Testing Diodes and Transistors Using a DMM in Diode Mode

Mechatronics IN4 Instructions Sample

Testing Diodes and Transistors Using a DMM

Safety considerations: • If diode is installed in a machine, follow relevant de-energizing procedures specific to that machine. • These instructions assume diode has first been removed from machine.

A DMM stands for digital Multimeter, it’s also called a VOM which stands for volt-ohm-milliammeter.

Sometimes a DMM is simply referred to by its brand name, such as the Fluke DMM because its capability to measure two or more electrical values distinguishes it from generic DMMs.

In Diode Mode Make sure the leads are connected to the correct ports on DMM • Set DMM to the diode mode setting Diode Symbol

* Red should be connected to the Ω / V port.

* Black should be connected to the COM port. http://www.electronicshub.org

Mechatronics IN4 Instructions Sample

• Figure out which side of diode is the anode and cathode by putting a probe on each side of diode. o The side that reads OL with red probe is the cathode. o The side that reads resistance with red probe is the One easy way to identify the anode. anode and cathode of diode is,

look for a white or colored line

which indicates the cathode ● Connect red probe to the anode end and black probe to side. the cathode end (Forward-Biased). o The meter should around .7 V for silicon diodes. o For germanium diodes around .3 V.

● Reverse the terminals (Reverse-Biased) and the meter should read OL.

If the meter reads 0 V in both directions the diode is shorted. In some cases, a shorted diode will have a very small voltage drop across it.

If diode is open it will read an OL in both directions.

NOTE: sometimes OL shows up as a 1 on the DMM. Mechatronics IN4 Instructions Sample

Using the DMM in Ohm Mode How to Test a Diode using a DMM Ohm Mode:

● Rotate selector of DMM to the ohm mode (Ω).

● Where resistor value is indicated put the selector in low resistance mode (May be 1k or 2k).

● Test diode forward biased. It should read very low resistance which means the diode is good.

● Test diode reverse biased in high resistance mode (100k ohm). The meter should read OL.

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If the meter reads OL in both directions it means the diode is open and bad. .

. If it reads 0 in both directions this means it is shorted and bad

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How can diodes fail? A diode can either become open or shorted. A common cause for diode failure is from heat. A poor soldering connection can contribute to diode failure. Mechatronics IN4 Instructions Sample

Diode Reference Data Rectifier diodes 1N4001-1N4007 1N4001 1N4002 1N4003 1N4004 1N4005 1N4006 1N4007 1.1 Vf PIV 50 100 200 400 600 800 1000 1.0 Io

(Vf) = Forward voltage:The minimum necessary voltage to pass a sustained current through diode. PIV = Peak-inverse-voltage: The maximum reverse-bias voltage that a diode can withstand. (Io) = Maximum forward current: The maximum average forward current, limited by how much heat the PN juction can handle.

Testing Bipolar Transistors • There two types of transistors PNP and NPN. • A bipolar transistor is basically two diodes and a base in the center.

Visual Reference of Both PNP and NPN Types

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Mechatronics IN4 Instructions Sample

On DMM the leads should be connected to the ohm/V and COM setting. The selector should be in the low resistance ohm (Ω) setting.

● Test with red/positive lead to base and black/negative lead to the emitter. A good NPN will read a low resistance and a good PNP will read OL. Also test to the collector from base which should read the same except there should be slightly less resistance.

● Now perform the same test with the meter leads reversed (black on base, red on emitter/collector) A good PNP will show some resistance and a good NPN will show OL.

● Now test from emitter to collector and vise versa. These should read OL on a NPN or PNP.

Transistor case style for 2N3904 & 2N3906. http://www.electronicshub.org

Transistor case style for 2N2222

Bottom view

Transistor Reference Data

● What is a FET?

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● A field effect transistor. FETs are widely used in circuits with discrete components. ● similar to a BJT in a field effect transistor there are 3 connections. Drain, source and gate. However current flows between drain and Source. The gate controls current using an electrical field. Mechatronics IN4 Instructions Sample

● What is a MOSFET?

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● Metal Oxide Silicon FET (MOSFET) ● A type of FET, the gate of a MOSFET is insulated by a thin layer of oxide. ● MOSFETs can consume very low levels of current and as a result are widely used in microprocessors, logic integrated circuits as well as switches and amplifiers.