A diode is a semiconductor device that allows electric current to flow in one direction only. It has two terminals: an anode (positive) and a cathode (negative). A diode is said to be forward-biased when the anode is connected to a higher voltage than the cathode, and reverse-biased when the opposite is true. A good diode should have low resistance in forward bias and high resistance in reverse bias.
Testing a diode is a simple and useful way to check its functionality and quality. It can be done with either an analog or a digital multimeter, which are instruments that measure voltage, current, resistance, and other electrical properties. In this article, we will explain how to test a diode with both types of multimeters, using different modes and methods.
What is Diode Test Mode?
Diode test mode is a special function on some digital multimeters that are designed specifically for testing diodes. It applies a small voltage (usually around 2 V) across the diode and measures the voltage drop, which indicates the resistance of the diode. Diode test mode is usually indicated by a diode symbol on the multimeter dial or display.
Diode test mode is often the best and easiest way to test a diode, as it gives a direct and accurate reading of the diode’s condition. However, not all digital multimeters have this function, and some diodes may require higher voltages than what the multimeter can provide. In such cases, other methods can be used to test the diode.
How to Test a Diode with Diode Test Mode
To test a diode with diode test mode, follow these steps:
- Turn off the power source of the circuit that contains the diode. If possible, remove the diode from the circuit for more accurate results.
- Set the multimeter to diode test mode by rotating the dial or pressing the button.
- Connect the positive (red) lead of the multimeter to the anode of the diode, and the negative (black) lead to the cathode. The diode is now forward-biased.
- Read the voltage drop on the multimeter display. A good silicon diode should have a voltage drop between 0.5 V and 0.8 V. A good germanium diode should have a voltage drop between 0.2 V and 0.3 V.
- Reverse the leads of the multimeter, so that the positive lead is on the cathode and the negative lead is on the anode. The diode is now reverse-biased.
- Read the voltage drop on the multimeter display again. A good diode should show OL (overload), which means infinite resistance or no current flow.
If the readings are different from what is expected, then the diode may be faulty or damaged. A low voltage drop in both directions means the diode is shorted (low resistance). A high voltage drop or OL in both directions means the diode is open (high resistance).
How to Test a Diode with Resistance Mode
Resistance mode is another function on digital multimeters that can be used to test diodes. It measures how much current flows through a component when a known voltage is applied. Resistance mode is usually indicated by an ohm symbol (Ω) on the multimeter dial or display.
Resistance mode can be used to test diodes if there is no diode test mode available, or if the diode requires higher voltages than what the multimeter can provide. However, resistance mode may not always give reliable results, as it depends on several factors, such as battery condition, lead resistance, and temperature. Therefore, it should be used with caution and only as a supplementary method.
To test a diode with resistance mode, follow these steps:
- Turn off the power source of the circuit that contains the diode. If possible, remove the diode from the circuit for more accurate results.
- Set the multimeter to resistance mode by rotating the dial or pressing the button. Choose a low range (such as 1 kΩ) for better sensitivity.
- Connect the positive (red) lead of the multimeter to the anode of the diode, and the negative (black) lead to the cathode. The diode is now forward-biased.
- Read the resistance value on the multimeter display. A good diode should have a resistance value between 1 kΩ and 10 MΩ, depending on the type and quality of the diode.
- Reverse the leads of the multimeter, so that the positive lead is on the cathode and the negative lead is on the anode. The diode is now reverse-biased.
- Read the resistance value on the multimeter display again. A good diode should show OL (overload), which means infinite resistance or no current flow.
If the readings are different from what is expected, then the diode may be faulty or damaged. A low resistance value in both directions means the diode is shorted (low resistance). A high resistance value or OL in both directions means the diode is open (high resistance).
How to Test a Diode with an Analog Multimeter
An analog multimeter is an older type of multimeter that uses a needle and a scale to indicate the measurement. It can also be used to test diodes, but it requires more skill and experience to read and interpret the results. An analog multimeter has a selector switch that can be set to different ranges of resistance, voltage, or current.
To test a diode with an analog multimeter, follow these steps:
- Turn off the power source of the circuit that contains the diode. If possible, remove the diode from the circuit for more accurate results.
- Set the selector switch of the analog multimeter to its resistance mode. Choose a low range (such as 1 kΩ) for better sensitivity.
- Connect the negative (black) lead of the multimeter to the anode of the diode, and the positive (red) lead to the cathode. The diode is now forward-biased.
- Read the needle position on the scale of the multimeter. A good diode should have a low resistance value, which means a high needle deflection towards the right side of the scale.
- Reverse the leads of the multimeter, so that the negative lead is on the cathode and the positive lead is on the anode. The diode is now reverse-biased.
- Read the needle position on the scale of the multimeter again. A good diode should have a high resistance value, which means a low needle deflection towards the left side of the scale.
If the readings are different from what is expected, then the diode may be faulty or damaged. A high needle deflection in both directions means the diode is shorted (low resistance). A low needle deflection in both directions means the diode is open (high resistance).
Conclusion
Testing a diode is a simple and useful way to check its functionality and quality. It can be done with either an analog or a digital multimeter, using different modes and methods. The main principle is to measure the resistance or voltage drop across the diode when it is forward-biased and reverse-biased, and compare it with the expected values for a good diode. A good diode should have low resistance in forward bias and high resistance in reverse bias. A faulty or damaged diode may have low or high resistance in both directions or no resistance at all.
