the diode with the leads hooked one direction and then in the other direction?Good means measuring infinitity (open) in one direction and some number (300-700) in the other direction.A zero means shorted.The first problem is you have to know what and how your particular model of meter is measuring.The diode range works by putting a constant current through the device under test.It then measures the voltage (in millivolts) that this current has caused.Unfortunately here is where things get complicated.If the constant current source is exactly 1 mA then a 100 ohm resistor causes 100 mV drop which goes to the display as "100".BUT a lot of meters don't use 1 mA, they could use ANYTHING.But they still want to make the diode range behave like the resistance range.So if I have a meter that uses 2 mA then a 100 ohm resistor causes a 200 mV drop, which gets SCALED to the display as "100".If you connect a normal diode (1N4148) on this that 600 mV drop will get scaled to the display as "300"(I just checked one of my meters, it uses 1.35 mA, no idea why.)The other unknown thing about your meter is what is the open circuit voltage that it generates?This only makes a difference if you want to check some types of LEDs, some white LEDs take more than 3 V.(My meter does 3.0 V open circuit.)So, check your meter with some good diodes so you know how it works.When you know what your meter is doing, you'll know what the readings mean.

author: Zero999
author_rank: Super Contributor
timestamp: December 10, 2020, 01:29:30 pm
content: Quote from: madeeb on December 10, 2020, 12:45:05 pmA short circuit means a very low resistance, typically under an Ohm.Open circuit means a very high resistance, i.e. hundreds of Megaohms.A good diode has a voltage drop of around 0.6V, irrespective of the current, when forward baised (anode to +V and cathode to -V) and a very high resistance when reverse biased.A bad diode will be open circuit, or a low resistance (100 Ohms or so) in both directions.