Help me to test this home voltage regulator

[mcinque]

Hello,

I've got this home voltage regulator and I'd like to measure it's behaviour when it does the corrections, switching from the "normal" tension range.
In particular, I'd like to investigate what's really it's reaction time and what kind of wave it provide as corrected output.

I have in mind to connect a load to one of its outlet and take the reading in parallel with the DSO.

But since I'm a beginner with DSO measurements on more than 12VDC circuits and obviously I don't want to fry my unit , I want to ask you one important question.

As I know, all DSOs are earth referenced instruments, so the with the BNC ground connected to the earth. Thus, doing measurements on devices that have main earth reference is dangerous if you do not pay attention an know what you are doing (as I see in Dave's "how no to blow your oscillloscope).

In this case I'm confused: I don't know exactly how this kind of devices works, and I ignore their connections and circuitry. I fear that I might do a short circuit connecting the probe ground to the N and the tip to the L of the voltage regulator outlet.

How do you suggest to connect the probe to do this test?

[AG6QR]

If your scope isn't capable of handling full mains voltage, or if you'd rather not try, a safe technique is to find an isolating step-down transformer to bring mains down to something reasonable, around 12VAC to 24VAC or so.  This kind of step-down transformer is available as a plug-pack for devices that want AC input.  I've seen them for doorbells, irrigation systems, and landscape lighting, among other places.  They're also included as the first stage of many linear power supplies.  With no load, the output of the step-down transformer will be proportional to the input voltage.  Just connect one probe across the output of the isolating transformer.  This is a relatively safe thing to do.

Another technique, assuming you have a two channel scope that is fully capable of handling the mains voltage on its inputs, is to put channel A on one side of the mains, and channel B on the other side of the mains, and set the scope to display A-B (or set it to display A+B and then invert one of the channels).  If you can, limit the bandwidth of your scope, because any high frequencies are just noise for this purpose.  Before you attempt this, be absolutely sure your scope is capable of handling the full mains voltage, be sure your probes are also rated for mains voltage, and to be extra careful, only touch the setup with one hand at a time.  Sit on your other hand.

But I'd probably use an isolating step-down transformer.

[alm]

Another technique, assuming you have a two channel scope that is fully capable of handling the mains voltage on its inputs, is to put channel A on one side of the mains, and channel B on the other side of the mains, and set the scope to display A-B (or set it to display A+B and then invert one of the channels).

Depending on the wiring, neutral is often just a few volts above ground. So you can get away with just measuring between ground and live. Note: this does not mean you can safely clip the ground lead to neutral!

But I'd probably use an isolating step-down transformer.

Agreed, especially if you're inexperienced in this kind of stuff.

[MrAureliusR]

Hmmm .. I just stumbled onto this thread by accident and I have a couple questions. My scope says on the front 400V Max. It doesn't specify AC or DC -- does it generally mean both? Both of my probes are CAT II rated, one specifically says at x1 it can handle 200Vpk and at x10 600Vpk. The other just says CAT II. Could I test my mains using the above method? What do I ground the scope probes to? The ground on the mains outlet?

Just curious, I always like to learn how to do things the safe way!

[EDIT: My mains is 120V 60Hz, I'm in North America]

[alm]

The CAT II rating is likely lower than this 600 Vp, since a CAT II rating also implies that it can survive a certain over voltage. If the probe is rated for something like CAT II 300 V, then they should be fine for mains. Use 10x probes, there is no good reason to use a 1x probe with mains signals. Don't clip the ground lead to anything except mains ground. You can either ignore the existence of neutral all together (note that this is not the most accurate method), or use the poor man's differential method that AG6QR described. In that case you can either clip both ground leads to ground or clip them together and not connect them for anything. Make sure they don't short anything out with the ground clips. If it's only low frequency work, then you may also able to get away with just not connecting the ground leads and relying on the long round wire to the scope.

[MrAureliusR]

Okay, I'll try the differential method, clipping the ground leads together. Why will that work? Assuming I put one probe in live, and one in neutral, and nothing in the ground, then clip the ground leads of the probes together, and switch everything on...  That should work okay? I want to avoid --> 

[EDIT: It worked fine. Very cool! If you try this make sure to be very careful! I used an IEC mains cord, with an old IEC socket I had pulled out of something as the probes aren't long enough to get into a standard mains socket or even IEC socket. I just clipped the probes onto the leads off the back of the IEC socket, clipped the grounds together, and then switched that circuit on. I also made sure to tape down the cables and the probe leads, to make sure live 120V didn't accidentally slip off the table and shock me or my cat!]

[alm]

It's indeed good to use hands free probing for mains if at all possible.

[mcinque]

If your scope isn't capable of handling full mains voltage, or if you'd rather not try, a safe technique is to find an isolating step-down transformer to bring mains down to something reasonable, around 12VAC to 24VAC or so.  This kind of step-down transformer is available as a plug-pack for devices that want AC input.  I've seen them for doorbells, irrigation systems, and landscape lighting, among other places.  They're also included as the first stage of many linear power supplies.  With no load, the output of the step-down transformer will be proportional to the input voltage.  Just connect one probe across the output of the isolating transformer.  This is a relatively safe thing to do.

Another technique, assuming you have a two channel scope that is fully capable of handling the mains voltage on its inputs, is to put channel A on one side of the mains, and channel B on the other side of the mains, and set the scope to display A-B (or set it to display A+B and then invert one of the channels).  If you can, limit the bandwidth of your scope, because any high frequencies are just noise for this purpose.  Before you attempt this, be absolutely sure your scope is capable of handling the full mains voltage, be sure your probes are also rated for mains voltage, and to be extra careful, only touch the setup with one hand at a time.  Sit on your other hand.

But I'd probably use an isolating step-down transformer.

Thank you AG6QR.

Even if I'm new to DSO measurements, luckily for me I know very well how to avoid an electric shock But thank you for your kindly advice, I appreciate it!
Yes, my DSO have two inputs 300VRMS, its probes can handle the same voltage at 10X and I can limit the bandwith.
If I test live circuits with probes on 10X the DSO should survive also with 500V spikes (X10 probes means X10 attenuation so 500V spike equals to 50V on DSO input). Don't you?
Well, I can easily find a step down transformer (as you said is included almost in any device) and I really like this suggestion, you can bet I'll use this method. It's safe and practical.
Thank you! 

Thank you ALM.

Depending on the wiring, neutral is often just a few volts above ground. So you can get away with just measuring between ground and live. Note: this does not mean you can safely clip the ground lead to neutral!

You're right, in both cases.

Don't clip the ground lead to anything except mains ground.

You can bet!

It's indeed good to use hands free probing for mains if at all possible.

That's exacly what I always do!

Thank you!

[mcinque]

[EDIT: It worked fine. Very cool! If you try this make sure to be very careful! I used an IEC mains cord, with an old IEC socket I had pulled out of something as the probes aren't long enough to get into a standard mains socket or even IEC socket. I just clipped the probes onto the leads off the back of the IEC socket, clipped the grounds together, and then switched that circuit on. I also made sure to tape down the cables and the probe leads, to make sure live 120V didn't accidentally slip off the table and shock me or my cat!]

great!