How to test a semiconductor diode with DMM?

The p-n junction forms a popular semiconductor device called p-n junction diode. The p-n junction has two terminals called electrodes, one each p-region and n-region.

  • p-region as Anode
  • n-region is Cathode.

The total voltage drop across a p-n junction diode forward biased condition is Vf and it is made up of
A. Drop due to barrier potential,
B. Drop due to internal resistance.
The total Vf is of the order of 0.7V for silicon and 0.3v for the germanium.
P-N JUNCTION=RECTIFIER.
The process of converting the alternating voltages and currents to a pulsating DC is called rectification. Thus is achieved using devices, which conduct in only one direction i.e. unidirectional devices like p-n junction diodes and Scr.
Half wave rectifier (HWR) single diode.
Full wave rectifier (FWR) 2 diodes and bridge rectifier 4 diodes.
                                                                   DIODE SYMBOL.
semi-diode-gif

 How to use the Digital multimeter? Digital multimeter selector switch to diode mode, (DMM means Digital MultiMeter)

DMM means Digital multimeter
TESTING WITH DMM – (Diode Mode
  •  Never exceed the protection limit values indicated in specifications for each range of measurement.
  • When the value scale to be measured is unknown beforehand set the range selector at the highest position.
  • When the meter is linked to measurement circuit, do not touch unused terminals.
  • Before rotating the range selector to change functions, disconnect the lest leads from the circuit under test.
  • Never perform resistance measurements on live circuit.
  • Always be careful when working with voltage above 60v DC of 30v Ac RMS. KEEP THE FINGERS BEHIND THE PROBE BARRIERS WHILE MEASURING.
  • BEFORE ATTEMPTING TO INSERT TRANSISTORS FOR TESTING, ALWAYS BE SURE THAT TEST LEADS HAVE BEEN DISCONNECTED FROM ANY MEASUREMENT CIRCUIT.
  • COMPONENTS SHOULD NOT BE CONNECTED TO THE hfe SOCKET WHEN MAKING VOLTAGE MEASUREMENTS WITH TEST LEADS.

IMPORTANT:

  1. If the resistance being measured exceeds the maximum value of the range selected or the input is not connected, an over range indication  will be displayed.
  2. When checking in-circuit resistance, be sure the circuit under test has all power removed and that all capacitors have been discharged fully.
  3. For measuring resistance above 1 Mohms the meter may take a few seconds to get stable reading., this is normal for high resistance measurements.

SELECT DIODE MODE IN THE DIGITAL MULTIMETER.
step-1.
Forward bias:

  • Connect DMM (+) Test Red lead to anode
  • DMM (-) Test Black lead to cathode
  • Verification: If the DMM reading is from  0.478v to 0.668v the condition is  GOOD.

Step-2.
Reverse bias:

  • Connect DMM (+) Test lead to cathode
  • DMM (-) Test lead to anode

Verification: If the DMM reading is OL (or) 1 or open (over load) then condition is  GOOD.
If you get reading in forward bias as 0000 or OL  or 1, and in reverse bias as 0000 (or) low values the diode can  be FAULTY and needs replacement.
IMPORTANT:   (0.478V Means = 478 mV RANGE )

    semi-diode-dmm
BRIDGE RECTIFIER TESTING with Digital multimeter. (DMM)
       How to test Bridge Rectifier diodes with Digital Multimeter? When test with in-circuit, Disconnect from the transformer secondary side to rectifier wire connection before your testing starts and without power supply ON condition.
Step-1.
Connect DMM probe to bridge diode
  • (+) Test Red lead to Ac pin
  • DMM (-) Test Black lead to  Ac pin

Verification: If the DMM reading is OL or ‘1’ or open the condition is GOOD.
Step-2.

Forward bias:
  • (- )Test lead to Ac pin
  • DMM (+) Test lead to Ac pin
  • DMM Reading shows = OL (over load) or ‘1’ or open
  • DMM Negative teat lead bridge diode (-) side
  • DMM positive Lead to Ac sides  = 0.487, 0.439v
  • DMM positive test lead to the bridge diode (+) side
  • DMM negative sides 0.480, 0.489v
Verification:   If the DMM reading is OL (or) 1 or open (over load) then condition is  GOOD.
If you get reading in forward bias as 0000 or OL  or 1, or open and in reverse bias as 0000 (or) low values the bridge diode can be FAULTY and needs replacement.

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