Diode Checking

• The condition of a semiconductor diode can be determined quickly using

  1. a digital display meter (DDM) with a diode checking function
  2. the ohmmeter section of a multimeter
  3. a curve tracer.
• Diode Checking on a Digital Meter:

  • Some digital multimeters have a specific setting for checking diodes. This is usually indicated by a small diode symbol on the dial.
• How to Use the Diode Check Function:

  • When you set the meter to the diode check position and connect the leads to a diode, the meter applies a small, constant current through the diode.
• Forward-Bias Voltage Display:

  • If the diode is good and you connect it in the forward-bias direction (positive lead to the anode and negative lead to the cathode), the meter will show a voltage reading. For a silicon diode, this reading is typically around 0.67 volts. This indicates that the diode is allowing current to flow (it is "on").
• Open Diode Indication (OL):

  • If the meter shows "OL" (overload) when connected in the forward-bias direction, it means there is no current flowing through the diode. This indicates that the diode is open or defective.
• Reverse-Bias Check:

  • If you reverse the leads (positive lead to the cathode and negative lead to the anode), a good diode will not conduct, and the meter should show "OL". This is because a diode blocks current in the reverse direction (it behaves like an open circuit).
• Interpreting the Results:


  f the meter shows "OL" in both forward and reverse directions, the diode is likely defective. It suggests that the diode is open and not allowing current to flow in either direction, which is not normal behavior for a good diode.
• Forward-Bias Resistance: In ohmmeter

  • When you connect the ohmmeter with the positive lead to the anode and the negative lead to the cathode (forward-bias), the diode should allow current to flow. The resistance should be low because diodes have low resistance when they are forward-biased.
• Reverse-Bias Resistance: In ohmmeter

  • When you reverse the leads (positive to the cathode and negative to the anode), the diode should block current. The resistance should be high because diodes have high resistance when they are reverse-biased.
• Ohmmeter Reading:


  The reading on the ohmmeter depends on the current provided by the ohmmeter's internal battery (usually 1.5V). More current results in a lower resistance reading
• • Low Resistance (Forward-Bias): If the diode shows low resistance in the forward-bias direction, it is functioning correctly.
• • High Resistance (Reverse-Bias): If the diode shows high resistance in the reverse-bias direction, it is functioning correctly.
• • High Resistance in Both Directions: If the ohmmeter shows high resistance in both directions, the diode is likely open (defective) and not allowing any current to flow.
• • Low Resistance in Both Directions: If the ohmmeter shows low resistance in both directions, the diode is probably shorted (defective) and allowing current to flow in both directions, which it shouldn't.
• • When testing a diode with an ohmmeter, you expect low resistance in the forward direction and high resistance in the reverse direction.
• Curve Tracer Overview:

  • A curve tracer is a device that can graphically display the electrical characteristics of various electronic components, including diodes. It shows how the current through a device changes with the applied voltage.
• Connecting the Diode: curve tracing

  • To test a diode, you connect it to the test panel of the curve tracer, typically found at the bottom center of the unit. The connections ensure the diode is properly placed for the measurement.
• Adjusting the Controls:

  • The curve tracer has controls that you adjust to set the measurement parameters. These settings will allow you to visualize the diode's behavior on the display.
• Interpreting the Display:

  • The display shows a graph with two axes: the vertical axis represents current (in milliamps, mA), and the horizontal axis represents voltage (in millivolts, mV).
• a curve tracer helps visualize the current-voltage relationship of a diode by displaying a graph. By connecting the diode correctly and adjusting the settings, you can see how the diode behaves under different conditions, such as how much voltage is needed to get a certain current through the diode.