How to Test a Triac Using an Analog Multimeter

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What is a triac, and how does it work? If you want to know more, keep reading. 

A triac is a three-terminal AC switch activated into conduction when a low current signal is distributed to its gate terminal. The triac conducts in any route once it is turned on compared to SCR. It is also very different from the SCR in that either a negative or positive gate signal activates it into conduction. 

Hence, the triac is a three-terminal, 4-layer, bidirectional semiconductor tool controlling AC power. At the same time, an SCR regulates direct current power or sends biased half cycles of AC in a load. Keep in mind that it has bidirectional conduction property. That’s why it’s widely utilized in the field of power electronics for control purposes.

Triac is the abbreviation for the three-terminal AC switch. Tri means the device has three terminals, and ac means the device controls AC or can conduct either route. 

Where Can You Use a Triac? 

Did you know that the triac is the most widely utilized member of the thyristor family? In most control applications, it has changed SCR by virtue because of its bidirectional conductivity. 

Phase control circuits, liquid level control, illumination control, temperature control, motor speed regulation, and power switches, among others, are some of its key applications.

Nevertheless, the triac is considered less versatile compared to the SCR when turn-off is considered. Remember that it can conduct in whichever direction. However, forced commutation through reverse-biasing can’t be used.

Turn-off is either by current starvation that is normally not feasible or by AC line commutation. You will find two limitations imposed on the application of triac at the present state of commercially accessible devices. 

The frequency handling ability is generated by the limiting dv or dt at which the triac keeps hindering every time no gate signal is used. That dv/dt is approximately 20 Vmicros-1 compared with the general figure of 200 Vmicros s-1 for the SCR. Hence, the limitation of frequency is at the power level of at least 50 Hz. 

The similar dv/dt limitation indicates the load to be regulated, possibly a resistive one. Take note that the back-to-back SCRs can’t be changed by the triac, especially when high frequencies and high dv/dt are involved.

  • AC power control

A triac control circuit controls ac power to load by switching on and off throughout the input sinusoidal signal’s negative and positive half cycles. 

D2 is opposite biased, while Diode D1 is forward biased, and the gate terminal is positive concerning A1. Bear in mind that this is throughout the positive half cycle of the input voltage. On the other hand, throughout the negative half cycle, diode D1 is reverse-biased, and diode D2 is forward-biased. 

Therefore, the gate becomes positive concerning terminal A2—the point of commencement of conduction is governed by changing the resistance R2

  • Half power lamp switching

The triac is cut-off, so the lamp is dark when switch S is in position 1. When the switch is placed in position 2, a tiny gate current flowing into the gate turns the triac on. Therefore, the light is turned on to offer rated output. 

How Do You Test a Triac? 

Are you now wondering how you can test if your triac is fully functioning? There’s one tool you can use in this matter—that’s a multimeter. 

You will find two types of multimeter: analog and digital. But no matter which option you go with, this device is utilized to measure the electricity of an appliance or object you are working with. They can measure current, resistance, and volts, among others. 

Digital multimeters are very famous for a reason. These devices often present more accurate readings compared to analog multimeters. The chance of misreading the measurement is lowered because the digital screen shows the exact numbers. 

Further, they feature an auto-polarity function. That indicates you won’t accidentally pick the wrong polarity if you connect a test circuit and your meter. 

Even though it might appear like going digital is the obvious option, depending on the type of project you are dealing with, an analog multimeter might be a good choice for you. Are you planning to read a fluctuating signal and need a range? Then analog is the perfect way to go. 

What’s more, you don’t need a power source or batteries to use an analog reader. This is useful if you are working on longer projects, not to mention you don’t need to purchase batteries or need external power sources, helping you cut down on expenses. 

Going back to testing a triac, we will use an analog multimeter. You can use this multimeter to test the overall health of your triac.

  1. Put the multimeter selector in a high resistance mode (for example, 100K).
  2. Attach the positive lead of your analog multimeter to the MT1 terminal of the triac and the negative lead to the triac’s MT2 terminal. You don’t need to be worried if you reverse the connection.
  3. The multimeter’s needle will then show a high resistance interpretation or open circuit. 
  4. Place the selector switch to a low resistance mode. Attach the gate and MT1 to the positive lead and MT2 to the negative lead.
  5. Your analog multimeter won’t present a low resistance reading and will indicate the switch ON. 
  6. If the tests above are positive, you can think that your triac is fine and doing well. Anyway, that test is not suitable for triacs that need high current and voltage for triggering. 

Final Thoughts

Electrical AC power control using a triac is highly efficient, especially when utilized correctly to control resistance-type loads like small universal motors, heaters, incandescent lamps, normally seen in portable power tools and small appliances. 

But keep in mind that such devices can be utilized and connected to the mains AC power source. Hence, circuit testing must be completed when the power control device is disconnected from the mains power supply. Whenever testing for triacs, always remember your safety first!

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