# Hello. I have a question about a dimmer circuit with a triac

#### johnboyman

Joined Jun 1, 2019
43
Hey I am a beginner in electronics and I have a project that involves a dimmer circuit with a Triac. I got this circuit off of the website it has a youtube video and a website explaining it. It uses these parts

Parts:- Triac BT136, Diac DB3, Capacitor .1uf/400v, 10K resister, variable Resister( variable Control) 500K

Here is a video

and here is the website https://ethcircuits.com/light-dimmer-circuit/

I also attached a diagram of this. It looks ok to me but I question it on the youtube video everyone in the comments says that its a great video. I question a few things about it and am looking for feedback. Aren't the resistors too small in the video they look like 1 watt resistors max and that potentiometer may be too low wattage for a light bulb. This just doesn't look right to me. The potentiometer and resistor look too small to handle that power is there a reason or something that i am not seeing. Thanks for any tips.

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Joined Jan 15, 2015
7,256
The resistors are not handling the power. The power is handled by the triac. That is the whole idea behind such a circuit. The triac does the heavy lifting (the real work of power handling).

Look up the data sheet for that TRIAC. Note the Gate Current min and max. I see about 2.5 to 10 mA. In fact one great feature of that TRIAC is the very low gate trigger current. The load current is not flowing through the resistor(s) in circuits like this.

Ron

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#### MisterBill2

Joined Jan 23, 2018
16,641
Post #2 is correct! The other parts that may possibly carry some load current are a filter inductance, which most dimmers do not bother with any more, and a series mechanical switch. So the fixed resistor, the capacitor, the variable resistor, and possibly a small diode ar all parts of the variable trigger circuit.

Joined Jul 18, 2013
28,069
That is a normal circuit for a resistive loads, if thinking of controlling small motors etc, then a L/C (supression) snubber is usually included.
Max.

#### johnboyman

Joined Jun 1, 2019
43
Oh I see the triac limits the current. I get it thanks for the advice. Triacs are awesome.

Joined Jul 18, 2013
28,069
The triac is used where bi-directional control is required, a SCR is used for uni-directional control.
Max.

#### MisterBill2

Joined Jan 23, 2018
16,641
If you use a triac light dimmer to control a smaller AC motor understand that it varies the speed by varying the torque, and that the control will not be as smooth as with a motor speed controller device. In addition, there is an area where the motor will stall, with possible overheating.

#### MisterBill2

Joined Jan 23, 2018
16,641
Yeah, the TRIAC will become your new best friend. Just keep in mind what was mentioned about inductive verse resistive loads. This is a good read on the subject.

Ron
The triac delays the start of the voltage rise on each cycle, so that there is less power delivered. In graphic terms, there is less area under the curve.

Joined Jan 15, 2015
7,256
The triac delays the start of the voltage rise on each cycle, so that there is less power delivered. In graphic terms, there is less area under the curve.
I believe that was covered, including pictures, in the link I left the thread starter. Matter of fact the area under the curve is very apparent in one animated graphic.

Now is that fancy or what. The Blue being the Gate.

Ron

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Joined Jul 18, 2013
28,069
Now is that fancy or what. The Blue being the Gate.
Ron
On a slightly allied note, I had started reverse engineering one of the popular PWM T.M. controllers.
The motor is PWM controlled via a micro which initially sets the speed at the requested rate via a Mosfet, then the micro controls an SCR on the DC supply to ramp up the motor DC supply via a charge pump in order to obtain a very smooth acceleration.
Max.

#### Willen

Joined Nov 13, 2015
333
I damaged 3 TRIACs in the circuit. So I would suggest a 100 ohms series resitor between Diac and Triac and 4.7k from gate of triac to ground.

#### MisterBill2

Joined Jan 23, 2018
16,641
I damaged 3 TRIACs in the circuit. So I would suggest a 100 ohms series resitor between Diac and Triac and 4.7k from gate of triac to ground.
Certainly there is a need to limit the gate current to a triac, and for this reason one needs to read the specifications to know that limit, and then make certain that the driving portion will not exceed that limit.