Mastech M-830B Diode Test

The Mastech M-830B is capable of testing a diode and providing the forward bias voltage, also known as the forward voltage drop. The meter uses a test voltage of 3 V, and a test current of 0.8 mA for this measurement.

For open or infinite resistance the display reading is "1" which appears when the diode is reverse biased. When the diode is forward biased, the reading consists of three digits indicating the voltage in millivolts.

Get the Latest Meter

Some of the more expensive meters will measure the diode in both directions without having to change over the test leads. They simply display "good" or "bad" on the LCD read-out. I managed to get one of those very cheaply from an online seller.

Diode Test

The side with the band is the cathode, and therefore, logically, the other side must be the anode.

When the red (positive) test lead connects to the cathode (negative), and the black test lead (negative) connects to the anode (positive), the display reading should be "1" indicating infinite resistance. With reversed probes, where the red test lead (positive) connects to the anode (positive), the reading should be a three-digit figure indicating the forward biased voltage drop.

Different Diodes

When testing a general-purpose diode such as the 1N4148, if the diode displays a '1' in both directions then it is defective and said to be "open". If a diode displays a voltage drop reading in both directions then it is defective and said to be "short". However, there are many different types of diodes, for example, high voltage microwave diodes, which exhibit a high resistance in both directions.

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