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Need help with zero-crossing.

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alphacat

New Member
Hello.

I programmed my unit so every time I want to switch an appliance with the relay, ON or OFF, the switching will happen at zero-cross of the line voltage.
I want now to check with the oscilloscope that the switching indeed happens at zero-cross.

I connected to the relay a light bulb, and connected the probe's pins to both legs of the light bulb.
Now, I dont know how do I tell the scope to freeze the picture when the 220Vac turns zero, so I can see at what voltage of the light bulb, it dropped to zero, hopefully it will be a very small voltage thats close to zero.

If it was 220VDC, I would put a the trigger line a little above zero, and would tell the 'scope to freeze the picture when the voltage crosses the trigger line from above downwards.
But how do you it with AC, when it always crosses the trigger line?

I'd appreciate any help on this.
Thank you very much.
 

alphacat

New Member
If instead of a light bulb, i'd use a AC to DC transformer as an appliance, would it be ok, or would it create some time delay, since in the transfoermer's output there's a large capacitor along with the rectifiers?
If its ok, I though of placing the probes on the DC output of the transformer, but again, i'm not sure if there'll be a phase delay between the input (AC) zero cross and the output (DC) voltage drop to zero.
 
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BrownOut

Banned
Don't use the internal trigger. Connect the "ext" to the appliance you're turning on. Then, the 'scope triggers only when you switch it on. You can capture the 22V with the probes.
 

alphacat

New Member
Thank you very much.
I've never used before the "ext" option of the 'scope (which is a digital scope).
I connected the appliance to the "ext" input of the 'scope.
Should I config anything in the scope or does it freeze the picture automatically when the "ext" voltage changes from zero volt?
 
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BrownOut

Banned
You have to choose external trigger on your "trigger source" selector. Then just set the levels as usual.
 

alphacat

New Member
Alright.
* I connected "EXT" to the light bulb.
* I connected CH1 to the light bulb also.
* I set the trigger source to "EXT".
* I set Coupling to AC.
* I positioned the vertical trigger line to the middle of the screen.
* I positioned the horizontal trigger line to 0V.
* I set the trigger's Edge Slope to Rising (Its either Rising or Falling, no other choice).
* The time scale is 10mSec (the line frequecny is 50Hz).

Experience's results:
1. When I turn on the light bulb, there's no trigger, the sine wave keeps going on the screen.
2. When I turn off the light bulb, then trigger happens, the problem is that sometimes you can see the cut off of the sine wave at the middle of the screen (and indeed the cut off happens close to zero :) ), but sometimes i see at the middle of the screen a complete sine wave, i dont see where its cut off.
3. I didnt manage to cause a trigger when I turned on the light bulb. I tried changing the trigger's Edge Slope to Falling but it didnt help.


I dont understand, when does the trigger happen?
What should happen in the "Ext" voltage for a trigger to happen?
Is there anything I can do to fix it (what happen in result #2)?

I really thank you for teaching me this powerful measuring tool.
 
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BrownOut

Banned
Adjust the horizonal trigger level until you get the scope to trigger where you want. Sometimes you just have to fool around with these things.
 
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alphacat

New Member
I'll try playing with it some more (hopefully the light bulb wont burn :))

Could you tell me please, when the "ext" creates a trigger?
For example, If I set the Edge Slope as "Rising", and place the horizontal trigger on 0V, then the "Ext" will create a trigger when its voltage crosses the "0V" upwards?

Its weird that I never get a trigger when I turn on the light bulb.
 
Last edited:

BrownOut

Banned
For example, If I set the Edge Slope as "Rising", and place the horizontal trigger on 0V, then the "Ext" will create a trigger when its voltage crosses the "0V" upwards?

Yes, but if your curcuit is switching on at a small voltage above zero, you might miss it.
 

BrownOut

Banned
You can go back and try to use Ch. 1 as the trigger input. When I suggested using the ext. I thought you were connecting Ch. 1 to the mains. Since Ch. 1 is in the "shitched" side, you can trigger with it.
 

alphacat

New Member
Thank you for the help!
i'll try it out.

By the way, here is the difference between using zero cross and not using it.

:)

I wonder, what is that little negative hill in the "without zero cross" picture?
Where could it come from?
Each square is 10V, and the probe attenuates the light bulb voltage by 10x, so I guess the hill's peak is around -20Vpeak.

with zero cross:
code-with-zero-cross-jpg.30761


without zero cross:
code-without-zero-cross-jpg.30760
 

Attachments

  • Code without zero cross.jpg
    Code without zero cross.jpg
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  • Code with zero cross.jpg
    Code with zero cross.jpg
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Last edited:

BrownOut

Banned
Hmmm... that little hill could be some artifact you get from the fast switching, which is what you want to eliminate with zero-crossing in the first place :)

Cool pics. Thanks for sharing.
 

Hero999

Banned
Are you using an isolation transformer?

You should be!

You could connect a small mains transformer to the mains and monitor the secondary with your scope.
 

alphacat

New Member
Hey,
If you suggest the small AC mains to AC transfoermer for safety means, then I'm already being much careful when dealing with AC mains.
 

Hero999

Banned
What do you mean? Your post is really ambiguous.
 
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