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How to Test A Circuit Board?

Simon / 2021-01-18

Contents [hide]

Why We Need to Test a Circuit Board

Electronic and technological devices contain circuit boards, and this makes them extremely valuable. From mobile phones to complex machines, circuit boards play integral roles in the functioning of such devices. In case of errors or malfunction issues, the circuit board will not function as it should. That is where testing comes in.

Manufacturers highlight that the testing stage of a circuit board's manufacturing is the most important. It is vital to test it because there could be errors that may have been unnoticed during the production phase. These errors have the capacity to cause defects in future. They can be really annoying and are a source of customer frustrations. Therefore, it is imperative for the right testing procedures to be put in place so that a circuit board and its components function at their best.

Circuit board design companies ensure that they execute thorough testing procedures during the production stage so that they identify errors early enough and work on them accordingly. It is these testing procedures that make sure that the final product is of the highest quality possible. If a circuit board is tested well, it will not end up being a waste for developing defects later on. Again, manufacturing companies know that bad circuit boards can injure their customers and end up ruining their names. Once the name of a company gets tainted because of producing faulty products, they may end up losing their competitive edge in their market space. It's for this reason that testing should be a priority.

Of great importance still, testing a circuit board is important to boost a product's chance of remaining in a good working condition for a long time. Every product owner wants to have products that will last for a long time. In the same vein, product designers and manufacturers have to make sure that their devices stay in a good condition for a long time. Hence, testing a circuit board is a good way of enhancing its long life span. Essentially, with proper testing, it's possible to detect any potential errors and defects. Once you discover any errors, you're able to take the right measures of correction and improve the longevity of a device. In the long run, this saves costs because there won't be a need to start repairing it when it malfunctions.

How to Test a Relay on a Circuit Board

You should test a relay to ascertain whether it is in good condition or not so that you can make an informed decision which will not compromise the circuit board. So how do you test a relay on a circuit board? Well, the easiest way to test it would be to measure resistance values of the relay. Once you find the resistance values, you will get to know whether a relay is in a good condition or not.

Testing the coil terminals of the relay

In order to ascertain the relay coil's resistance, place the multimeter in the ohmmeter setting and position the probe leads of the multimeter on the two terminals of the relay's coil. If the value you read is almost the same as the rated resistance, the coil should be able to function well. On the other hand, if you happen to read a very low and very high resistance, the coil is not in a good working condition and you should replace it as soon as you can.

Apart from testing the coil terminals of the relay, you can also ascertain whether the other terminals are in good condition as well. These other terminals include: the Normally Open Terminal, the COM Terminal, and the Normally Closed Terminal. For you to test these contacts in the best way possible, you should measure the resistance readings between them.

How to Test Diodes on a Circuit Board

Digital multimeters are used to test diodes. There are two methods you can use to test diodes. The methods include: diode test mode and resistance mode. Diode test mode is known to be the best approach for testing diodes. For the Resistance Mode, it is usually used if a multimeter does not have a Diode Test Mode.

An important point to note here is that you may be required to take away one end of the diode from the circuit for you to test the diode.

There are a couple of important things that you should be aware of as you use the Resistance mode to test diodes. One of them is that the Resistance mode will not always show whether the diode is in a good working condition or not. Again, you should not use the Resistance Mode when a diode is connected in a circuit. This is because it has the possibility of generating a false reading. Lastly, you can use a Resistance mode to confirm that a diode is not in a good working condition once a Diode Test reveals that a diode is not in a good working condition.

For you to test a diode in the best way possible, you measure the voltage drop across the diode with a forward bias. Generally, a forward-biased diode performs as a closed switch, allowing current to flow. It's imperative to note that a Diode Test mode generates a small voltage between test leads. The multimeter shows the voltage drop after the test leads are joined across a forward-biased diode. Here is how you undertake a Diode Test process:

  1. Ensure that the power current flowing to the circuit is turned off and that the diode does not have voltage. Remember, voltage can be in the circuit as a result of charged capacitors. If this is the case, make sure that the capacitors are discharged. Afterwards, ensure that the multimeter measures ac or dc as it is required.

  2. Make sure that the dial is on Diode Test Mode. You will notice that there may be another function on the dial, so make sure that you turn the rotary switch to the Diode Test mode.

  3. It's now time to have the test leads connected to the diode. A measurement will be shown. Record it.

  4. The test leads should be reversed at this point. Record the measurement that appears.

After doing a diode test, it's imperative to do an analysis to ascertain whether a diode is good or bad for your needs. The following analysis will help you to know whether a diode is good or bad.

  • A good forward-biased diode shows a voltage drop ranging from 0.5 to 0.8 volts. Normally, this is for silicon diodes, which happen to be the ones used the most. There are germanium diodes that show a voltage drop that ranges from 0.2 to 0.3 V.

  • The multimeter will show OL in the case of a good reverse-biased diode. The OL measurements show that the diode is operating as an open switch.

  • In the case of a bad diode, you will notice that it does not permit current to move in either direction.

  • A shorted diode contains similar voltage drop reading. It is estimated at 0.4V in both directions.

Take Note: The process of conducting a Resistance mode procedure is the same as that of a Diode Test. The only difference is that in Step 2, you have to switch the dial to Resistance mode.

How to Test a Transistor on a Circuit Board

Transistors are not known for wearing out gradually. So, they are either functioning or not. Essentially, any component of a circuit that develops defects or malfunctions can easily cause a transistor to stop working. Transistors play an important role in circuits, thus if they are not in good working condition, they can cause a circuit to stop functioning as it should. Therefore, it is important to test transistors to ensure that they are functioning at their best. In case of faulty electronics, it's advisable to test the transistors to determine whether the faults can be addressed by replacing them. Here are the steps to take when testing a transistor on a circuit board:

  1. Make sure that the power to the circuit is off before you begin the testing process. You choose to either unplug the AC power cord or take out the battery supplying power. Again, make sure that all the capacitors on the board are drained of power. Afterwards, simultaneously hold the terminals on a capacitor with an insulated metal screwdriver to remove stored power.

  2. Find the collector, base, and emitter leads on a transistor. You will be able to identify the leads by checking the orientation of the circuit. If you're not able to identify the precise orientation of the leads, you should check a supplier's catalog for easier identification.

  3. For a digital meter, use a diode setting. For analog meters, use ohms scale setting. Take note that the ohms scale setting has to be low for analog meters.

  4. It's now time to check collector readings. You have to check in both directions. Once you check the readings, you should turn the leads around. For a good reading, it will reveal infinity in one direction and show a reading of about 600 for the other.

  5. In both directions, confirm the base to emitter readings. Handle one lead to the base and the other to the emitter. Have a look at the meter and turn the leads around to read the reverse direction. A good reading will reveal infinity in one side and about 600 in the other direction.

  6. If you realize that the numeric readings are distant from 600, take away the base lead. Take note that the readings of a transistor can be affected by components like a resistor. Make sure that the lead board is not connected to the circuit. To do this, you have to use a soldering iron to separate it. Once you are through with that, use the meter to test from base to collector as well as base to emitter. Once you have taken the meter readings, ensure that the base lead is back to the board

  7. If the readings of base to emitter or base to collector are zero or infinity in both directions, you should change a transistor. Normally, zeros are a sign of a short and infinities reveal an open diode within the transistor.

How to Test a Fuse on a Circuit Board

It is easier to test a fuse on a circuit board than other electrical components. Unlike a fuse, other electrical components can have complicated wiring systems that may present challenges when testing them. The gadget you use to test a fuse is a multimeter. You will realize that most fuses will allow you to have a visual verification that a fuse is in a good functional condition. You are able to see if the wire is intact or not. In case you notice the translucent area turning black, this is a clear indication that the fuse is not in a good state. However, the blackened area may appear because of overheating. If you notice that a device is not working properly, it is prudent to test the fuses. If you find out that the fuses are in a good working condition, the only logical explanation is that something else is the problem.

When you are testing a fuse, you should turn off the device and take out the fuse. When removing the fuse, you just have to pull it out of its compartment. The next thing you have to do is to turn the meter on. Ensure that the dial on the meter is directed on the continuity setting. Just before you begin the testing procedure, join the positive and negative leads and listen for a beeping sound from the meter. The beep is an indication that it is working well.

Afterwards, you should put a lead on both ends of the fuse and check the display. Now you're ready to test the fuse. As you hold the probes against the fuse, the multimeter should produce a sound if the fuse is good. If you do not hear any sound from the meter, then it's obvious that the fuse is not working. It should be replaced.

It's important to take note that when you are replacing a bad fuse or one that you're suspecting, never replace it with a higher rated one. Rating makes sure that power current moves across the wiring in a safe way. Therefore, it is highly advisable that you replace a burned out fuse with one having the same rating or a lower one. This is good for maintaining safety.

How to Test a Capacitor on a Circuit Board

Capacitors store energy as electric charge. We will explore two ways to check whether a capacitor is in a good working condition or not.

Method 1: Using multimeter with capacitance setting

This is regarded as a quick and easy way to test a capacitor. A digital meter containing capacitance meter component is required for this testing. Most of the high-end digital multimeters will have this feature. The following steps should be followed for you to test a capacitor:

  • Ensure that the capacitor is disconnected from the circuit board and have it discharged completely.

  • Set the knob on the digital meter to capacitance settings

  • Make sure that the probes of the multimeter are connected to the terminals of the capacitor. For a polarized capacitor, join the red probe to the positive terminal of the capacitor. The black probe should be joined to the negative terminal. For a non-polarized capacitor, you can connect it either way since they don't have polarity.

  • Afterwards, look at the readings at the digital multimeter. If the readings are not far from the actual values, the capacitor can be regarded as a good one.

  • However, if there is a big difference between the actual values and the measured reading, the capacitor is a bad one and should be replaced.

Method 2: Using multimeter without capacitance setting

Most of the cheap digital multimeters do not have the capacitance setting. Nevertheless, it is still possible to test a capacitor using them. The following steps should help you do the testing:

  • Take the capacitor from the circuit board and ensure that it is discharged

  • The next step is to set the multimeter to measure resistance. Set the knob to resistance settings

  • Join the leads of the capacitor with the probes of the multimeter. In case of polarized capacitors, connect red to positive and black to negative

  • The digital multimeter will display a reading of resistance and also show the resistance of an open circuit. Take note of the reading.

  • Disconnect the capacitor from the multimeter and have the test repeated severally

  • If the capacitor is good, the results displayed will be the same for every test

  • The capacitor is not in a good working condition if, for the further tests, you notice no alteration in the resistance

While this method might not be perfect, it is able to make a distinction between a good and a bad capacitor.

How to Test a Resistor on a Circuit Board

A bad resistor can cause other components of a circuit to become faulty. It can also cause the entire circuit to malfunction. That is why it is good to test a resistor on a circuit board to establish whether it is in a good functioning condition or not. You can use a multimeter to test it. These are the steps you need to follow when testing a resistor on a circuit board:

  • The first thing you need to do is to join the red and black probes to the correct terminals on the multimeter. The black probe is attached to the COM' terminal on the multimeter while the red probe is attached to the terminal with an ohm mark for resistance.

  • Make sure that the multimeter dial is on the resistance setting.

  • Ensure that the circuit with the resistor you want to test is off.

  • Make sure that the circuit doesn't have any charged capacitors.

  • Touch the probes where the lead is joined to the circuit.

  • Check the reading on the display. A resistor in a good working condition should test within its rated limit. On the other hand, a resistor that is not in a good working condition will display infinite resistance or a reading that is way higher than its rated resistance.

A point worth remembering is that if you don't put power off from the circuit or discharge capacitors, you could end up damaging your multimeter as well as getting misleading readings.


It is quite evident from the above information that electronic devices have multiple components that work to ensure that the devices function at their best. Despite the fact that these components have different functions, they work together to ensure that a device operates properly. A point that comes out quite clearly is that if one component fails to function as it should because of a fault, it can easily affect the other components. That is why testing these components to determine whether they are in a good condition to function properly is of great significance.

In the event that any of the components is bad or not functioning as it should, the best thing to do is to replace it if repairing is not possible. Another important point to note is that there are some of the components that have complex testing procedures while others are easy to test. Either way, it's imperative to test them to ensure that they function at their best and also avoid the setbacks of developing costly faults in future. Lastly, you should always be very careful when conducting tests on a circuit board because you are dealing with electricity. For this reason, be very cautious to avoid injuring yourself in the process of testing the electric components.