How to Test an SCR with a Multimeter

In the intricate world of electronics and circuitry, the Silicon Controlled Rectifier (SCR) holds a crucial role. Whether you’re an electronics enthusiast, a DIYer, or a technician, knowing how to effectively test an SCR is a valuable skill. SCRs are semiconductors used for switching applications in various electronic devices, and ensuring their proper functioning is essential for maintaining the integrity of electrical circuits.

In this blog, we’ll delve into the step-by-step process of testing an SCR using a multimeter, demystifying the procedure and empowering you to diagnose, troubleshoot, and replace faulty SCRs with confidence. Whether you’re embarking on a new project or tackling a malfunctioning circuit, this guide will equip you with the knowledge to navigate the intricacies of SCR testing like a pro.

Have you ever seen a silicon-controlled rectifier, often known as an SCR? It might be used by some people instead of relays and switches.

After the transistor and diode, an SCR is a 3D semi-conductor switching device and one of the most important components. We’ll go over using a multimeter to test the SCR in this article. Understanding SCR testing in the simplest way is essential for troubleshooting electronic cards, especially when working with the power circuit diagram. The anode, cathode, and gate leads of an SCR are the first things we need to understand.

Symbol for SCR

In case you didn’t know, the SCR symbol is extremely similar to the diode’s and has a gate terminal. SCRs are unidirectional devices that allow electricity to flow in one direction while obstructing it in the other. Take  note that SCR offers three different terminals:

• Anode (A)
• Cathode (K)
• Gate (G)

Controlling the biasing conditions or the gate input could be used to switch on and off these terminals.

Keep in mind that the SCR and Thyristor symbols are similar.

How to Identify the Terminals of the SCR

• We will now use the SCR TO-92 package as a guide for understanding.
  We are able to determine the polarity of the multimeter leads using a P-N junction diode. Unidirectional junction diodes have low forward biasing resistance and high reverse biasing resistance.
• The multimeter leads should now be connected to the diode’s anode and cathode. The multimeter lead attached to the diode’s anode is positive if the resistance reading is low, and the other lead is negative.
• The reverse bias state of the diode is indicated by a high resistance reading on the ohmmeter. In this situation, the multimeter’s positive lead is linked to the anode while the multimeter’s negative lead is attached to the anode.
• In most cases, the multimeter’s positive lead is connected to the red socket there.

How Does It Function?

The harmful nitrogen oxides in the exhaust of diesel cars are changed into safe water and nitrogen gas vapour using SCR. Before the catalyst, the SCR solution is fed into the exhaust system. The nitrogen oxides in the exhaust gas are produced when the urea hydrolyzes to ammonia.

How to Test an SCR with a Multimeter? 

Here are a few important points you should have in mind before moving on to the steps you must follow when testing your SCR with a multimeter:

• When the meter is attached to the measurement circuit, avoid touching any unused terminals.
• Never measure the resistance on a live circuit.
• Any voltage greater than 60 volts DC or 30 volts AC RMS should always be handled with extreme caution.
• Avoid exceeding the protective limit values listed in the specifications for each measurement range.
• The test circuit must be disconnected from the test leads before the range selector can be turned to alter functions.
• It is also important to note that some SCRs will not operate with just the current provided by an ohmmeter with the R x 10K value.
• Use the R x 100 or R x 1000 setting on your ohmmeter to see if the SCR you’re testing can handle a bigger amount of current.

Testing for Anode to Cathode for Thyristors And Diodes

• Make sure the probes are connected for a voltage test and that your multimeter is set to short circuit test.
• By attaching the black and red probes to Pin 2 and Pin 1 and Pin 3 and Pin 1, you may test the SCR in both directions.
• If the multimeter beeps, it means your SCR has failed and there is a short circuit. The meter should not beep if it is functioning properly.
• If you test the diode in the forward direction, you should hear a beep.
• Cathode-to-anode reverse bias testing should be silent.
• If the multimeter beeps a sound if the diode has failed.
• Both the reverse bias test and the forward bias test on the SCR will not result in a beep.

Test for Resistance to Locate a Short Circuit

• Put the Ohm (resistance) test mode on your multimeter.
• You can see hundreds of kOhm to mOhm values by measuring the anode to cathode on both devices.
• If the impedance is slow, there has been a partial failure.

Test for SCR Gate Cathode Resistance 

• Check Pin 5 to Pin 2 and Pin 6 to Pin 3 using the resistance test once more.
• The impedance must be between 10 and 50 ohms or less than 10 ohms.
• If it is really high, it has failed. The SCR control or firing card will most likely experience that failure mode if the board has failed. Lightning strikes or transitory surges may potentially be to blame for such.
• Additionally, certain SCRS fail to exhibit the latching behaviour during multimeter testing. When the jumper wire is cut, the resistance rises to a higher reading. The SCR may have a higher holding current than what the internal battery supply of the multimeter can support in the circuit, which is the source of this.

SCR Testing with a Multimeter Has Limitations

The supply current of the multimeter affects both the results of the SCR test and how it is conducted. We can test the SCR using a multimeter if it provides enough current for SCR conduction.

Conclusion

You have it now! All you need to do is adhere to the instructions on this page. In any event, these tests pass with no issues, but you continue to have issues. What ought you to do? Then it’s time to contact a professional and find out what assistance they can provide for your issues. By using the specialized test gear they employ in production, they may go one step further.

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