How to Test a Diode Using Digital and Analog Multimeters? . Diodes are one of the basic and important components in electronic circuits that are used for protection, rectification, switching, and many other applications. They are one of the first components to be damaged in case of a fault and hence, it is necessary to know how to check whether a diode is properly working or not.
What is a Diode ?
A diode is a semiconductor device, typically made of silicon, that essentially acts as a one-way switch for current. It allows current to flow easily in one direction but severely restricts current from flowing in the opposite direction.
Diodes are also known as rectifiers because they change alternating current (AC) into pulsating direct current (DC). Diodes are rated according to their type, voltage, and current capacity.
Diodes are available in various configurations. From left: metal case, stud mount, plastic case with band, plastic case with chamfer, glass case.
Read More : Difference Between Schottky Diode and Shockley Diode
Troubleshooting Electronic Devices and Components
Troubleshooting in electronic and electrical engineering is an important part and one must know the basic skills and knowledge about the components to design and troubleshoot a circuit. It is advisable to test component before assembling and placing in a circuit.
Sometimes, we get unexpected results and we must perform some tests to find whether the component and device is working properly or we should replaced with a new one. For this purpose, we have started some Digital and Analog multimeters tutorials where we will discus how to test different electrical and electronics components. Today, we will have to discuss how to test a diode using DMM and AMM .
How to Test a Diode?
The diode is a two terminal semiconductor device that allows current to flow only in one direction. These are found in different applications like rectifiers, clampers, clippers and so on.
When the anode terminal of the diode is made positive with respect to cathode, the diode is said to be forward-biased. The forward-biased diode voltage drop is typically 0.7V for Silicon diodes. This is the minimum potential difference between Anode and Cathode of the Diode to become forward biased.
Before testing a diode, we have to first identify the terminals of the diode i.e., its Anode and Cathode. Most of the PN Junction diodes have a white band on its body and the terminal near this white band is the cathode. And the remaining one is anode. Both through-hole and surface mount Diodes have this marking.
Read More : What is the Difference between Diode and Transistor?
Some diodes may have a different color band (for example, some Zener Diodes have a Black marking on its Red / Orange body), but the terminal near this colored mark is almost always the cathode.
The testing of a diode can be carried in different ways, however here we have given some basic testing procedures of the diode.
NOTE: The below mentioned testing procedures are only for normal PN diode.
NOTE: If the diode you want to test is already in a circuit (on the PCB), then you can perform the following mentioned tests by removing / de-soldering only one lead of the diode.
Tools Needed to Test a Diode
Before starting, gather the following tools:
- Digital Multimeter (DMM): A modern, precise tool with a dedicated diode testing mode.
- Analog Multimeter (AMM): Useful for basic testing with a needle display.
- Additional Tools: Test leads, safety gloves, and a clean workbench.
How to Test a Diode using a Digital Multimeter?
The diode testing using a Digital Multimeter (DMM) can be carried in two ways because there are two modes available in DMM to check the diode. These modes are:
- Diode Mode
- Ohmmeter Mode (or Resistance Mode)
The Diode Test Mode is the best way to test a diode as it relies on the characteristics of the Diode. In this method, the diode is put in forward bias and the voltage drop across the diode is measured, using a Multimeter. A normally working diode will allow current to flow in forward bias and must have voltage drop.
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In the Resistance Mode Test of the diode, both the forward and reverse bias resistances of the diode are measured. For a good diode, the forward bias resistance should be few hundreds of Ohms to few Kilo Ohms and the reverse bias resistance should be very high (usually indicated as OL – open loop in a multimeter).
Diode Mode Testing Procedure
- Identify the anode and cathode terminals of the diode.
- Keep the Digital Multimeter (DMM) in diode checking mode by rotating the central knob to the position where the diode symbol is indicated. In this mode, the multimeter is capable to supply a current of approximately 2mA between the test leads.
- Connect the red probe of the multimeter to the anode and black probe to the cathode. This means the diode is forward-biased.
- Observe the reading on multimeter’s display. If the displayed voltage value is in between 0.6 to 0.7 (for a Silicon Diode), then the diode is healthy and perfect. For Germanium Diodes, this value is in between 0.25 to 0.3.
- Now, reverse the terminals of the meter i.e., connect the red probe to cathode and black to anode. This is the reverse biased condition of the diode where no current flows through it. Hence, the meter should read OL or 1 (which is equivalent to open circuit) if the diode is healthy.
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If the meter shows irrelevant values to the above two conditions, then the diode is defective. The defect in the diode can be either open or short.
Open diode means the diode behaves as an open switch in both reverse and forward biased conditions. So, no current flows through the diode in either bias condition. Therefore, the meter will indicate OL (or 1) in both reverse and forward-biased conditions.
Shorted diode means diode behaves as a closed switch, so the current flows through it irrespective of the bias and the voltage drop across the diode will be between 0V to 0.4V. Therefore, the multimeter will indicate zero voltage value, but in some cases it will display a very little voltage as the voltage drop across the diode.
Ohmmeter (Resistance) Mode Testing Procedure
Similar to the Diode Test method, the Resistance Mode is also a simple method to check the diode whether it is good, short or open.
- Identify the terminals of the diode i.e., anode and cathode.
- Keep the Digital multimeter (DMM) in resistance or ohmmeter mode by rotating the central knob or selector to the place where ohm symbol or resistor values are indicated. Keep the selector in low resistance (may be 1K ohm) mode for forward-bias and keep it in high resistance mode (100K ohm) for the reverse bias testing procedure.
- Connect the red probe to the anode and black probe to the cathode. This means diode is forward-biased. When the diode is forward-biased, the resistance of the diode is so small.
Read More : Why Does a Diode Allow Current in Only One Direction?
If the meter displays a moderately low value on the meter display i.e., a few tens of ohms, then the diode is not good. But if the resistance reading is few hundred ohms to few kilo ohms, then the diode is good and working properly.
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- Now reverse the terminals of the multimeter such that anode is connected to black probe and cathode to red probe. So the diode is reverse biased.
- If the meter shows a very high resistance value or OL on meter display, then the diode is good and functions properly. Since in reverse biased condition diode offers a very high resistance.
From the above it is clear that for proper working of the diode, DMM should read some low resistance in the forward-biased condition and a very high resistance or OL in reverse-biased condition.
If the meter indicates a very high resistance or OL in both forward and reverse-biased conditions, then the diode is said to be opened. In other hand, if the meter reads a very low resistance in both directions, then the diode is said to be shorted.
How to Test a Diode using Analog Multimeter?
Most analog multimeters usually do not have a dedicated Diode Test Mode. So, we will be using the Resistance Mode in Analog Multimeter, which is similar to the testing of diode using DMM ohmmeter mode.
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- Keep the multimeter selector switch in low resistance value
- Connect the diode in the forward-biased condition by connecting the positive terminal to anode and negative to the cathode.
- If the meter indicates a low resistance value, then it says that the diode is healthy.
- Now put the selector in high resistance position and reverse the terminals of the meter by connecting positive to the cathode and negative to anode. In this case, the diode is said to be in reverse bias.
- If the meter indicates OL or a very high resistance, then it refers to the perfect condition of the diode.
- If the meter fails to show above readings, then the diode is said to be defective or bad.
This is about simple PN diode-testing using digital and analog multimeters. These testing procedure may not be applicable for all types of diodes. So, now let us see how to test an LED and a Zener diode.
How to Test LED (Light Emitting Diode)?
As discussed above, before testing any diode we must know its pins (terminals). The terminals of the LED can be identified by the length of the leads. Longer one is anode and the shorter one is the cathode. Also, another method is using the surface structure wherein a flat surface indicates the cathode and other one is the anode.
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To test an LED with digital or analog multimeter, follow the instructions given below.
- Disconnect the LED from the circuit and power supply if it is already connected in a circuit.
- Find the LED terminal i.e. anode and cathode (as shown in above fig)
- In case of digital multimeter, set the meter on “Diode Test” Mode (in case of analog multimeter, set the multimeter on Resistance or continuity mode) by turning the rotary switch of multimeter.
- Connect the LED in forward biased with multimeter test leads i.e. cathode to black (-ve) and anode to Red (+ve) test leads.
- If the LED glows, it doesn’t need to say it is in good condition and working properly, otherwise LED is defective and it should be changed.
- In reverse biased (LED anode to Black (-ve) and cathode to Red (+ve) test leads), it won’t work and multimeter wont show any reading as LED will not flow current through it i.e. it acts like an open switch same as diode.
How to Test a Zener Diode?
When compared to the testing a normal diode, testing a Zener diode needs some extra circuitry. Because, the Zener diode conducts in reverse-biased condition and only if the applied reverse voltage is more than the Zener breakdown voltage.
Read More : What Is the Difference Between Analog and Digital Multimeter?
To check a zener diode with digital or analog multimeter, follow the instructions below.
- Disconnect the zener diode from the circuit and power supply if it is already connected in a circuit.
- Find the zener diode terminals i.e. anode and cathode as it is same as normal LED and PN junction diodes (as shown in above fig)
- Connect the zener diode across a variable or known (such as 12V DC) supply voltage source in series of 100Ω resistance and then connect the zener diode reverse bias (cathode to Red (+ve) and anode to Black (-ve) test leads of multimeter as shown in fig.
- In both of digital or analog multimeter, set the meter on “DC Voltage” test mode by turning the rotary switch of multimeter.
- Increase the supply voltage gradually to the zener diode and note the meter reading displayed on screen. The meter reading should increase until the breakdown voltage of zener diode (in case of 12VDC supply voltage, the breakdown voltage is 6V) when you increase the supply voltage step by step from low to high. After this, meter should not show additional value i.e. it must shows a constant value (Such as 6V in case of 12VDC supply voltage). When meter stop at specific value and not showing another value when you still increasing the supply voltage, you should not continue to increase the supply voltage otherwise, diode may destroy.
- If it is the case, the zener diode is in good condition otherwise, the zener diode is faulty and need to be changed.
Common Issues and Troubleshooting Tips
Testing diodes may present challenges. Here’s how to address them:
- Multimeter Errors: If results seem inconsistent, check the multimeter’s battery and connections.
- Circuit Testing: Always test diodes outside the circuit to avoid interference from other components.
- Damaged Leads: Inspect test leads for damage that could affect readings.
Read More : Analog to Digital Converters (ADCs)
Applications of Diode Testing
Understanding how to test diodes is valuable for:
- Circuit Repair: Diagnosing and replacing faulty diodes in power supplies and amplifiers.
- Educational Purposes: Gaining insights into electronic components’ behavior.
- Preventive Maintenance: Ensuring components are functional before installation.
FAQs About Testing Diodes
1. What voltage drop should I expect from a good diode?
For silicon diodes, expect a forward voltage drop between 0.6V to 0.7V, while germanium diodes typically show 0.2V to 0.3V.
2. Can I test a diode while it’s in a circuit?
It’s best to test diodes outside the circuit to avoid incorrect readings due to parallel components.
3. What does “OL” mean on a digital multimeter?
“OL” stands for “Open Loop” and indicates infinite resistance, which is expected in reverse bias testing.
4. How do I test a Zener diode?
To test a Zener diode, you’ll need a power supply and resistor to simulate reverse bias conditions and measure its breakdown voltage.
5. What happens if a diode shows low resistance in both directions?
Low resistance in both directions indicates a shorted diode that needs replacement.
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