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How to test a diode using a digital multimeter

  • 1.0 What is a diode
  • 1.1 Diode Symbol
  • 2.0 How to test a diode
  • 2.1 Diode Mode Testing
  • 2.2 Diode test analysis
  • 2.3 Resistance Mode Testing
  • 2.4 How to test a LED (Light Emitting Diode)
  • 2.5 How to Test a Diode using Analog Multimeter?
  • 3.0 Diode Short or Open

In this tutorial, we will explain all the necessary information about diode. If you want to know about the diode, its symbols and how to test diode with DMM in detail, then read this post carefully till the end.

1.0 What is a Diode

A diode is a two-terminal semiconductor device that enables just one direction of current flow. These may be found in a variety of applications such as rectifiers clippers, and clampers etc.
Because they convert alternating current (ac) to pulsing direct current (dc), diodes are also termed as rectifiers (dc). 

1.1 Diode Symbol

Anode (positive wire) and cathode (negative wire) establish the polarity of a diode. When positive voltage is given to the anode, most diodes enable current to flow.
A forward-biased diode permits electricity to pass through it. When a diode is reverse-biased, it does not permits the current to pass through it.

The arrow in the diode symbol points in the opposite direction of electron flow.  Engineers created the sign, and their schematics depict current which flows from the positive (+) to the negative (-) end of the voltage source (-). The arrow points in the authorized path of "conventional" passage, and against the allowed direction of electron flow.

2.0 How to test a diode 

Diode testing using a digital multimeter (DMM) may be done in two ways since the DMM has two modes for checking diodes: diode mode and resistance mode. Diode mode is the best practice. If a multimeter does not have a Diode Test mode, ohm meter mode is usually used.
Note: In order to test the diode, it may be required to remove one part of the diode from the circuit.

2.1 Diode Mode Testing 

The voltage drop across a diode while it is forward-biased is the best way to test it. A diode with a forward bias works as a closed switch, allowing current to pass. The Diode Test mode on a multimetergenerates a tiny voltage between test leads. When the test wires are linked across a diode when forward-biased, the multimeter displays the voltage drop. The Diode Test is carried out in the following manner:
●Ascertain that a) all voltage to the circuit is turned off and b) the diode has no voltage. Due to charged capacitors, power may be present in the circuit. So, we had to discharge the capacitor first to carry on. As needed, set the multimeter to record ac or dc voltage.
●Set the pointer to the diode measuring setting by pressing the function button.
●Connect the diode to the test leads. And record the reading appearing on the screen of the multimeter.
●Rep the test leads in the opposite direction. And record the reading appearing on the screen of the multimeter.

2.2 Diode test analysis

●For the most widely used silicon diodes, a suitable forward-based diode has a voltage drop of 0.5 to 0.8 volts. The voltage loss on certain germanium diodes is between 0.2 and 0.3 V.
●When a good diode is reverse-biased, the multimeter reads OL. The diode's OL value indicates that it is operating as an open switch.
●Current cannot flow in either direction via a faulty (opened) diode.
●In both directions, a shorted diode has almost the same potential difference loss measurement (about 0.4 V).

If a multimeter does not have a Diode Test mode, a multimeter set to Resistance mode (Ω) can be used as an extra diode test. 

2.3 Resistance Mode Testing

When testing diodes, there are a few things to keep in mind concerning the Resistance mode:
It is not always possible to tell if a diode is excellent or bad by looking at it. When a diode is connected in a circuit, this test should be avoided since it might result in a misleading readout. After a Diode Test reveals that a diode is defective in a given application, it can be used to validate that the diode is bad in that application.
Following are the important steps:
●Make sure that a) all power to the circuit is turned off and b) the diode has no voltage. Due to charged capacitors, voltage may be maintained in the circuit. So, we had to discharge the capacitor first to carry on. As needed, set the multimeter to measure ac or dc voltage.
●Set the pointer to the Resistance setting (Ω) by pressing the function button.
●After the diode has been removed from the circuit, connect the test leads to it. And record the reading appearing on the screen of the multimeter. Then connect the test leads in the opposite direction. And record the reading appearing on the screen of the multimeter. When testing diodes using the Resistance mode, compare the values to a known good diode for the best results.

2.4 How to test LED (Light Emitting Diode)

Before we can test a diode, we must first identify the anode and cathode terminals. Anode (+) is the longer terminal of a diode, whereas cathode (-) is the shorter terminal (-). In other circumstances, the cathode is the flat terminal of the diode, while the anode is on the opposite side.
Follow the procedures below to test an LED using a digital multimeter.
●If the LED is already attached to a circuit, disconnect it from the circuit and power supply. ●Locate the LED terminals (anode and cathode)
●By pressing the function button of the multimeter, set the meter to "Diode Test" Mode.
●Connect the LED to the multimeter test leads in forward bias, i.e., cathode to black (-ve) and anode to red (+ve).
●If the LED illuminates, it is in excellent condition and functioning correctly; otherwise, the LED is faulty and should be replaced.
●It will not operate and the multimeter will not display any readings if the LED is reverse biased (anode to Black (-ve) and cathode to Red (+ve) test leads). This is because the LED will not flow current and will act like an open switch, similar to a diode.

2.5 How to Test a Diode using Analog Multimeter

The majority of analogue multimeters lack a specialized Diode Test Mode. As a result, we'll use the Analog Multimeter's Resistance Mode, which is analogous to utilizing the DMM's ohmmeter mode to test diodes.
●Maintain a low resistance value on the multimeter selector switch.
●Connect the positive terminal to the anode and the negative terminal to the cathode to make the diode forward-biased.
●If the meter displays a low resistance value, the diode is considered healthy.
●Reverse the terminals of the meter by linking positive to the cathode and negative to the anode, and set the selector to high resistance. The diode is said to be in reverse bias in this instance.
●When the meter reads OL or a very high resistance, it means the diode is in great working order.
●The diode is deemed to be damaged or bad if the meter does not reflect the aforesaid readings.

3.0 Diode Short or Open

There are two types of diode defects: open and short. In both backward and forward biased states, an open diode functions as an open switch. As a result, no current passes through the diode. As a result, under both backward and forward-biased settings, the meter will display OL.
When a diode is shorted, it acts as a closed switch, allowing current to pass through it when the voltage decreases across the diode is zero. As a result, the multimeter will show zero voltage, but in certain situations, the potential difference across the diode will show a very small voltage.

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