I googled a lot before posting here. And i plan to recreate few circuits on the web.
I completely understand this circuit.
But i don't understand this one. Please if someone could explain?
The phase control method using zero cross detection and pwm, is not the way i want to execute the job.
Instead i thought of using a digital pot to control the firing angle. This way i can control the dimming (linearly or any way i want) much easily.
Problems-
I don't know the power rating for the pot required to get the work done.
And they are expensive. Is there a way to use a 30v pot in this circuit?
How can i use a digital pot to control the firing angle? And if, that's not feasible, what are the other better ways to control it, which work in the same fashion?
Forget what I said before. I understood your first diagram, you just got to get the correct pot and capacitor to regulate the rc constant and change the angle. I dont know which voltage must the pot be rated but Id say the same voltage of the ac supply.
The second circuit is the same as the first circuit, the differences are that it has a filter on the input to the diac, and it has a snubber circuit R5 & C3 to reduce the interference it generates.
Instead i thought of using a digital pot to control the firing angle.
To do that you will need to completely isolate the pot from any other circuit. Also I do not think you will find a digital pot that can handle those sorts of voltages anyway.
You have to forget about using an analogue delay and use timed pulses to fire your triac.
I don't think a digipot will do this I have found it very hard to isolate the pot for that kind thing. A zero cross timed circuit is much better and its not hard to do there's plenty of info out there and lots of ways but really the principal is all the same. Find zero wait some delay time turn on triac turn it back off at zero.
The phase control method using zero cross detection and pwm, is not the way i want to execute the job.
There's a good reason most digital dimmers use phase detection. (BTW - You can't use "regular" PWM with a TRIAC.)
Instead i thought of using a digital pot to control the firing angle. This way i can control the dimming (linearly or any way i want) much easily.
] How about a regular pot controlled by a servo motor? ] You can actually buy a [u]Motor Driven Variac[/u],
but they are expensive, bulky, and not very practical.
When I built a digitally controlled dimmer a million years ago with a different microcontroller, I used the power supply's transformer for isolation and I detected the (near) zero-crossing from the transformer's secondary. The TRIAC was isolated with an opto-isolator especially made for use with a TRIAC (as usual).
mart256:
I dont know which voltage must the pot be rated but Id say the same voltage of the ac supply.
okay that's one answer, thanks.
Grumpy_Mike:
You have to forget about using an analogue delay and use timed pulses to fire your triac.
Thanks mike, i understood the circuit. I'm avoiding PWM, it takes a lot more components. I have a feeling PWM wouldn't be that accurate, there's got to be some other way.
drksam:
What is your load?
I understand PWM method, i want to avoid taking that path.
I want to drive tungsten filament bulbs or a ceiling fan atmost.
There's a good reason most digital dimmers use phase detection. (BTW - You can't use "regular" PWM with a TRIAC.)
Is it? I didn't know that. This method is relatively easy though. What's different for TRIACS?
How about a regular pot controlled by a servo motor?
Initially i thought of that too . Impractical indeed.
Its not exacly PWM. You could always use a pre made dimmer if you feel uncomfortable making the boards. Something like a dmx dimmer may be a way to look.
Yes not exactly PWM but more of a trigger pulse.
I'll wait for a few days for a solution. After that i'll start digging into the PWM method.
Thank you.
I do not think you have understood what I said. I did not say use PWM , mainly because you can not do pwm using a triac and AC mains. The problem is that once on a triac stays on for the rest of the half cycle.
What I suggested was to digitally control the delay between the zero crossing point and the triac firing.
The only way to proportionally control an AC load with a triac is by using phase shift modulation.
Grumpy_Mike:
I do not think you have understood what I said. I did not say use PWM , mainly because you can not do pwm using a triac and AC mains. The problem is that once on a triac stays on for the rest of the half cycle.
What I suggested was to digitally control the delay between the zero crossing point and the triac firing.
The only way to proportionally control an AC load with a triac is by using phase shift modulation.
I understand what you mean, when you said we can't do PWM with triacs. And what you suggest is the conventional way to do it.
But the RC circuit used to trigger the triac here does all the work. I just need to find a way to digitally control the time constant of the RC circuit.
JohnLincoln:
If you are going to be using a Triac, then you need to completely forget about using PWM.
Triacs (and their close relative, Thyristors) work in a completely different manner to FETs and bipolar Transistors, as far as turning them on and off is concerned.
For FETs and Transistors you apply a control signal to turn it on, remove the signal and it turns off. Simple to understand.
By using the Arduino's PWM outputs you can use FETs or transistors to make a DC dimmer by varying the ratio of the on time to the off time. This occurs at a frequency of 490 Hz or 980Hz for Arduinos. The exact frequency doesn't matter, as long as it is fast enough for the human eye to not be able to see any flickering.
For a Triac or Thyristor, you apply a signal to turn it on. If you remove this signal, the Triac/Thyristor remains conducting, until the current through the load falls below a certain threshold. If you are using the device with a DC supply, that may never happen! However if you are using it with an AC supply, that will happen at each zero crossing of the AC cycle.
To make an AC dimmer you need to be able to consistently trigger the triac at a particular part of the AC cycle, either near the end of each half cycle for dim, or nearer to the start of each half cycle for brighter.
The frequency of your trigger pulses is now critical, you need to trigger at twice the mains frequency, so that your triac conducts on both the positive and negative half cycles, and your pulses need to be synchronised with the AC mains frequency.
Yes john, I understand and agree with every word you said. I was confused with the terminology, I should have used the term, pulse triggering. I understand that we can trigger a triac with a pulse but can't stop it. We are helpless for a half cycle if we have triggered it once.
But what i'm looking for here is a way to control the RC circuit so i can change the time constant.
One idea strikes my mind but it's pretty impractical. I can use a transformer to reduce down the voltage to a level, where i can use a digitally controlled pot to control the RC circuit.
Here's what i am thinking.
Started this reply some ten hours ago, got busy doing something else and then - sensibly - went to bed.
You seem to be getting rather confused here. You are having difficulty "getting the message".
You want a "simple circuit"? Well, BillHo has given you the simple circuit. It gets no simpler than that, and it is the correct solution. {Well, the bridge rectifier can be dispensed with by using a specific-purpose optocoupler such as the EL814}
This problem has nothing whatsoever to do with "digital pots". They are not in any way a general-purpose substitute for a mechanical potentiometer. They are specifically for use as volume controls in audio circuits. There may be other purposes you can find for them, but it is a matter of finding a purpose for which their particular characteristics suit, not for attempting to find one to suit an application that happens to use a mechanical potentiometer. I am just trying to make this clear. They are simply a particular variation of an digital to analog converter.
If then you can successfully forget about "digital pots", we can examine the actual problem. You wish to implement "phase control dimming". A very (and I do mean - very) crude way to do this is the circuits you first indicated. They use analog components to implement a timing function which is gated by the DIAC to pulse the Triac; a basic variable time delay.
If you want to do this using a microcontroller such as the Arduino board, then you want to implement a variable time delay - in software. And that is what MCUs such as the Arduino are exceptionally good at! It is simple software - interrupts are not useful for this, but you need to know how to write "loop" ("non-blocking") code properly.
Actual PWM on AC - using IGBTs - is going to be even more troublesome. In general, the point is that you require strict isolation of the control circuits, and that is why you cannot get it simpler than the circuit BillHo offers.
T1 conducts when the optocoupler sends a pulse to its base. C1 discharges through the collector-emitter line.
The trigger current is limited by R2 to around 40 mA.
The discharge current time of C1 is less than 1 ms.
How about a regular pot controlled by a servo motor? ]
. Impractical indeed.
