what I understand, the snubber isn't really needed for a purely resistive load, but it would be a good thing to have anyway (it can't do any harm and may prevent some false firing).

Two comments:

1. High values of dV/dt will turn on a triac. A snubber prevents this.

2. Triacs require a require a minimum load. Providing one means an open load means you measure 120V where the load would be even when off.

3. If your doing phase angle firing particularly, you have load options to consider.
1. Resistive heating - make P (0-100%) relate to V^2
2. Tunsten - e.g. UV heating
3. Inductive - Thing turns off at V=0 and I=0; Current limit is required

P is typically the variable for the output of a PID controller. It can vary -100 to 100% for heating/cooling application.

So, for resistive heating, you have to relate phase angle to 0 to 100%, to average voltage, squared. Since R is resistive and a constant, the 0-100% is linear related to power.

 

I'm quite confused as to why R8 is required in this circuit.
Here is a simulation with R8 in:


and here is a simulation with R8 out:


The top plot is the voltage across the load (both plots OK). The middle plot is the gate current (I put R9 in there just to measure the current). The bottom plot is the Gate to A2 voltage.

So the second (without R8) seems correct (to my limited understanding!): there is a current pulse at the gate, which goes high wrt A2. The current plot in the with-R8 simulation seems very weird. Could just be a simulation error, of course.

 

Hi Eric ... did you have a chance to try out the LTSpice model (post #20)?

(Here they are again).

Robert

 

hi R,
Missed it earlier, downloaded it now, let you know.
E

The MOC emitter current exceeds the 30mA recommended.

Have you checked the power dissipation in the Triac.???



It looks OK, the delay from ZCD to Trig seems longer than expected.

EDIT:
It may not be an inductive load, but as a MCU is being used and possible long wiring runs, I would fit a R/C snubber.
Use a X2 rated mains cap.

 

Hello,

Another thing to check is the gate power dissipation. This is often overlooked when a constant gate signal is used. It may be ok though for this circuit.

If you turn the load on for several seconds and off for several seconds you dont need phase control. I think this is what someone else was mentioning, Then the power control is linear in that 1/2 time on and 1/2 time off leads to 50 percent power. With phase control however it is not linear at all because the voltage varies as the sine or cosine function throughout the range of phase angle. Thus 45 degrees does not lead to 1/2 the power that 90 degrees provides, although 90 degrees does lead to 1/2 the power that 180 degrees provides. So the power ratio is not linear over the whole phase range.

 

hi R,
Missed it earlier, downloaded it now, let you know.
E

The MOC emitter current exceeds the 30mA recommended.

Have you checked the power dissipation in the Triac.???
View attachment 161917

WIP

Thanks Eric ... I think the MOC3020 is OK. The max repetitive rating is 1A for 1ms, 120pps, so as long as the firing pulse is 1ms or less it should be OK (I think).

Yes, the BTA16A rating is about 7W at 7A(RMS) which should be OK for this heater. I'll need a big heatsink! https://www.jameco.com/Jameco/Products/ProdDS/1384296.pdf

 

https

Hello,

Another thing to check is the gate power dissipation. This is often overlooked when a constant gate signal is used. It may be ok though for this circuit.

If you turn the load on for several seconds and off for several seconds you dont need phase control. I think this is what someone else was mentioning, Then the power control is linear in that 1/2 time on and 1/2 time off leads to 50 percent power. With phase control however it is not linear at all because the voltage varies as the sine or cosine function throughout the range of phase angle. Thus 45 degrees does not lead to 1/2 the power that 90 degrees provides, although 90 degrees does lead to 1/2 the power that 180 degrees provides. So the power ratio is not linear over the whole phase range.

Hello MrAI,

I don't want to turn the load on and off for seconds at a time because the application is a coffee roasting machine: air is blown over the heater and this heats the beans. Because of the air flow, turning off the heater element for a short time causes a fast temperature drop, which isn't good. So what I'm planning to do is to fire the triac for full half-waves (so little EMI and low gate power), but to 'knock out' half-waves as needed. So for 50% heat it would be On/Off/On/Off, 75% would be On/On/On/Off etc. Over half a second this will give me 2% control (but as power control under 50% is not needed, I can go from Off to 50% and 50%..100% in steps of 1% using 50 half-cycles). Sorry for the obfuscated explanation

 

hi R,
As you are using burst control for the Triac On period, after the first ZCD sync pulse into the MCU I would keep the Triac On until the required Off time and then re-sync the Off trigger. [ ie: remove the emitter DC On voltage from the MCU].
E

EDIT:
Just seen post 27, while I was typing.!

 

hi R,
As you are using burst control for the Triac On period, after the first ZCD sync pulse into the MCU I would keep the Triac On until the required Off time and then re-sync the Off trigger. [ ie: remove the emitter DC On voltage from the MCU].
E

EDIT:
Just seen post 27, while I was typing.!

Yes, what I'm aiming at is half-way between phase control and burst mode ... just to be difficult! Also, I would like to keep the option open to proper phase control, although I don't think I'll need it.

My problem with this circuit is that I don't have a proper understanding of a triac. For example, I don't understand why R8 appears to be needed - the simulation doesn't, but presumably there is a reason it should be there. Also, with R8 in, there is current into the gate for the whole cycle, which is not all good (going back to your point about the MOC current rating and also MrAI's point about gate power dissipation).

 

hi R,
As the Load is connected to MT2 of the triac in the Mains Line I would not have R8 in circuit, if the Load is in MT1 ie: the Mains Neutral I would fit R8.
E

 

hi R,
As the Load is connected to MT2 of the triac in the Mains Line I would not have R8 in circuit, if the Load is in MT1 ie: the Mains Neutral I would fit R8.
E

Is there an advantage to either options? Obviously, if I can leave out a resistor so much the better. What is the function of R8?

The other thing is that there's over 30V drop across the triac ... which I hope is wrong, or I'll have some fried triacs to sell!

 

https


Hello MrAI,

I don't want to turn the load on and off for seconds at a time because the application is a coffee roasting machine: air is blown over the heater and this heats the beans. Because of the air flow, turning off the heater element for a short time causes a fast temperature drop, which isn't good. So what I'm planning to do is to fire the triac for full half-waves (so little EMI and low gate power), but to 'knock out' half-waves as needed. So for 50% heat it would be On/Off/On/Off, 75% would be On/On/On/Off etc. Over half a second this will give me 2% control (but as power control under 50% is not needed, I can go from Off to 50% and 50%..100% in steps of 1% using 50 half-cycles). Sorry for the obfuscated explanation

Hi,

Ok good. Did you check to see if the heater element can handle DC too? Some elements can only handle AC because they have an inductive part that only works with AC. They might say "AC only" stamped on them somewhere.

 

The other thing is that there's over 30V drop across the triac ... which I hope is wrong, or I'll have some fried triacs to sell!

hi R,
It is possible the Triac LTS model is not correct, that dissipation is high compared to the d/s.
E

 

Hi,

Ok good. Did you check to see if the heater element can handle DC too? Some elements can only handle AC because they have an inductive part that only works with AC. They might say "AC only" stamped on them somewhere.

Hi, would only positive half-waves (say) constitute DC? I suppose so. Unfortunately it will take me a while to get the information on the heater.

What I could do is to allow full waves to get over the possible issue.

 

hi R,
It is possible the Triac LTS model is not correct, that dissipation is high compared to the d/s.
E
View attachment 161921

Yes, like 120W versus around 6W!!

I've had a look at the MOC3083 from the LTSpice group. You may be interested in having a look (I've done some cut-and-pasting on the Htr_4a.asc file in the yahoo group (attached).

By varying the pulse period the voltage can be nicely controlled. The only thing is that synchronization would be needed, so some form of basic zero-cross would be required (no need for hysteresis I don't think).

 

Well I think the MO308x could be used like this:
1. Get interrupt at around zero cross (say +/- 3ms)
2. If half-wave after this coming one needs to be on, wait 4ms then turn on opto for 10ms.

This shows an example which assumes the zero-cross was detected 3ms after actual zero-cross:

The top plot is just the AC, for reference. The middle plot is the half-wave that has been turned on. The bottom plot is the opto trigger signal.

It's more convoluted software-wise than the MOC3020 method, but it has the advantage that the whole half-wave will definitely be turned on, at the right time.

 

hi,
So have you decided to use burst mode with the MOC3083.?
E

 

hi,
So have you decided to use burst mode with the MOC3083.?
E

Hi Eric,

No, definitely not burst mode, and at the moment I'm not sure about which way to go ... just checking out the options.

Here is an example using a PWL file (my first time trying this!) to show two half-waves being turned on at different times. The 'zero-cross' was detected 3ms late for the first half-wave, and 3ms early for the second half-wave:



What do you think? Pros and cons, I guess.

Here is the Lite-On application example for the MOC308x:



Is there an advantage, to your mind, of putting the load on the hot or neutral side? They seem more or less equivalent and I can't really see any pros and cons.

BTW ... many thanks for all your help!! This is an area I'm very dodgy on, and switching 2KW 240AC loads isn't something I particularly like to do ... no doubt I will have the smell of burning hair before it's over

 

Hi Eric,

No, definitely not burst mode, and at the moment

Here is the Lite-On application example for the MOC308x:

Any particular reason?
The Moc308x is designed for zero cross?
It is customary to switch the live side with the load between switching device and neutral..
Max.

 

Any particular reason?
The Moc308x is designed for zero cross?
It is customary to switch the live side with the load between switching device and neutral..
Max.

Hi Max,

Well, definite is a relative term for me. But the reason I said that I didn't want burst-mode is that it's important for the heater not to be off for more than a very short time. Air is blown over it and the hot air heats coffee beans, so the temperature drops rapidly as soon as the heater is turned off. So my intention was to turn half-cycles on as needed (so for 50% it would be On/Off/On/Off ... etc)

However, I can probably get away with the heater off for 2 seconds (absolute max). In that case I could work off 100 cycles and for 50% I would turn on for 1 second and then off for 1 second. For 90% I would turn on for 1800ms and off for 200ms. The issue with that is that if I want 95% I'm at 1900ms and 100ms (or 5 cycles): so 5% control is the best I'll be able to achieve.

On the other hand, if I selectively turn half (or even full) cycles on and off I can get to 1% control accurately (which is better than I need, but 5% is a bit high).

I can use either the MOC302x or MOC308x to turn on half-cycles selectively (as I show above). The advantage of the MOC308x is that it handles the zero-cross switching, so my timing doesn't need to be so precise (and I can simplify the zero cross detect for the microcontroller, which will still be needed for synchronization). I don't know if that makes sense or not?

I'll modify the circuit for hot-line switching (although I still don't understand the pros and cons of having the load on the live or neutral side).

Cheers

Robert