How to measure firing angle after Zero cross with RIGOL1054z

[rstofer]

What is the Trigger type that i should go for in here. Pulse was what is was thinking to use. Are there anything else that i should be looking at ?

What signal are you triggering on?  If you go to the trigger menu and then select source=AC the scope will automatically trigger on the AC line.

If you are triggering on the zero crossing interrupt, I would use edge triggering and the positive going slope.

[anishkgt]

Thanks rstofer. I've managed to get this wave form
ch1 is probed to Pin 4 on the Atmel, showing the pulse during zero cross. ch2 to pin 5 where the trigger is send to the TIRAC via the MOC3023. Ch3 a second transformer as AC sine wave reference. when a pulse is send out to ch2 from pin5 of the ATmel, the wave form is like the one attached. Why is that ? what sort of wave form is that ? how can it be triggered ?

[alsetalokin4017]

(Pin 5 for the CH2 trace? But Pin 5 on the schematic is NC. Do you mean Pin 3 perhaps?)

OK, here's what I think. I'm guessing about the programming of the microprocessor.

I think the circuit is programmed to turn the triac ON for a certain time interval, which is set by the potentiometer and indicated by lighting one of the LEDs that have time durations noted next to them.

So your CH1 yellow trace is showing the zero crossings of the AC source signal. The spikes are 10 ms apart which corresponds to each zero crossing of a 50Hz sine wave. So that's good.

Now one of the detected zero crossings tells the microprocessor to turn on, and keep on, the triac for the time interval selected by the potentiometer. This is shown on the CH2 trace (if it is at Pin 3, not Pin 5). Since you are setting your scope at 5 ms/div, you are not seeing the full length of the "on" signal sent to the triac, which is going to be between 100 ms and 450 ms.

If I were sitting at your scope I'd set the Timebase at something like 50 ms/div, and use the Edge triggering on the CH2 signal. This should allow the capture of a full "ON" pulse to the triac (2 to 9 divisions). Then you can use the zoom feature to zoom in to see the finer details of where the pulse starts and stops wrt the actual zero crossings.
 
Also, your CH2 trace should probably be DC-coupled and displayed at a more sensitive V/div setting.

[rstofer]

When you trigger on the edge of Ch2, you can move the trigger point over to the left edge of the screen and get more room to display the entire duration of Ch2.
Once you get the entire screen filled with Ch2 ON, you can count the pulses on Ch 1.  Then multiply by 10 ms to get the duration.  You can also get the ON time from reading the graticule but I would be very interested in the number of pulses as I changed settings.

[nuno]

I have never used it, but doesn't the scope have a trigger mode that automatically triggers on mains zero-crossings? I guess you won't see the wave, but you'll know that the trigger point is the zero crossing.

[rstofer]

I have never used it, but doesn't the scope have a trigger mode that automatically triggers on mains zero-crossings? I guess you won't see the wave, but you'll know that the trigger point is the zero crossing.

Yes it does and it works well.  Trigger Menu -> Source -> AC

I haven't had an opportunity to see how close it triggers to zero crossing.

[anishkgt]

Well i guess i am looking at the correct wave now. is this how it is ? what are the readings i can get from this or what should i understand from this ?I can see that the pulse starts at the peak of the sine.

[that_guy]

I'm not normally mr. health and safety but I really get worried when a beginner says he wants to mess with a MOT. Those things have current enough to kill instantly and enough voltage to bridge gaps that would be good insulators at safer voltages.

[JS]

  I built a dimmer with a µC few weeks ago, for zero crossing a H11AA1, with a few resistor to mains, then a big resistor on the colector, basically the same approach as shown in the schematic a few posts ago.

  I then used 2 interrupts, one triggered by the ZCD to reset a timer, a second one with the timer to trigger the opto triac. The triggering time was very short and gave me a hard time trying to estimate the length of the pulse, best approximation was about 50 cycles of the 16MHz µC but could be wrong. The brightness of the lamp was controlled changing the timer comparator value, for min brightness the precision was really great, the last value barely made the filament red, no flickering. At the other end of the range wasn't so stable but I guess was because of not having holding current at the moment it triggered, I guess longer pulses could been used but I didn't see no real reason to do so, just call maximum a few µs longer timer.

  Normally inductive loads are driven without much of a change in the circuit, but the changes in the power control circuit. you could time the interrupts in your µC to be very precise once you know how your zero crossing works, in the time domain, with a scope shouldn't be a problem, maybe look at the ZCD interrupt moving a pin of your µC and showing it in the scope, to have a number for the delay. If you want to hold it as the max power, leaving the LED on is an option, will trigger as soon as the voltage reaches the minimum and would be very short so the spike isn't terrible, even if it is, the power triac would be triggered for the dV/dt or excess voltage in the worst case. Do you have the smulations of your load (MOT) how behaves when triggering at different times/phases of a sinewave?

JS

[alsetalokin4017]

Well i guess i am looking at the correct wave now. is this how it is ? what are the readings i can get from this or what should i understand from this ?I can see that the pulse starts at the peak of the sine.

OK, you are getting there. (although you have apparently changed the channel connections to the circuit.) Your "on" pulse can be seen to be 100 ms duration, starting and stopping at peaks rather than zero crossings of the CH1 sine wave. Now you need to determine whether that CH1 sine wave is really "in phase" with the actual power that the triac is supposed to be controlling.
 
Also, why are you still using AC coupling on the CH1 signal? Normally we would use DC coupling unless there is a very good reason for using AC coupling. In this case it probably isn't very important (AC coupling removes any DC offset that may be present and there may not be any DC offset in this actual signal) but it is better to be sure rather than guessing. AC coupling isn't "for measuring AC", rather it is (mostly) used for removing DC offset.

[anishkgt]

Hi JS,

you could time the interrupts in your µC to be very precise once you know how your zero crossing works

Did not understand what you meant by how it works ? the zero crossing is detected by the H11AA1 and the scope is probed on pin5 of it which goes to pin4 on the ATmel.

maybe look at the ZCD interrupt moving a pin of your µC and showing it in the scope, to have a number for the delay.

Moving a pin - you mean move it detect on another pin ?. There is actually two pulse the pic shows the first pulse.
nope no simulation and did not understand the last paragraph

Hi alsetalokin4017,

Your "on" pulse can be seen to be 100 ms duration, starting and stopping at peaks rather than zero crossings of the CH1 sine wave

From what i learned is that an inductive load should not be switched on at the zero cross, so now from what you said it should switched OFF at the zero cross, am i correct ? i've corrected the switch off at zero cross now. pic attached. How do i check if ch1 is in sync with the TRIAC ?


Thanks guy for the answers.

[oldway]

You have only control on "switch on" time of the triac.
When gate signal goes to 0, the triac does not "switch off" imediately, the triac stay conducting until the MT1/MT2 current drop below the holding current for a sufficient time.

[JS]

Hi JS,
Did not understand what you meant by how it works ? the zero crossing is detected by the H11AA1 and the scope is probed on pin5 of it which goes to pin4 on the ATmel.
[...]
Moving a pin - you mean move it detect on another pin ?. There is actually two pulse the pic shows the first pulse.
nope no simulation and did not understand the last paragraph
...
Thanks guy for the answers.

  You are probing your analog signal, that way you know how the H11FF1 works, nothing about how fast the µC responds. Make an ISR when the µC detects the zero crossing and with it write a pulse in an output pin. Then time the difference between one signal and the other, or even better, the actual zero and the µC output. That is the time you have as delay, it should be short enough it doesn't represent a problem working at 120 pulses/second, from the 60Hz mains, but is good to be sure.

  About how it works I'm referring to the timing, once you have enough data to do the timing between the analog and digital behavior, that behavior should be consistent. Note that there isn't any more info in plotting 12 pulses than just 2, but when plotting 2 pulses you can see much better what's going on, also you should use some markers one the sine wave to compare how well you are timing from it to the pulses.

  I hope that clears the digital part a bit, for the power handling, which was the last paragraph all about. When working with inductive loads, the thing to be carefull about is when you switch off, since it kicks back as a mule, trying to keep the same current raising in voltage as much as is needed to do so, a 5V relay can kick several times those 5V if disconnected without using the protection diodes. When you are working on AC you can't just hook up a diode to solve this, so it's easier to just switch when the current is low enough so the kickback isn't a problem. Good luck the triac doesn't turn off till the current is low enough, so using a triac solves that by it self, you only need to be little careful. If you go to the datasheet of the triac you are using you will see the typical application circuit (Figure 13 in the link) which has 2 extra resistors and 2 caps that the one resistive load doesn't need. For the turn on, there are no spikes for voltage or currents with an inductive load, the transient of the inductor will take care of that, for capacitive loads you could have a huge current spike, look at the initial charge of a filtered rectifier.

http://www.mouser.com/ds/2/149/MOC3023M-196223.pdf

  You would probably still need some trial and error to see the optimal times for your interrupts to trigger the triac, it will depend on how inductive the inductive load it is, for resistive loads the fastest you turn it on (as far as the holding current is reached) the higher the voltage in the load. If the load is only inductive, the zero crossing current will be at the peak of the voltage, so you need to trigger after that to have the holding current, also the triggering pulse should be longer (than a resistive load) as the inductive load will take longer to reach the holding current.

  You still have the problem of your load changing as what you are doing with the other side, the load with the secondary open will be very inductive, the load with the secondary shorted (while soldering) will be very resistive. Maybe is a good idea to sense the zero crossing in current, maybe both.

JS

[anishkgt]

Thanks sounds clearer

Now to see how long the TRAIC is switched on and when it switches off would it be ok to connect a 1.5k resistor at MT2 and another at the neutral side then probe it ?

[anishkgt]

silly me, 1.5k wouldn't work, maybe 1M.

[JS]

  As you have a transformer I'd recommend to probe the secondary which is insulated, and the voltage should follow pretty closely the primary. You still have a 4 channel scope with math capability, you can use 2 channels as a differential one and make the subtraction in software. Of course you should only use the signals to do the probing (in 10X for this application) and leave the grounds alone as they are already connected internally.

  Another option to work safe with mains measurements is to use a transformer, maybe a tiny 12V would do the job, first measure the mains and the secondary of the transformer, make the division and write the number on the transformer, this would be the turns ratio, then you just plug the primary to mains and the insulated sec to your instruments. The limitation with this is the bandwidth, which should extend up to a few kHz. Other limitations of this method is the low input impedance which is not a problem for mains, there could be saturations as the transformer is unloaded and not at nominal load and as I said the bandwidth. What I'm trying to say here, the method isn't good for anything else other than mains measurements, but for the kind of things one is looking to see in mains the transformer is a valid method.

  I'm not saying the resistors wouldn't work but, it isn't safe, you would have no idea what are you measuring level wise as the attenuator ratio would be unknown, etc

JS

[anishkgt]

Thanks JS.
 The transformer that you mean is the MOT, correct ? It has about 600 to 800Amps. would that be a problem for the O'scope even when measured at 10X ? or can it measure that much amps ?

...............2 channels as a differential one and make the subtraction in software.

how is that done ?

[JS]

Thanks JS.
 The transformer that you mean is the MOT, correct ? It has about 600 to 800Amps. would that be a problem for the O'scope even when measured at 10X ? or can it measure that much amps ?

...............2 channels as a differential one and make the subtraction in software.

how is that done ?

 O'scopes doesn't care about no currents bro, they just care about voltage, if your sec is unloaded the sec would have a tiny fraction of the pri voltage and no current, even if it can handle 1kA. If your sec is shorted the voltage won't be any good as you are in the limit (or out) of the transformer's confort zone.

  Press the button on your scope that says 'math', from there the menu should help you do the rest. Do not connect the grounds to anything. Just the tip of your probes to whatever you are measuring, let's say channel 1 and 2. Then you select math, and there you'd have the option to subtract, select the 2 channels 1 and 2 and it will plot a new signal in a new color representing Cmath=C1-C2. You can turn off the plots of C1 and C2 as they no longer represent any information for you, and just use the Cmath to work as usual. Input triggering, gain, sensitivity and time base should still be made over the physical channels, not the math one. Remember to use the same sensitivity range in both input channels for the difference to be true. In any case you should give it a proper read to the user manual of the scope, it will explain all this plus a lot of other functions it has, some of them referring how not to kill the dude using it. It's not as door handles, you aren't supposed to know how to use one of this, it's ok to go to the manual even if you have used a bunch of scopes, as the manual will revel some functions you didn't even know were there.

JS

[anishkgt]

So i've connected the MOT secondary on ch4 and here is how it looks. The MOT switches on at the peak but as suspected switches off half way after zc.

i heard aternistors like the http://www.farnell.com/datasheets/1723975.pdf?_ga=1.14963187.479192878.1474453029 are better at switching off at the zc and they are said to be snuberless.

How good would those be here other than it can handle high current ?