Using Triacs for Controlling Inductive Loads

Here we try to investigate a few enhanced triac based phase controller circuits which can be recommended for controlling or operating inductive loads like transformers and AC motors much safely than earlier traditional triac based circuit dimmer circuits.

Using Triacs for Controlling AC Loads

A Triac is a semiconductor device used for switching AC loads. Normally it is recommended that the loads that needs to be operated through triacs should be resistive in nature, meaning loads which incorporate coils or capacitors heavily, must be avoided.

Therefore in general loads which convert energy into heat like incandescent bulbs or heaters etc only become suitable with triacs as the switch and devices like transformers, AC motors and electronic circuits are a big NO!

However recent developments and researches have improved things to great extents and today new triacs and the involved improved circuit configurations have made it absolutely safe even for the triacs to be used for switching purely inductive loads.

I won't be discussing the technical areas of the configurations, keeping the new electronic hobbyists in mind and for the sake of simplicity.

Let's analyze a few of the researched designs which boast to support triacs with inductive loads.

Triac Control Circuit Only Suitable with Resistive Loads

The first circuit shows the general way of using a triac and a diac combination for implementing the required controlling of a particular load, however this design is just not suitable with inductive loads.

The circuit incorporates the principle of triggering with synchronization across the triac. The configuration is the simplest in its form and has the following advantages:

The design is very simple and cheap.

Use of only two end terminal wire and absence of any external power supply.

But one big disadvantage of this design is its incapability of working with highly inductive loads.

Triac Control Circuit Reasonably Suitable for Operating Inductive Loads

However a little contemplation shows that the above circuit can be simply modified into the design shown in the next diagram.

The principle here now gets transformed to triggering of the triac with synchronization by the mains voltage.

The idea to very extent neutralizes the above issue and becomes very much coordinated even with inductive type of loads.

Please note that in the above design very interestingly, the position of the load and the resistor connection has been changed for acquiring the intended results.

The advantages can be assessed as follows:

Again a simple design and also is very low cost.

Better control of even loads which are inductive by nature.

As usual no external power source is required for the functioning.

The disadvantages though are the involvement of 3 terminal wire ends for the intended connections.

The operations become very asymmetrical and therefore the circuit cannot be used for controlling highly inductive loads like transformers.

Triac Control Circuit Ideally Suitable for Highly Inductive Loads like Transformers and AC Motors

An intelligent tweaking of the above circuit makes it very much desirable even with the most tabooed inductive loads like transformers and AC motors.

Here another small sensitive triac is cleverly introduced for rectifying the major issue that's primarily responsible for making triacs so unsuitable with inductive loads.

The second small triac makes sure that the triac is never switched OFF and blocked completely, by generating a pulse train, keeping the triac alive and "kicking" all the time.

The advantages of the above final design may be marked with the following points:

Very simple design,

Superb accuracy while controlling highly inductive loads,

No use of external power supply.

The above circuit was exclusively developed by the SGS-THOMSON Microelectronics applications laboratory and used with success for a wide range of equipment.

COURTESY: