How to Use Triacs for Controlling Inductive Loads like Transformers and AC Motors

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. 

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.


35 thoughts on “How to Use Triacs for Controlling Inductive Loads like Transformers and AC Motors

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Hi, thanks for this article. How can one work out some sensible values for the components? I have found the original document by Durbecq but his application circuit is more complex.

    Many thanks, Albert

    • Hi, Thanks!

      If you notice the last diagram, it's actually an ordinary light dimmer circuit, the only difference being the inclusion of an extra triac across the pot.

      The configuration can be tested by procuring a raeadymade dimmer switch unit and by integrating an additional triac (BT169) in the manner shown in the above article.

    • Indeed, you are right. That's a helpful insight. Just wondering what'd be the appropriate value for Rd. Also, do you think this kind of circuit is suitable for controlling a 2000 W/220 V universal motor if using the BTA41 triac?

      Many thanks

    • Thanks:)

      Rt can be = 10k
      P= 220k pot,
      Rd = 6k8,
      C = 0.1uF/250V
      Ts = BT169

      According to me it should definitely be able to handle to 2000w motor if a BTA41 is used as the triac.

  3. Many thanks for your reply, you have been very helpful. Keep up the good work.

    I'll drop a line to report back when the project is finished.

  4. i will made this circuit with bta41 but when give supply bt 41 and bt169 triac come blast ,you can show circuit with pin numburs or easy way to show this circuit please

    • Hi Ekeh,

      I think Rd = 6k8 is incorrect, it should be a low value resistor positioned just for protecting the diac from current surges…..a 100 ohm 1/4 watt resistor will probably do the job.

  5. Hello, I love your page, I have a question what values do I need for Rt,Rd,Ts,P,T,D,C if I want to use a transformer of 1100 Watts aprox? (in mains I have 220 Volts and 50 Hz)

    • hello, I am not sure about the exact component values that may be suitable for your application, however today the modern triacs mostly have in-built snubbers and protection against inductive loads, so I think you can comfortably use any standard fan dimmer circuit and use a high power triac such as a BTA41/800 with it for the mentioned load without any concerns.

    • hi again,if I use the values that you wrote up in the blog:
      Rt can be = 10k
      P= 220k pot,
      Rd = 6k8,
      C = 0.1uF/250V
      Ts = BT131

      and instead the BTA41 a TRIAC of 10 Amps max. should it work? I do not know very much of electronics and I dont´t have any idea on how calculate the values of the components I just dont want to burn anything.
      Thanks !

    • You can try those values but i cannot guarantee the results.

      a 10amp triac might work if adequately heatsinked.

      nothing would burn actually since your transformer is in series with the circuit, and if at all anything burns it would be one of the components not your house wiring.

      Make sure you have an experienced/qualified technician with you while you attempt this experiment.

  6. Hi
    I need to switch a transformer using a triac controlled by a controller. the transformer is rated 230/120 V 1.1 KVA. Ineed to switch only during the first few cycles. Can you help me out with this?

  7. Hi, I am building an arc welder and would like to be able to dim the power input to the transformer. This would be a much higher power application than any mentioned before. Whet component values would I need to make this work? the welder has a 230:30v transformer, and I wouldn't be surprised about pulling 30amps into the transformer

  8. Hey Swagatam, thanks for you website, clear descriptions and all the time and effort you share. But… I do have a question ;).
    I built the last of the above circuits, and it works fine, but with Rd = 120 Ohm. The inductive load is just a 60 Watt ventilator (I plan to use the circuit for heavier inductive loads still). However, when using it with this 60 W load I can not get the rpms below 60% of full power. Can I simply increase Rd to lower the output power further? Thanks

    • Thanks Wtg, According to me increasing the value of the pot might help to increase the range on the lower side, because the speed basically depends on how fast or how slow the capacitor is allowed to charge and discharge, therefore by slowing down the charging rate of the capacitor we would be able to decrease the speed of the motor proportionately.

      However you can experiment with the other components also, there are hardly anything in the circuit so identifying the right one will be just a matter of minutes 🙂

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