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:
Anonymous says
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
Swagatam says
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.
Anonymous says
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
Albert
Swagatam says
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.
Julius X says
BT169 seems a SCR and not a triac, am I missing something?
Swagatam says
Yes BT169 is an SCR, …..it should be BT131 instead.
Anonymous says
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.
Cheers
Albert
Swagatam says
You are welcome!
Anonymous says
d valve what?
Anonymous says
220k pot have a 3 pin circuit show 2 pin . can u explain it?
Swagatam says
only two pins need to be used, the center and any of the remaining ones.
Anonymous says
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
TheReviewer says
can you provide the link or original application note for SGS THOMSON
Swagatam says
Google the words given in the courtesy image at the bottom of the page, you will get it immediately.
EKEH EMMANUEL CHIJIOKE says
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
Swagatam says
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.
AZIZ JAMALI says
I HAVE REQUIRED 100 AMP AC SINGLE PHASE MOTER SPEED CONTROL CIRCUIT DIA GRAM ,
THANKS
AZIZ JAMAL;I
SarAnG shAstRaKaR says
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
Swagatam says
Hello Sarang,
inductive load becomes an issue only when it's switched with a frequency through a triac, just for ON/OFF timer operation it won't pose any problem. you can use the following circuit:
https://homemade-circuits.com/2013/07/simple-triac-timer-circuit.html
SarAnG shAstRaKaR says
Timer On time is 3min, and off time is variable 2 to 8 min.
Swagatam says
you can use the following circuit, just replace the relay with a triac
https://homemade-circuits.com/2012/04/how-to-make-simple-programmable-timer.html
Unknown says
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)
Swagatam says
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.
Daniel Acciarri says
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 !
Swagatam says
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.
Swagatam says
…..confirm the set up using a 200 watt bulb first, after that you can replace it with the trafo
Satvik Sharan says
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?
Satvik
Swagatam says
you can use any triac based timer and use it for switching your transformer for the desired amount of time:
Anonymous says
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
Swagatam says
Hi, for dimming purpose you could try a standard triac dimmer circuit as shown in the last diagram from the following article
https://homemade-circuits.com/2012/04/how-to-make-simplest-triac-flasher.html
The triac should be a BTA41 600 for controlling massive currents even upto 40 amps
the dimmer circuit must be employed in series with the mains primary side of the transformer
W tG says
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
Swagatam says
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 🙂
Anonymous says
Hello sir, please what is the value C, rd and rt for resistive loads, i tried some values but never worked. thanks
Swagatam says
hello, resistive loads you ca simply use the connects explained in the following article
https://homemade-circuits.com/2012/04/how-to-make-simplest-triac-flasher.html
Swagatam says
typo correction: I mistakenly wrote connects instead of concepts
ainhan says
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.
Swag says
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
Godson says
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.
Swag says
Hi Godson, I won’t recommend the above experimental designs, instead you could try the last diagram having 5 step rotary switch controlled fan dimmer from the following article
https://www.homemade-circuits.com/how-to-make-simplest-triac-flasher/
Godson says
I have checked the link. It is just perfect for my application. Thank you very much sir.
Keep up the good work!