How to Test a DIAC with a Multimeter

This site contains affiliate links to products. We may receive a commission for purchases made through these links.

In today’s guide, we will help you understand more about the crucial semiconductor component we referred to as the DIAC. Further, this post will help you understand its symbol, construction, working characteristics, and so much more. 

Are you ready? If yes, let’s jump in! 

Introducing the DIAC 

Did you know that a DIAC is very much the same as a transistor, so more strictly known as a transistor than a thyristor? Nonetheless, it has a very critical role in Triac triggering and other thyristors-based circuits. Numerous gate triggering circuits utilize this component for accomplishing higher triggering stability as well as noise immunity. 

In case you didn’t know yet, DIAC is short for the Diode AC switch. It features a two-terminal bidirectional switching device. Such terminals aren’t called cathode and anode in the case of a normal diode. That suggests this device can be utilized in either direction. 

The symbols of a DIAC feature two arrows in both directions, meaning it conducts for either polarity of the supply storage. Keep in mind that a DIAC does not have any controlling terminal like a gate in the case of thyristor devices. 

What is the working mechanism of a DIAC?

Only a tiny leakage current flows through the devices as soon as the supply voltage—whether negative or positive—is applied across the DIAC’s terminal. Hence, the device functions in either reverse or forward blocking modes.

An avalanche breakdown happens at the reverse-biased junction when the applied voltage is raised to a value such that it’s equivalent to the breakover voltage. It then begins conducting and shows negative resistance qualities such as the increasing current with the declining values of applied voltage.

Take note that the voltage drop throughout the conduction is quite less and is equivalent to the ON state drop of the diac itself. The current flow surges right away when it arrives at the conduction setting. Hence, a resistance is attached in series with the diac for a safe and smooth operating level of this conduction current in either direction. 

How about construction? What is the construction of a DIAC?

Keep in mind that a DIAC’s construction looks the same as a transistor? Nevertheless, you will find a few variations among the two. Some of those are the following:

  • The three regions you will see in a diac are the same in size.
  • A transistor is a three-terminal device, while a diac is a two-terminal device.
  • The doping concentrations in every layer are the same, while the transistor features an extremely doped emitter, moderately doped base, and lightly doped collector. 

It’s worth mentioning as well that the diac could be fabricated into a five-, four-, or three-layer structure. Keep in mind that a three-layer structure is more typically utilized than other structures. It can also be designed in either NPN or PNP structures. 

When it comes to the PNP arrangement, two terminals are linked to the exterior silicon P-regions separated by the N region. That structure is very much the same as the PNP transistor without any base connection. 

Now, let us contemplate the PNP crystal structure, in which terminals 2 and 1 are linked to the P2 and P1 outer layer accordingly, which are then separated by the N layer. Moreover, junction P1-N is forward biased, and P2-N is reverse biased when the terminal T1 is positive with respect to terminal 2. 

In case the breakover voltage of P2-N is acquired, the whole structure changes into the conduction mode. Therefore, diac conducts them from terminal 1 to terminal 2. This is where then the reverse will take place if terminal 2 is positive with respect to terminal 1. 

What are the common applications of a diac?

As you may already know, triac needs either negative or positive gate pulse to arrive at the conduction stage. Even though it can be initiated or triggered by a basic resistance firing circuit, a diac is utilized in series with the gate for a dependable, smoother, and faster turn ON. 

Therefore, the diac is typically and mainly utilized as a trigger device to the trial. In today’s modern market, you will find many different diac-triac matched pairs accessible for various control circuits. Have we already mentioned that the diac is utilized as a trigger device in phase control circuits of light dimmers, motor speed control, heat control, and other things? 

The gate control circuits of triacs could be enhanced by including a breakover device within the gate lead, like a diac. Utilizing a diac in the gate-triggering circuit provides a crucial benefit over basic gate-control circuits. 

The benefit is that the diac offers a pulse of gate current instead of a sinusoidal gate current. That leads to a better-controlled firing sequence. Hence, discs are utilized exclusively as triggering devices. Always keep that in mind. 

What are the steps to testing a diac?

One of the best ways to determine if a diac is good or bad is through a digital multimeter. Follow the steps below if you like to test a diac.

  1. Get your digital multimeter and set it on the Ω scale.
  1. Attach the leads of your digital multimeter to the leads of the diac and record the resistance reading.
  1. Now, reverse the leads of your multimeter and record the resistance reading on display. 

Remember that both of the resistance readings you have performed must give high resistance, as the diac is two Zener diodes connected in series. Testing a diac in this fashion only demonstrates the concept is shorted. 

Do you believe your fac is being open? Then do not hesitate to test it using an oscilloscope too. 

Final thoughts

There’s no doubt that a diac is a very crucial device within the thyristor family. It doesn’t switch sharply to a low voltage condition at a low current level as performed by a triac or SCR. It also has a low voltage drop until its current falls below the current holding level. 

We hope this guide provides you with all the information you need about diacs and how to test them using a multimeter.

About The Author