How to Test a Diode with a Multimeter (2-Method Guide)

Diodes are vital in almost all modern electronics for different things like switching ac to dc, isolating and mixing signals from the supply, and many other functions. Here we will discuss the process of testing the diode with a multimeter in case you suspect there may be an issue with your diode.

Testing a Diode

A diode is a semiconductor device that has two terminals. It allows the current to flow in one direction only. Diodes can be seen in devices like clippers, clampers, rectifiers, and so on. It is advisable to identify the terminals of the diode before testing it.

Detect the anode (+ve) and cathode (-ve) terminals for the diode. The terminals are distinguished by the color bands.

The color band indicates the cathode (-ve), and the non-color one indicates the anode (+ve).

anode and cathode

You can test a diode with a multimeter in two ways: The diode test mode and resistance test mode. In diode testing mode, you can test the diode with an analog and digital multimeter. The terminals of the diode are connected to the terminals of the multimeter and the resistance is detected either in the form of low or high value. In resistance testing mode, the terminals of the diode are identified and after the identification, the value is detected in ohms.

Diode Test Mode

You can efficiently execute diode testing using both an analog and a digital multimeter.

Diode testing with an analog multimeter

You can test the diode through the following steps:

  • Set the value of the multimeter selector switch to low resistance.
  • Connect the terminals of the diodes to their respective side. The positive terminal of the diode will connect to the positive terminal of the multimeter and the negative diode terminal will connect to the negative terminal of a multimeter.
  • If the meter detects the low resistance value, then the diode is working fine.
  • Transfer the diode to a reverse bias state. You can reverse it by connecting the positive terminal side to the cathode and the negative side to the anode. Don’t forget to put the selector in a high resistance position.
  • The indication of OL or high resistance on the meter indicates the perfect conditioning and working of the diode. The diode is said to be in a defective state if the meter is not able to show such readings or values.

analog multimeter

Diode Testing with a Digital Multimeter

You can test the diode with a digital multimeter through the:

  • Diode testing mode
  • Resistance testing mode

Diode Test Mode

Follow the below-mentioned steps:

  • Identify both sides of diode terminals as cathode and anode.
  • Rotate the central knob towards the indicated place of the diode symbol. In this way, you can keep the digital multimeter to the diode checking mode. The multimeter supplies a current of approximately 2mA in such mode.
  • If the diode is a forward-biased one, you can connect the red probe to the anode side and the black one to the cathode side.
  • Check out the voltage values displayed on the meter. A value falling between 0.6 to 0.7 indicates a healthy and ideal state of the diode if it’s a silicon diode. The value will show in between the range of 0.25 to 0.3, in the case of germanium diodes. (1)
  • You can reverse the condition of the diode by connecting the red probe to the cathode side and the black probe to the anode side. In such a situation, no current flows through the device. The meter will indicate the OL in the case of a healthy diode and it is equal to an open circuit.
  • The meter showing irrelevant values indicates that the diode is defective, maybe open or short. No current flows through an open diode. It’s pretty much like an open switch that allows zero current to flow through the diode.
  • When the voltage drop across the diode is zero, it will indicate the flow of current and is called a shorted diode. In such a case, zero voltage value is indicated on the multimeter.

analog multimeter testing the anode and cathode

Resistance Testing Mode

Resistance testing mode is also helpful to indicate a short, open, or good state. You can implement the following steps to perform resistance testing:

  • The first step is the identification of the terminals of the diode, which are the anode and cathode.
  • If you want to keep your digital multimeter in the ohmmeter mode, you can rotate it to the central knob indicating the ohm symbol or resistor values. Ohmmeter mode is also called the resistance mode. It is advisable to keep the selector in a low resistance mode that is about 1K ohm in case of forwarding bias. In the case of the reverse bias method, keep the selector at high resistance mode around 100K ohm.
  • The diode is indicated as forward-biased when you connect the red probe to the anode and the black probe to the cathode. The resistance of the diode in such a case is small.
  • The diode is in good working condition if the resistance reading stands between a few hundred ohms to a few kilo-ohms. If the resistance reading is in between the hundred to a few kilo-ohms, then the diode is not working properly.
  • You can reverse the terminals of the multimeter by connecting the black probe to the anode and the red probe to the cathode. It will make the diode reverse biased. The diode indicates working functionality if the meter displays show a high resistance value.
  • If the meter displays a moderately low value on the display section i.e., a few tens of ohms, then the diode is not good. But if the resistance reading is a few hundred ohms to a few kilo-ohms, then the diode is working fine. 

Diode Test Analysis

The above discussion shows you the clear path of diode testing using a multimeter. If you want to check whether the diode is in proper working condition or not, then it’s advisable to read the low resistance on the meter in forward-biased conditions.

In reverse-biased condition, read the high resistance.

The diode is an opened one if the meter reading of the resistance is in the high condition against both the forward and reverse-biased conditions.

The diode gets shortened in case of low resistance. This is why both diode and resistance testing are vital while detecting a diode with a multimeter. (2)

Take a look at some of our related articles below.

(2) analysis –

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About Sam Orlovsky

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Education: University Of Denver - Electric Engineering
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Electrical engineering is my passion, and I’ve been in the industry for over 20 years. This gives me a unique ability to give you expert home improvement and DIY recommendations. I’m not only an electrician, but I also like machinery and anything to do with carpentry. One of my career paths started as a general handyman, so I also have a lot of experience with home improvement I love to share.

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