Hello,
I am trying to find the best way to isolate a circuit from mains for probing with an oscilloscope. I generally run the mains 120v into a variable transformer for testing. I was thinking that I could connect a 1:1 isolation transformer between mains and the variable transformer. Does anyone have a better method?
I also found that isolation transformers can be extremely expensive. Especially since my needs are 120v 15-20A (1800VA-2400VA). I don't necessarily need 15-20A but I would feel much more comfortable if it was rated near the values of the variable transformer.

I don't know much about transformers. Would it be possible to set up something relatively inexpensive? For example using a vary large ferrite toroid?

Any good ideas would be appreciated!

 

You do not need such a large iso-transformer for the O'scope.

You would VAPORIZE it.

You can tap off your existing secondary with a few dollar 1:1 transformer.

You only need a few mA to get the scope a signal.

 

Ah, I was actually trying to isolate the entire power supply for the circuit not just the scope but not a bad idea.

 

Any transformer isolates. Isolation transformers are an odd name for 1:1, but all they do is isolate.

A 12v:120v transformer should be called an Isolation Step-up Transformer

And 220v:24v Should be called an Isolation Step-down Transformer

Transformers, by nature, isolate.

 

Yes, but unfortunately I am using a variac for the power supply for testing and was told that it doesn't isolate like other transformers. Correct me if I am wrong. As I said I know little about transformers.
I am trying to isolate the variac from mains.

 

Then yes, you will need a 1:1 transformer which is rated at or above your peak usage level.

 

Can you draw a schematic of what you are trying to A schematics say more than 2000 words

 

Battery powered, all singing, all dancing, scopes are fantastic for connecting to mains and noisy circuits to get clean waveforms, compared to a mains powered scope or even a scope powered through an iso transformer (as there is still residual capacitive coupling). That said, the act of 'probing' is still highly dangerous, so best to set up the scope probes with the circuitry de-energised.

 

I never did isolate equipment under test, although I have used a variac to power the equipment under test.

I ALWAYS isolated the o'scope when isolation was needed though. I suppose it's 6 of one, and half a dozen of the other, in this case.

 

It's always a good idea to know what you're probing. I watched an IBM tech vaporize about 1/8" of a probe tip while poking into a piece of IBM equipment.

I don't know how, but the scope (H-P USM107) lived.

 

Here is what I had in mind.

Mains(120v outlet)---->isolation transformer---->variac(variable transformer)---->circuit being tested---->oscilloscope probes.
I do not have a bench supply so I have been building circuits with filtering and using the variable transformer for the power supply. I was trying to find a way isolate everything from mains either just before or after the variac without purchasing an expensive 1:1 transformer.

 

You can use two transformer alike to do the same thing. Say two 120v - 12v transformers, they must be the same rating/size. Connect the 120V to the primary of the first trans, then connect the secondaries of both transformers together. Then the primary of the second transformer will give 120V out to the project/scope. For some reason the step down transformers are cheaper and easier to find. I would get a bigger pair of transformers than you think you will ever need and keep them set up just for this.

 

You don't say what it is you're trying to accomplish. Why are you trying to probe circuits with line voltages with your scope? This is something that really should only be done by experienced folks who know the hazards and have the proper tools to minimize the hazards.

One approach is to connect two like-rated transformers "back-to-back" to make your own isolation transformer like shortbus recommended. Another approach is to look on ebay -- I got a very nice, almost new 1 kVA medical-grade isolation transformer for around $70 delivered. If you're going to be working on line-powered stuff, an isolation transformer is a good idea.

However, remember that an isolation transformer certainly won't protect you from the hazardous voltages that will be present. I like to see folks use a decent differential amplifier to make their measurements with a scope -- and it has the benefit of not tying up two channels of a scope. That said, you can also use the poor-man's differential transformer of connecting two scope probes to the points you want to measure the voltage between, then subtracting the two signals.

For each measurement, you should know exactly what you're measuring, why you're measuring it, and what decision you'll make with the data once you have them.

In addition, you need to have slow, safe, methodical, thoughtful measurement procedures. For each measurement, this should involve shutting the AC power off, verifying that it's off, making the scope probe connection(s), then turning on the AC power while you're safely away from the circuit under test. You should also make a habit of never putting a ground lead on a scope probe while you're probing around in areas with line voltage. This is a good practice that I had to learn the hard way -- the only reason I didn't get a shock was because of the rubber boot around the alligator clip. But I did get a nice, blue spark with a big bang. Oh, also have an experienced person standing by who can help you if something goes wrong.

As to the hefty rating of the isolation transformer, I would be surprised if you need it. You can certainly use e.g. a 500 VA isolation transformer with your more heavily-rated Variac -- just make sure you have proper fuse protection on the isolation transformer.

 

Thank you for the lengthy response "someonesdad". I appreciate everyone taking a moment to post and i appreciate your safety concerns. I am familiar with proper safety techniques and protocols necessary when dealing with electricity. I am used to dealing with much higher voltages then mains. What I am not familiar with however, is probing circuits connected directly to mains without an isolation transformer. And since some circuits that I will be working with will require various input voltages and currents higher then what I can produce with batteries or my small lab supply, I am trying to determine a safe, "inexpensive" and " fairly universal" approach which will allow me to test these circuits without purchasing or building a larger bench supply.

Thank you all for your input! I am sure that I will be able to find what I am looking for with more time invested searching online.