# I would like to understand and learn about Triacs, Diacs, and AC Dimmer Circuits.

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#### Guest3123

Joined Oct 28, 2014
404
Can someone please teach me about the different parts of maybe starting out with a simple dimmer circuit using either two Diacs, resistors, capacitors.

I understand that I need an isolating transformer for my circuit, because working with mains voltage is dangerous.

So I'd like someone to present a simple dimmer circuit, and work let's work from there. I want to have a better understanding of how capacitors, resistors, Diacs, Triacs work in a dimmer circuit. I'll post a simple circuit, and if it's not simple enough, or if anyone has a better starting point, I'll work from there. Thanks.

Just going to throw this in here as a starting point. I don't understand this circuit at all. But I'd like to understand it. Like for instance, the paths the electricity takes. Talking about what happens when the electrons hit an electronic component. etc.

#### DickCappels

Joined Aug 21, 2008
10,227
I understand that I need an isolating transformer for my circuit, because working with mains voltage is dangerous.
Uttering the magic incantation does not give magic protection from safety being reviewed.

Most lamp dimmers don't need a transformer in order to be allowed.

First, watch this short video which makes the use phase control clear.
The circuit is well described at the following link:
http://home.howstuffworks.com/dimmer-switch3.htm

#### Guest3123

Joined Oct 28, 2014
404
Uttering the magic incantation does not give magic protection from safety being reviewed.

Most lamp dimmers don't need a transformer in order to be allowed.

First, watch this short video which makes the use phase control clear.
The circuit is well described at the following link:
http://home.howstuffworks.com/dimmer-switch3.htm

Wow.. So an autotransformer is awesome. Or variable transformer. Why the heck are they so expensive.. Sounds like a simple device.

Anyways, I'm actually trying to start at the beginning. Learning about Capacitors, and Resistors. I have a pretty good understanding how resistors work, how resistors in series work (Adds Ω), and how resistors in parallel work.

Yes, I draw all my stuff I show on AAC in InkScape. So here's an example of my understanding of Resistors in parallel.

I do not understand how capacitors work with resistors, known as an RC Circuit. I tried learning about a simple RC circuit and I don't understand it.

The most I understand is that 1 Coulomb of charge is equal to 6,241,509,750,000,000,000 Electrons. According to Wikipedia, 1 Coulomb is also equal to 1 ampere of current. Which is 6,241,509,750,000,000,000 electrons passing a given point per second.

6.241 Quintillion electrons.

So.. 27uF has, correct me if I'm wrong, 0.000027uC of charge, or.. 0.000027 x 6,241,509,750,000,000,000 E = 168,520,763,250,000 Electrons stored in it.. idk, I could be wrong. I also understand that one of the biggest uses for a capacitor is storing potential energy (Voltage). When the circuit needs it, it gives it. I don't know, or understand how fast it gives it. It has to do with it's capacitance I think.

Did I watch the video, and read the webpage? Yes, I did. The job of the triac is kinda easy to understand, I just don't really understand Capacitors, and how they work with resistors. I've seen videos showing an LED, Resistor, and Capacitor, and how the LED uses the voltage in the capacitor to slowly discharge it.. etc.

1 Amp = 6,241,509,750,000,000,000
1 Coulomb = 6,241,509,750,000,000,000
Yes, the ampere is equivalent to one coulomb.

C Coulomb = 1 Coulomb
mC milli-Coulomb = 0.001 Coulomb
uC Micro-Coulomb = 0.000,001 Coulomb
nC nano-Coulomb = 0.000,000,001 Coulomb
pC pico-Coulomb = 0.000,000,000,001 Coulomb
fC femto-Coulomb = 0.000,000,000,000,001 Coulomb

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#### Guest3123

Joined Oct 28, 2014
404
So RC Circuit is R x C, or Resistance x Capacitance.

If the pot R1 was at 500k, or 500,000Ω and since it's in series with the R2 3,300Ω, then it's actually 503,300Ω x 0.1uF = 50,330 Coulombs?

So 50,330 Coulombs = 50,330C / 6,241,509,750,000,000,000 Electrons = 8.0637541261551341804761259885879e-15, which is..

0.0000000000000080637541261551341804761259885879 Electrons passing a given point per second.

Which is NOTHING.

So did I figure it out? When the pot it turned to MAX 500k Ω Resistance, the amount of electrons is 0.000,000,000,000,008 Amps.

8.063 Quadrillionths of an Ampere. Or.. 8.06 femto amperes. Not very much.

Am I on the right track?

#### kubeek

Joined Sep 20, 2005
5,796
So.. 27uF has, correct me if I'm wrong, 0.000027uC of charge, or.. 0.000027 x 6,241,509,750,000,000,000 E = 168,520,763,250,000 Electrons stored in it..
No, not at all.
1 Farad is equal to 1 Coulomb per Volt. Therefore a capacitor charged to 1V will have 6.24*10^18 electons more on one electrode than the other. However a capacitor charged to 1000V will have only 6.24*10^15 electrons more on one side than the other.
Nop,e
If the pot R1 was at 500k, or 500,000Ω and since it's in series with the R2 3,300Ω, then it's actually 503,300Ω x 0.1uF = 50,330 Coulombs?
Nope you can´t just mutliply R and C and get any meaningful results for that kind of operation. R*C is called a time constant, and it represents how long it takse for the C to charge to 0.63V after you apply 1V to the resistor and capacitor in series, so in this case it would be 503.3kΩ * 100nF=50.3ms.
The current through this will start at 1V/503.3Ω=1.98uA and will exponentialy decay to zero.

However, these calculation not very useful for calculating this circuit when the input is 12V at 50Hz. What you need then is steady state analysis, which is described here http://www.allaboutcircuits.com/textbook/alternating-current/chpt-4/ac-resistor-circuits-capacitive/ in chapter 4 of the AAC book.

#### MrAl

Joined Jun 17, 2014
11,565
Hi,

A triac is like an SCR that conducts in both directions, so it makes sense to try to understand an SCR before a triac.

The SCR has three terminals:
1. Anode
2. Cathode
3. Gate

When the gate is energized with a signal, the SCR turns on so the device conducts from anode to cathode. In a simple view it is like a relay that turns on and the contacts are the Anode and Cathode. The SCR then stays turned on even when we remove the signal at the gate. That is because it 'latches' on and stays on.
The only way to turn it off is to reduce the current through the Anode and Cathode to zero which usually means reducing the voltage. Once that happens the SCR turns off and stays off even if we reapply the external voltage to the Anode and Cathode. To get it to turn on again, we have to apply another signal to the gate.
Note that the Anode has to be more positive than the Cathode to get it to conduct when the gate signal is applied.

The triac is almost the same, and is the same if we never reverse the polarity of the Anode and Cathode, which are now called MT2 and MT1. If we reverse the polarity of MT2 and MT1 then the current can flow in the other direction when the gate signal is applied.
The only catch here is that some triacs can turn on with both positive and negative gate signals, while some can only turn on with a negative gate signal when the polarity of MT1 and MT2 is reversed. Some can turn on with either polarity, but the gate current requirement varies for these two different polarities.

MT1 stands for "main terminal 1" and MT2 stands for "main terminial 2".

So that covers the basic SCR and triac operation.

In an AC circuit the polarity usually reverses by itself and that is what allows the triac to turn off, because once the current goes close to zero it turns off by itself if the gate signal had also been removed.

#### Guest3123

Joined Oct 28, 2014
404
Hi,

A triac is like an SCR that conducts in both directions, so it makes sense to try to understand an SCR before a triac.

The SCR has three terminals:
1. Anode
2. Cathode
3. Gate

When the gate is energized with a signal, the SCR turns on so the device conducts from anode to cathode. In a simple view it is like a relay that turns on and the contacts are the Anode and Cathode. The SCR then stays turned on even when we remove the signal at the gate. That is because it 'latches' on and stays on.
The only way to turn it off is to reduce the current through the Anode and Cathode to zero which usually means reducing the voltage. Once that happens the SCR turns off and stays off even if we reapply the external voltage to the Anode and Cathode. To get it to turn on again, we have to apply another signal to the gate.
Note that the Anode has to be more positive than the Cathode to get it to conduct when the gate signal is applied.

The triac is almost the same, and is the same if we never reverse the polarity of the Anode and Cathode, which are now called MT2 and MT1. If we reverse the polarity of MT2 and MT1 then the current can flow in the other direction when the gate signal is applied.
The only catch here is that some triacs can turn on with both positive and negative gate signals, while some can only turn on with a negative gate signal when the polarity of MT1 and MT2 is reversed. Some can turn on with either polarity, but the gate current requirement varies for these two different polarities.

MT1 stands for "main terminal 1" and MT2 stands for "main terminial 2".

So that covers the basic SCR and triac operation.

In an AC circuit the polarity usually reverses by itself and that is what allows the triac to turn off, because once the current goes close to zero it turns off by itself if the gate signal had also been removed.
Thank you, but I need to learn about RC Circuits first.

Joined Jul 18, 2013
28,761
To further explain the circuit you show of a simple dimmer, for usage on AC the point of conduction of the AC cycle has to be known in order to trigger the SCR/TRIAC and execute at exactly the same time for a required level of control.
This is done in a Dimmer circuit be way of the AC trigger being phase shifted in a variable fashion by the R/C circuit.
In other forms of phase shift Triac control the zero crossing point of the AC has to be known in order to calculate the time between the zero point and the firing point.
For a clear description of this, see Fairchild App note AN-3006.
Max.

#### bertus

Joined Apr 5, 2008
22,289
Hello,

Have a look at the attached application note.

Bertus

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#### Guest3123

Joined Oct 28, 2014
404
Nope you can´t just multiply R and C and get any meaningful results for that kind of operation. R*C is called a time constant, and it represents how long it takes for the C to charge to 0.63V after you apply 1V to the resistor and capacitor in series, so in this case it would be 503.3kΩ * 100nF=50.3ms. The current through this will start at 1V/503.3Ω=1.98uA and will exponentially decay to zero.
503.3kΩ * 100nF = 50.3ms.
503,300Ω x 0.0000001F = 0.05033 seconds.

Other than that, I don't understand anything you said.

However, these calculation are not very useful for calculating this circuit when the input is 12V at 50Hz. What you need then is steady state analysis, which is described here http://www.allaboutcircuits.com/textbook/alternating-current/chpt-4/ac-resistor-circuits-capacitive/ in chapter 4 of the AAC book.
I am requesting a walkthough : http://www.allaboutcircuits.com/textbook/alternating-current/chpt-4/ac-resistor-circuits-capacitive/

The whole thing.

Thank you

#### Guest3123

Joined Oct 28, 2014
404
To further explain the circuit you show of a simple dimmer, for usage on AC the point of conduction of the AC cycle has to be known in order to trigger the SCR/TRIAC and execute at exactly the same time for a required level of control.
This is done in a Dimmer circuit be way of the AC trigger being phase shifted in a variable fashion by the R/C circuit.
In other forms of phase shift Triac control the zero crossing point of the AC has to be known in order to calculate the time between the zero point and the firing point.
For a clear description of this, see Fairchild App note AN-3006.
Max.
I am not ready to discuss DIAC/TRIAC's.

I am requesting a walkthrough with : http://www.allaboutcircuits.com/textbook/alternating-current/chpt-4/ac-resistor-circuits-capacitive/

I understand a little about how the Triac and Diac works, but I need to learn about Resistors and Capacitors (RC Circuits) first.

Thank You.

Joined Jul 18, 2013
28,761
Well before posting a question regarding Diacs/Triacs, you should obviously have studied up on the theory pertaining to R, C, & L in both DC and AC circuits prior to posing your questions.
Max.

#### Guest3123

Joined Oct 28, 2014
404
Well before posting a question regarding Diacs/Triacs, you should obviously have studied up on the theory pertaining to R, C, & L in both DC and AC circuits prior to posing your questions.
Max.
Yes, you're correct. I made a mistake. I need to study the basics first. I also need guidance, or clarification. Some of the problems or equations I do not understand when a write up talks about equations, they don't explain in detail. That's where I get stuck.

As far as guidance, and clarification on difficult discussions, what do you suppose I do? Knowing how to handle moments like this will greatly increase my understanding of electronics, and will push me forward to learning more advanced discussions, and topics.

In the and some other threads I got some really good help. People that actually knew what they were talking about and understood electronics far more than I could, have walked me through some tough subjects & discussions.

I like that. We need more people like that on AAC.

Joined Jul 18, 2013
28,761
Yes, you're correct. I made a mistake. I need to study the basics first. .
Well we can hardly be blamed for you posing the wrong questions in the first place.
Some have answered your OP questions and as a result somehow we are questioned on the ability to help! ??
You specifically mention Diac's and Triac's in your post title!!
Max.

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#### Guest3123

Joined Oct 28, 2014
404
Well we can hardly be blamed for you posing the wrong questions in the first place.
Some have answered your OP questions and as a result somehow are questioned on the ability to help! ??
You specifically mention Diac's and Triac's in your post title!!
Max.
I apologize for posting this thread. It was a mistake. People do that.
Not everyone is a computer simulation called Max headroom. People make mistakes.
My name is Guest3123, I'm a guest here at AAC. I'm uneducated, and will be requesting help on something more specific to what I need help with.

I will NOT be requesting it to be deleted, when the next thread is created.

Regardless if the answers I did get on this thread were correct, I'm still grateful, and would like to thank those that did try to help.

Thank you.

#### Sinus23

Joined Sep 7, 2013
248