How to Test an SCR with a Multimeter

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Have you ever seen a SCR, also known as Silicon Controlled Rectifier? Some may utilize it as an alternative to relays and switches. 

An SCR is a 3D semi-conductor switching tool that is one of the most crucial elements after the transistor and diode. Designed in 1957, this device can be utilized as a controlled switch to do different functions like:

  • regulation of power flow
  • inversion
  • rectification

The SCR has been considered of great importance in electronics, as it can be made in versions to handle currents up to a few thousand amperes and voltages over 1 kV. Furthermore, SCRs are made from silicone and are typically utilized for converting AC current to DC current (also referred to as rectification). These devices can deal with high voltage and current values and thus are utilized for many industrial purposes. 

What is the Symbol for SCR? 

In case you didn’t know, the symbol for SCR is very much the same as the diode and has a gate terminal. SCR is a unidirectional tool that enables the current to flow in one direction and opposes it in another direction. 

Take note as well that SCR features three different terminals:

  1. Anode (A)
  2. Cathode (K)
  3. Gate (G) 

These terminals could be turned off and on by controlling the biasing conditions or the gate input. 

Remember that the Thyristor symbol and SCR are alike. 

What is the Construction of SCR? 

The SCR is composed of four layers of a semiconductor device that creates a PNPN or NPNP structure that creates three junctions that include J1, J2, and J3. The anode is a positive electrode among SCR’s three terminals and will be on the P-layer.

A cathode is considered a negative electrode and will be on the N-layer of the SCR. Ultimately, the Gate functions as SCR’s control terminal. 

Take note that the outer N and P layers where the two electrodes are located will be massively doped, and the middle N and P layers will be casually doped. The gate terminal will be linked to the P-layer in the center. 

Moreover, the SCRs are designed along with three different types:

  • press pack type
  • mesa type
  • planar type 

How Does It Work?

SCR converts the dangerous nitrogen oxides in diesel cars’ exhaust into harmless water and nitrogen gas vapor. The SCR solution is metered into the exhaust system in front of the catalyst. The urea hydrolyzes to ammonia that then responds with the nitrogen oxides in the exhaust gas. 

How to Test an SCR with a Multimeter? 

Before we continue to the steps you need to do when testing your SCR with a multimeter, here are some important things you need to keep in mind: 

  • Don’t touch unused terminals when the meter is connected to the measurement circuit
  • Don’t do resistance measurements on a live circuit
  • Always be extra cautious with voltage more than 60v DC or 30v AC RMS. 
  • Don’t go beyond the protection limit values specified in specifications for every range of measurement. 
  • Disconnect the test leads from the circuit under test before rotating the range selector to adjust functions. 

It is worth mentioning as well that a few SCRs will not work with only the amount of current supplied by an ohmmeter set to the R x 10K setting. 

If the SCR you’re testing could deal with a higher quantity of current, you can try using the R x 100 or the R x 1000 setting on your ohmmeter. 

Follow these steps below to test your SCR with a multimeter:

Testing for Anode to Cathode for Thyristors & Diodes 

  1. Set your multimeter to short circuit test and ensure the probes are connected for voltage test. 
  2. Test both directions of the SCR by connecting the black and red probes to Pin 2 to Pin 1 and Pin 3 to Pin 1. 
  3. Your SCR has failed if the multimeter creates a beep sound, and hence, there’s a short circuit. No beep from the meter indicates it’s working well. 
  4. For the diode, expect to hear a beep when you test the forward direction.
  5. Reverse bias test with Cathode to Anode shouldn’t give any sound. 
  6. The diode fails if the multimeter creates a beep sound. 
  7. For the SCR, you won’t get a beep for both reverse and forward bias tests. 

Test for Resistance to Locate for Short Circuit 

  1. Switch your multimeter to Ohm (resistance) test mode. 
  2. Measure the Anode to Cathode on both devices, and you’ll see hundreds of kOhm to mOhm values. 
  3. It’s a partial failure if the impedance is slow. 

Test for SCR Gate Cathode Resistance 

  1. Utilize resistance test again and check Pin 5 to Pin 2 and Pin 6 to Pin 3. 
  2. The impedance must be below 10 ohms or at least 10 to 50 Ohm. 
  3. It’s failed if it’s very high. That failure mode most likely happens when the SCR control or firing card has suffered from board failure. That can also happen because of transient surges or lighting strikes. 

Also, some SCRS don’t demonstrate the latching behavior when testing using a multimeter. The resistance returns to a greater reading when the jumper wire is disconnected. The reason for that is because the SCR might have a greater holding current than what the multimeter’s internal battery supply can sustain in the circuit.

Final Thoughts

There you have it! Just follow the steps mentioned in this post, and you are good to go. However, in any case, these tests come up okay, but you’re still experiencing problems. What should you do? Then it’s time you get in touch with a professional and inquire about the support they can offer to your problems. They could take it a step further using a piece of specialist test equipment they use in production.

We hope you find informative and helpful ideas while reading this post. Are you ready to do your own test on your SCR with a multimeter? Share your thoughts with us by leaving your comments below!

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