triac phase control

[anandpatel27]

i want to control the phase angle of the triac by ic moc3041. is it possible to vary the phase angle by PWM output of the microcontroller.?? what shound be the input to the moc3041 ic?

 

[eblc1388]

Wrong choice.

The MOC304x series is designed for Zero-Crossing switching thus has internal circuit to ensure firing at zero crossing so no phase angle control is possible.

You need to use MOC302x series instead which is designed for phase angle control.

MOC3020, MOC3021, MOC3022 and MOC3023 Non-Zero-Crossing Triacs

 

[anandpatel27]

ok i ll tel u my appplication . i want to control the voltage to the primary of the transformer whose secondary is connected to a heating element . i want to control the temperature with the help of microcontroller. so to synchronize the a voltage and firing pulses as in i would have to use an external zero crossing detector circuit. so what di you suggest. is it possible by PWM?

 

[Nigel Goodwin]

As you're wanting to do 'temperature' control, try using 'burst fire' control instead of 'phase' control - your zero-crossing opto-isolator would be fine for this.

Generally you don't use phase control for heating!.

 

[anandpatel27]

thank you sir for the suggestion but could u please elaborate what is burst fire control . have not heard of it before..

 

[Nigel Goodwin]

thank you sir for the suggestion but could u please elaborate what is burst fire control . have not heard of it before..

It's the standard way of varying the power of a heating element - you turn it ON for a while, then OFF for a while - the ratio of the two is the power you're applying to the load (it's basically slow PWM).

So if you had a 1000W heating element, and you turned it ON for 5 secs, then OFF for 5 seconds, the load would receive 500W.

The thermal inertia of the element or load smooths the pulses out.

An obvious way to do it with AC mains to count the number of mains cycles, and turn ON and OFF accordingly.

 

[anandpatel27]

It's the standard way of varying the power of a heating element - you turn it ON for a while, then OFF for a while - the ratio of the two is the power you're applying to the load (it's basically slow PWM).

So if you had a 1000W heating element, and you turned it ON for 5 secs, then OFF for 5 seconds, the load would receive 500W.

The thermal inertia of the element or load smooths the pulses out.

An obvious way to do it with AC mains to count the number of mains cycles, and turn ON and OFF accordingly.

but i want to increase the temperature linearly to a fixed value and maintain it there.. say 500 degree cel .. from room temperature i want to rech there. is it possible by this on off method .. also i have transformer and the heating element is attached to the secondary of it.. so isnt it difficult to constantly on -off an inductive load

 

[ronsimpson]

Anandpatel27,

1) If you control the phase angle of the SCRs you are switching an inductive load on/off.

2) If you use Nigel’s method of PWM at a frequency of 0.1 hertz you are switching an inductive load on/off. This method is used in ovens and stoves. It has been used before you were born.

3) You might consider switching on/off faster. One cycle on, one cycle off, two on, one off etc. Do not switch on for ½ cycle!

Method 2 & 3 use zero crossing and have low loss. All three will work to control the temperature.

 

[Nigel Goodwin]

Anandpatel27,

1) If you control the phase angle of the SCRs you are switching an inductive load on/off.

2) If you use Nigel’s method of PWM at a frequency of 0.1 hertz you are switching an inductive load on/off. This method is used in ovens and stoves. It has been used before you were born.

3) You might consider switching on/off faster. One cycle on, one cycle off, two on, one off etc. Do not switch on for ½ cycle!

Method 2 & 3 use zero crossing and have low loss. All three will work to control the temperature.

You haven't mentioned the BIG advantage of burst fire control, in that you don't require mains filters - because the higher the power, the more expensive the filters - so it's not very practical to have heater sized filters.

But as he appears to wanting to simply maintain a stable temperature, I don't see as he requires any form of 'power' control? - just a thermostat type control, and if he wants it to be as stable as possible, using P.I.D. to control it.

 

[Hero999]

A fully loaded transformer isn't very inductive, it behaves more like a resistor.

If you're worried then switch the secondary or use a snubber network.

 

[Nigel Goodwin]

A fully loaded transformer isn't very inductive, it behaves more like a resistor.

If you're worried then switch the secondary or use a snubber network.

It's even less of a concern, because he's using (or will be) zero-crossing.

 

[anandpatel27]

thanks a ton for everyone's help

 

[anandpatel27]

need help again

ok guys i need your help again..

i found something similar on the net to what i want..

**broken link removed**

in the fig 1. first the have used a ready solid state relay and second a separate zero crossing circiut.

if i use moc3041 and triac separately is it possible for me to achieve the same..

 

[ronald_jansen]

Follow-up question

I have a similar question.
We also have to control a heater with transformer. The heater is used in a sealbag sealer.
The heater is a steel ribbon with very low resistance. The transformer is a ring-core transformer.
I got this setup from a previous designer, I am not sure which were the design considerations.

The old controller (which we are replacing) applied power to the heater for a set time for each sealing cycle.
The problem was that when the heater was still warm from last cycle it would overheat, damaging the sealbag and even the heater itself.

To solve this I designed a controller with a fast temperature sensor and a triac to switch the heater (+ transformer). I use a MOC3021, but with software zero crossing detector.

Everything seemed to work fine, but when the customer tried it he found some problems.
At his test bench the fluorescent lights start to flicker as a result of the frequent switching of the heater. (period is 10 mains cycles).
I thoroughly discussed this problem on a dutch forum.
They told me that as it is an inductive load I should not use zero-crossing. Also the transformer might saturate during start-up.

I do not think phase cutting is the way to go. The transformer is heavily loaded with an ohmic load. I think it will behave mainly ohmic, making phase cutting a large EMI risk.
I did see current surges of appr. 4A even when the heater was detached from the transformer. So the transformer start up problem seems to be a real problem.

What I also found, and which I don't understand, is the following:
When powering the transformer only for single cycles I get the following phenomenon.
I see the triac current rise in a sinusoidal way, then when it is at appr. 2/3 of the amplitude suddenly it drops to (almost?) zero. Then after a while at again about 2/3 amplitude it jumps back to being a sinusoidal shape.

What is going on here? Could this be some measuring error? I use a 50mr shunt resistor and set the oscilloscope to differential measurement. Could it be that the common mode rejection fails periodically?

 

[Hero999]

How much power does the heater require?

It think it's probably more likely that the mains voltage is dropping slightly when the heater is turned on causing the lamps to dim slightly.

You could put the heater on a different circuit to the lighting or increase the crossectional area of the cable feeding both the heater and lighting.

 

[fixitguy]

heat control

You might try a simple infinite heat control that is used on many electric ranges. Make sure it isn't a solid state type.
repairclinic.com is a great source and they would probably help you find one. It would be great just to get a used one and try it out to see if it's what you need.

 

[Hero999]

As far as I'm aware all modern stove controls are solid state.

It would still cause the same problems, I don't think it's EMC that's the cause here. He's using zero crossing so it should be better EMC wise than switching the heater with a relay.

 

[ronald_jansen]

Actually there are two versions of the same product.
one uses a 220V heater, the other uses the low voltage, high current heating ribbon through a transformer.
My customer says that he sees the problem with both variants.

Meanwhile I took a shunt resistor of 1R rather than 50mr. Since then I did not see the "cut sinus" phenomenon again. Of course after not to long the shunt starts to smoke as a result of high dissipation. So until I get a high power shunt I can only test for short periods.

I disconnected the heater from the transformer and measured the current. This peaks unregularly during transformer startup. The most powerful ones are about 30A. This sound like a good explanation for dimming lights.

However, with the resistive-only heater this cannot explain. I am starting to believe that the zero crossing detector might not work correctly.
I will try testing this this week.

If so, it might be that it also uses half phases, giving a rectifier-like effect. In that case I can imagine the transformer current going out of hand.

If that is not the case, I might have to conclude that it is not possible to frequently switch a transformer.
Or I should go to phase cutting. But in that case I would definitely need a big inductor to keep it EMC.

 

[Hero999]

Yes, the zero crossing might not be working properly.

Also make sure you have the same number of positive and negative cycles going through to avoid the rectifier effect.

 

[ronald_jansen]

Ok, will do some testing this week and tell you what I found