Do you know how to test a diode? This component is widely used as a rectifier and its operating status can be easily determined with the use of a digital multimeter and a circuit board, as will be seen below.
Basic aspects of the diode
The diode is a semiconductor device composed of a junction between a type P crystal and a type N crystal. The type P crystal represents the positive polarity of the diode (anode), while type N represents the negative polarity of the diode (cathode).
When materials of type N and P are joined, the electrons and gaps in the junction region combine, forming the so-called depletion region. There is an absence of carriers close to the junction, and only when a voltage is applied to the diode terminals greater than the potential difference in the depletion region (also called the potential barrier) will it be possible to conduct current. While no voltage is applied to its terminals, there is no charge flow through the diode in either direction.
When the diode is directly polarized, that is, with the source positive connected to the anode and the negative connected to the diode cathode, the electrons of the type N material and the gaps of the type P material are moved to the junction region, overtaking it a if the source voltage is greater than that of the depletion region. Under these conditions, the diode is in a conduction state.
However, when the diode is polarized inversely, that is, with the positive of the source connected to the cathode and the negative connected to the anode of the diode, the electrons of the type N material and the gaps of the type P material are removed from the junction region by attraction of the loads to the source terminals, the depletion layer increases and the diode starts to act as an open circuit, preventing current conduction.
How to test diode with digital multimeter?
The digital multimeter is an essential tool for anyone working in the field of electricity, and there are three ways to use it to test the semiconductor diode.
How to test the resistance of the diode with multimeter?
Even without being connected to a circuit, it is possible to test the diode using a digital multimeter by selecting the resistance scale. Resistance is measured by placing the multimeter’s probes at the diode terminals, and inverting them to obtain resistance in the other direction. After obtaining the two resistances, the results must be analyzed.
- If the measured resistances are low in both directions, the diode is shorted;
- If the measured resistances are high in both directions, the diode is open;
- If the resistance value obtained is relatively low in one direction (forward bias) and high in the other (reverse bias), the diode is functioning normally.
The values of high and low resistances vary depending on the diode and the resistances obtained with the multimeter for this test do not correspond to those found when the diode is in practical operation.
How to use multimeters with specific measurement function?
Some digital multimeters have a diode-specific test function. When this function is selected, the multimeter injects a slightly higher current into the diode than that used in the resistance test, generating a conduction in the device.
The test is done by connecting the diode at specific points on the multimeter, or by connecting the multimeter’s test leads to the diode terminals. The indication of the diode status is given directly on the multimeter’s display, without the need to analyze measured values.
How to test diode on the board?
After being inserted into the circuit, the diode is tested with the digital multimeter’s probes, placing the red tip of the multimeter on the anode and black probe (common terminal) on the cathode to verify direct polarization, inverting them to check reverse bias. According to the values obtained with the multimeter, there are four possibilities to be analyzed:
- If the forward voltage is equal to the potential barrier and the reverse voltage is greater than or equal to the barrier voltage, the diode is operating normally;
- If the forward voltage is greater than the barrier voltage, the diode must be operating as an open circuit;
- If the voltage in both directions is zero, the diode is shorted;
- If the voltage in both directions is greater than that of the potential barrier, the diode is functioning as an open circuit.
Testing of similar devices
The tests previously described are also applicable to devices and circuits that function similar to the semiconductor diode. Some of them will be shown below.
How to test bridge rectifier?
The tests previously described can also be applied to more complex circuits involving diodes, such as a rectifier bridge , which is a circuit used to convert alternating electrical quantities (alternating current and voltage) into continuous. The video below provides a more comprehensive approach to the rectifier bridge
All the tests described above can be used, since the essential components of the circuit are diodes.
How to test transistor with digital multimeter?
Resistance and voltage tests are also applicable to another semiconductor device, such as a transistoR . Because it consists of three semiconductor regions alternating between type P and N, its base-collector and base-emitter junctions can be treated as two diodes connected in series. After obtaining the test measurements, it can be verified whether the transistor operates in the active, saturation or cut-off region using the resistance and voltage tests previously described for the rectifier diode.
How to test the Zener diode?
Another type of diode that can be tested in these ways is the zener diode, which is a component used as a voltage regulator in circuits, where a specific output voltage is required. Using the resistance test it is possible to check the state of the junction, obtaining the same results as for a common diode.
Using a test circuit and a multimeter on the DC voltage scale, it is possible to check the function and the zener voltage of the device. The possible conditions are presented below:
- Obtaining the zener voltage with the multimeter, the zener diode is operating normally;
- If the voltage read is equal to zero, the zener diode is inverted;
- If it is well above the expected zener voltage (based on the datasheet value), the diode is operating as an open circuit;
- If the voltage read is 0.7 V, the diode is being measured in the direct direction, requiring inversion of the multimeter’s probes and taking new measurements.
Finally, an observation must be made about the resistance measurement of the zener diode. It must be done by a multimeter that applies a voltage less than the zener voltage to the zener diode, because when this occurs the voltage in the zener diode becomes constant regardless of the current being injected into it.