Need help with zero-crossing.

[alphacat]

Maybe I didnt get you.
What is the isolation transformer for?
I mean, could it make the measurements better (somehow) or just safer (not disrespecting safety)?

 

[Hero999]

Are you 'scoping the mains?

If the answer is yes then you should use an isolation transformer for safety reasons.

Connect the item which you are 'scoping to the mains via an isolation transformer.

 

[alphacat]

I see, thank you for this advice.
By the way, I always cut off the ground wire of the cable which powers up the 'scope, since i dont want the 'scope's earth to influence the DUT.

 

[BrownOut]

I see, thank you for this advice.
By the way, I always cut off the ground wire of the cable which powers up the 'scope, since i dont want the 'scope's earth to influence the DUT.

That's not the best solution. By cutting off the ground, you've left yourself vulneralbe to your scope's chassis becomming energized at the mains voltage, a potentinally lethal situation. I never use a scope to measure mains voltage, but if I had to measure and didn't have an isolation transformer, I would not connect the ground lead of my scope anywhere in the circuit. In fact, I would detach it from the probe so it wouldn't accidently touch the circuit. You can always use two probes and set up the scope for differential measurement.

 

[Hero999]

Never disconnect the safety earth, if a live shorts to the 'scope's case you're dead.

The only time it's safe to use an unearthed 'scope is when you're running it from an isolation transformer, even then you shouldn't remove the earth to the scope but plug it into an unearthed socked which is permanently connected to the isolation transformer.

If you're just triggering the scope from the mains then you could just connect the live to 'scope probe. Better still hold the probe in your hand and it'll pick up a couple of Volts of mains hum which should be enough to trigger it.

Another method of safely viewing mains signals is to use the differential function, the disadvantage is it uses two inputs.

 

[alphacat]

I didnt know it was that unsafe.
I'll take it under consideration next time I perform such measurements.
Thanks guys.
I hope I could bring some more pics like the ones here, but this time when turning on the light bulb