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Using Triacs for Controlling Inductive Loads

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.



About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

35 thoughts on “Using Triacs for Controlling Inductive Loads”

  1. Hi – many years ago I had a 160 amp arc welder in which the transformer primary was controlled by an electronics circuit but the circuit and or components (scr) were not up to the job and soon packed up leaving the machine on max current. The inductive control circuit above would seem to be a good solution to restoring controllability. What components and values would be needed to control a 7kva transformers? Thanks in advance for your help.

  2. Please, the last circuit for inductive load, does it mean it will reduce the power consumption of the load like freezer or what, why is necessary for inductive loads. I have tried your circuit on dimmer, it’s great in reducing the power consumption. Thanks so much for this

    • Dayo, yes it will reduce consumption, as well as the output power of the unit. But I am not sure about the resultant impact on appliances like freezers.
      I am glad you could build the linked dimmer circuit successfully

  3. Hello sir Swagatam,
    Thank you very much for sharing this article. I want to use the circuit to control my a.c ceiling fan, so I have some questions:
    1. Can use it for that purpose without any problem? If yes,
    2. I want to connect the circuit in such a way that I will use discrete resistor values (say, 4 resistors) instead of the 220k pot, such that I will use a rotary switch to switch between the resistors to give the fan different rotating speeds (four stages of equal intervals).
    Is this possible? If yes,
    3. What resistor values do you suggest that I use, and what should be thier power ratings?
    4. I want to have a fifth state that will bring the fan to a total stop, i.e. four stages for different speed levels and the fifth stage to switch off the fan. How do I achieve this?
    Anticipating your usual prompt response. Thank you.

  4. Thank you for introducing some of the basic and advanced Triacs circuits. Typically, when adjusting the input of the tranphomer, it produces very loud noise. The circuit you introduced by SGS-THOMSON. I see That’s great, but I’m asking you to try it. If I want to try this circuit and put in
    using arc welding machine control
    Is it good? Please give me some time. Thank you very much.

    • you mean to say you want to control a welding transformer power with this circuit? That may be possible, just make sure to use a heavy duty triac (100 amp, 400V) and a compatible diac…if you are unable to get a suitable diac then you can try replacing it with high grade neon bulb

  5. 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 🙂

  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. 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.

  8. Hello sir,
    I just need a circuit like timer. but the load is of 60 W 340V AC Inductive.
    Please help me in building the circuit using BT169D or any best suitable scr or triac for inductive load

  9. greetings sir pls if am wrong pls correct me. Rt i guess is a normal resistor of 10k? then how about Rd? that has a value of 6k8? i dont get that part pls

    • 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.

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