Hello,

I am going to measure the 120ac and 240vac by a scope. But for a safety issue, an ac voltage step down needed to to be used. My question is which way to reduce the Ac voltage to 12vac at safe level?

Thanks

ken

 

You don't only need to step the voltage down, you also need to isolate the scope from the mains.

A 20: 1 mains transformer will accomplish both safely.

 

Using a couple 10 k Ohm resistors instead of transformer, should it be ok?
Is using 2 pronge plug to isolate the scope?

Thanks

 

Using a couple 10 k Ohm resistors instead of transformer, should it be ok?
Is using 2 pronge plug to isolate the scope?

Thanks

NO. Don't do that. What you are suggesting is a recipe for a serious accident, possibly a fatal one. You are also likely to ruin your oscilloscope.

Removing the mains ground from an oscilloscope, then linking its input to the mains, even via a bit of resistance, will make the whole thing live.

Use a step-down isolating transformer to get a safe isolated voltage, and leave the grounding arrangements on your oscilloscope alone.

Even better, forget the whole idea of measuring the mains waveform. Is there some specific reason for measuring it?

 

Even if you did accomplish measuring the120 and 240 volt mains with a scope (which I also strongly discourage for the already stated safety reasons), and you display it at full scale it would be 8 divisions. Each full division would then represent 15 and 30 volts respectively. Each sub division would represent 3 or 6 volts respectively. For that level of voltage, use a voltmeter! It is far more accurate.

 

Besides the above two posts being very seriously true, the sine wave out of the secondary of a transformer will look exactly the same. Other than wave shape, you get no useful information with an oscilloscope.

A voltmeter is more accurate by orders of magnitude as to voltage, and a frequency counter is much more accurate than the number of transitions per grid. As the frequency of the AC is held to something like one part in a million, there is not much information to be gained there.

Protect yourself and your test equipment and stay away from the mains.

 

Never Use a scope without an Isolation transformer to measure live voltages.

I found it the hard way.

KaPow!

 

The advice given above is excellent and should be followed by beginners and newbies. Like R!f@@, years ago I once made a line voltage measurement with my scope and had a big bang. I stopped, sat down, and figured out what happened. My face still gets a bit red from that.

There are ways to make line voltage measurements, but a newbie isn't likely to have the equipment for them. And there are times when you do want to monitor the line's waveform or its current. But, again, these situations aren't what newbies should be playing with. One way is to use a scope probe that is rated for the higher voltages associated with the line. Here's an example of a probe that's rated to over a kilovolt.

The mistake I made that I mentioned above is that I wanted to measure the AC line current and I had a shunt resistor in the circuit for this. I thought I had connected the shunt on the low side, but it was on the high side (I had used an unpolarized plug for the AC connection). Then, foolishly, I connected the scope probe to one side of the shunt and the probe's ground lead to the other side. When I switched the power on, bang! I had shorted the line to ground.

By the way, that's one important safety precaution when you work with higher voltages: attach your instrumentation with the power turned off. In fact, get anal and pull the danged plug out of the wall, even if there's a switch. Another safety precaution I had failed to follow was that my scope should have been powered from an isolation transformer. I got myself one immediately after that little wake-up call.

What I should have done to make that measurement was to use two of those high voltage probes and connect them to both sides of the shunt resistor. Then I'd use the scope's subtract function to display the current waveform. This is a more proper way to make an AC current measurement with a shunt.

One disadvantage of the previous method is that it takes up two scope channels. Thus, if you want to display the power waveform, you can't with a two channel scope. Instead, a better tool is to use a proper differential amplifier. The old Tek 5000 and 7000 series scopes had nice differential amplifiers, as did some of the older vacuum tube models. The Tek TM500 instruments made in the 1970's had the AM502 differential amplifier. They can still be found on ebay and have a 1 MHz bandwidth (I have one and it still works well). There's at least one Chinese manufacturer who makes a selection of solid state differential amplifiers; Cal Test Electronics sells them, as do other folks. They are not cheap (they can cost more than your scope), but how much is your life worth? A few years back I got myself one and it's a pleasure to use. They typically come with shrouded banana plugs on the front, so you can choose the device you want to connect to your circuit with.

Use the proper tools (both the instrumentation and knowledge) and you won't blow up either yourself or your scope.