How do you scope the mains to get the zero cross?

[Flyback]

Hi,
When you scope the mains, how do you know that where the sine crosses the zero graticule , its actually the zero cross?. After all, the mains may have a DC shift, etc etc....or the scope may be out of calibration. How would you surely know that its the zero cross where it crosses zero?

 

[Papabravo]

It depends on your setup, but If you are AC coupled the scope will remove any DC offset if indeed the power company allows any. You don't say how your probe is connected to the mains and it is possible to damage your scope if you do it carelessly.

Heard you were hiring. Is that true?

 

[Tony Stewart]

To avoid common mode noise with respect to (wrt) ground noise, you would calibrate two 10:1 probes and measure between Line & Neutral on ac input. Otherwise you could measure Neutral wrt. Gnd and see if it was necessary using 1 probe.

 

[danadak]

Scope the mains w/o blowing up your scope :

Be careful probing SMPS, you can easily damage scope if you do not
observe ground loops and isolation issues :

https://download.tek.com/document/51W_10640_1.pdf

https://download.tek.com/document/Voltage on Power Supplies_App-Note_51W-60161-3.pdf


Regards, Dana.

 

[Flyback]

Thanks, on our scope, where it shows the zero cross...gotten by superimposjng mains vac in with its negative......that isnt the zero cross...we have to add an resistor-2-diode branch to the mains and scope that on low volts per div to give us the actual zero cross

 

[Tony Stewart]

sounds like your scope has an offset under those conditions.

 

[MrAl]

Hi,
When you scope the mains, how do you know that where the sine crosses the zero graticule , its actually the zero cross?. After all, the mains may have a DC shift, etc etc....or the scope may be out of calibration. How would you surely know that its the zero cross where it crosses zero?

Hi,

The AC mains should not have any DC offset or it would blow some equipment out due to high DC current.

With a scope you can usually short out the leads and then adjust the vertical to where the scope line trace is at the 0v point on the scope graticule. You can then test the line provided you have the proper isolation. I would use a battery powered scope.
After you zero the scope, you can test it by applying a small DC voltage like 0.1v and -0.1v to make sure the line you think is at zero is now really at zero.
One thing you did not mention yet is how accurate you need to detect the zero crossing. For a 120v AC mains the peak would be close to 170v, and the negative peak around -170v. The absolute zero level would of course then be at 0.000000v down to the microvolt. However, you may only need to go down to 0.001v, or even just 0.1v, or even just 1v. This is a specification you'd have to think about. What is the allowed tolerance of the measurement.

 

[Tony Stewart]

Normally, ZVD means zero volts and not the midpoint of the inverted peak crossover.

You may want to create a leading edge pulse at 0V or centred before and after the zero point.

In my case I wanted the leading edge at zero, so I used a low sine voltage into a limiter at logic level out into an XOR with the PW = RC delay. That was an AC-coupled logic inverter and XOR gate.

Keep in mind AC coupled HPF if only 1 decade below the fundamental results in a 5.7 deg phase lead and 2 decades below line freq is 1/10th of that but then has a long RC time constant.

 

[hevans1944]

Hi,
When you scope the mains, how do you know that where the sine crosses the zero graticule , its actually the zero cross?. After all, the mains may have a DC shift, etc etc....or the scope may be out of calibration. How would you surely know that its the zero cross where it crosses zero?

I would ask you this question: why do you care where the zero-crossing of the mains sine wave occurs? In other words, what are you trying to DO?

Alternating current is always distributed by means of transformers which BLOCK DC. The only DC component in your mains power would occur if you half-wave rectified the line voltage, thereby injecting a DC component into the secondary of the "pole pig" transformer. OTOH, a full-wave rectified line voltage draws power from both the positive-going as well as the negative-going half of each line cycle, creating zero DC component currents.

You definitely do not want a DC component in the current taken from your mains supply. A DC current in the winding of a transformer will "walk" the magnetization up toward saturation. If the DC current component becomes large enough, the magnetic core of the transformer will saturate, causing the reactance of its windings to go toward zero, the current then being limited only by the wire resistance of the windings. This almost always lets the "magic smoke" out and bricks your transformer.

The zero crossing of the AC line voltage can be used for timing purposes. If this is your intent, then any DC that might inadvertently be present on the mains, i.e., it isn't your fault because you don't have a home-brew half-wave 600A welding rig connected to your mains power line, then a 0.1 microfarad capacitor can be used to block DC from entering your 'scope.

 

[MrAl]

Hi,

You just reminded me that almost all scopes have an "AC" coupling setting.
A very, very, very old scope I had many, many, many years ago had ONLY an AC coupled input setting, no DC coupling. That was a big giant pain in the neck as well as other places.
I seriously needed DC coupling so I had to build a chopper so I could input both ground and the signal to be measured at the same time. Big pain.
At the time I could not get a better scope.
These days I have a low end but decent digital scope that runs on batteries or USB. That's great for doing power line measurements.