The first and foremost thing before start developing or troubleshooting the electronic circuits or projects is that one must require a sound knowledge on basic electronic components and their working if they are placed in energized circuits. The complete knowledge on the way in which components are tested is the key for having good troubleshooting skills of electronic circuits.
Before the assembling of basic components, it is advisable to test the components about their working condition or functioning rather than after assembling, to avoid the condition of getting the undesired outcome. In another case, after successful assembling of the circuits, usually we anticipate desired outcome from the circuit.
But sometimes we get unexpected results. In both of these cases, we need to conduct some testing procedures for main components in the circuit to know their proper working. So, Let us look on how to test diodes.
The diode is a two terminal semiconductor device that allows the current only in one direction. These are found in different applications like rectifiers, clampers, clippers and so on.
When the anode terminal is made positive with respect to cathode, the diode gets forward-biased and the forward-biased diode voltage drop is typically 0.7V for silicon diodes. The testing of this device is made to know its proper working conditions in forward and reverse bias modes.
Before testing of the diode we have to identify the terminals of the diode that is anode and cathode. Most of the PN diodes have the white-band on its body and this white-band side terminal is the cathode. And the remaining one is anode.
Some diodes may have a different color band, but the color band side terminal is 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 that the below testing procedures are only for normal PN diode.
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 and ohmmeter mode.
- Identify the terminals anode and cathode of the diode.
- Keep the digital multimeter (DMM) in diode checking mode by rotating the central knob to the place where the diode symbol is indicated. In this mode multimeter is capable to supply a current of 2mA approximately between the test leads.
- Connect the red probe to the anode and black probe to the cathode. This means diode is forward-biased.
- Observe the reading on meter display. If the displayed voltage value is in between 0.6 to 0.7 (since it is 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 that means 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 (which is equivalent to open circuit) if the diode is healthy.
If the meter shows irrelevant values to the above two conditions, then the diode is defective. The diode defect can be either open or short. Open diode means diode behaves as an open switch in both reverse and forward-biased conditions. So, no current flows through the diode. Therefore, the meter will indicate OL in both reverse and forward-biased conditions.
Shorted diode means diode behaves as a closed switch so the current flows through it and the voltage drop across the diode will be zero. 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.
Similar to the above method, it is also a simple method to check the diode whether it is good, short or open.
- Identify the terminals anode and cathode of the diode.
- 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.
If the meter displays a moderately low value on the meter display, which may be a few tens to few hundred ohms, then the diode is good and healthy.
- 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 a very 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.
This method is similar to the testing of diode using DMM ohmmeter mode.
- 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. Testing procedure not same for all types of diodes. So, now let us see how to test an LED and zener diode.
As discussed above, before testing any diode we must know the polarities. The LED polarities can be identified by the length of the leads. Longer one is anode and the shorter one is the cathode. Also, another method is the surface structure wherein flat surface indicates the cathode and remain one is the anode.
Let us see how to test an LED using a digital multimeter.
- Identify the terminals anode and cathode of the LED.
- Place the multimeter selector in diode mode.
- Connect the probes of the meter to LED such that it is forward-biased.
- If the LED is working properly, then it glows otherwise the LED is defective.
- Reverse-biased testing cannot be possible with LED since it doesn’t work in reverse-biased condition.
As compared to the testing of normal diode, Zener diode testing needs some extra circuitry. Because the Zener diode conducts in reverse-biased condition, only if the applied reverse voltage is more than the Zener breakdown voltage.
- Identify the terminals anode and cathode of the Zener diode and its identification process is similar to the normal PN diode.
- Connect the test circuit as shown in figure.
- Place the multimeter knob in voltage mode.
- Connect the meter probes across the Zener diode as shown in figure.
- Gradually increase the input supply to the diode, and observe the voltage on the meter display. This reading on the meter must be such that as we increase the variable supply, meter output should increase until the breakdown voltage of the diode. And beyond this point meter should show a constant value of voltage irrespective of any increase of the input variable supply. If it so, then Zener diode is healthy otherwise defective.
Suppose, if we apply a 12V to the Zener diode (breakdown voltage is 6V) from the battery through a resistor, then multimeter must show a reading which is approximately equal to the 6V, if the Zener diode is healthy.