# Using An Oscilloscope

#### oldfart

Jul 24, 2017
10
Ok, so I have an old analog Tektronix oscilloscope that is powered by standard 120v outlet. I want to measure voltages in a circuit that is also powered by 120v. Before I do that, is there any precautions that I need to take to avoid bad things happening? I read a discussion about using an isolating transformer but not sure if that is needed or wanted. Can you please give me some guidance?

#### Minder

Apr 24, 2015
3,366
The ideal is a 1:1 isolation transformer as it is possible to cause high current in the scope common lead if placed at the wrong point or cause a short circuit.
M.

#### oldfart

Jul 24, 2017
10
The equipment I want to test is hardwired and the oscilloscope is plugged into the wall. If I have an isolation transformer that plugs into the wall and then the scope plugged into that is that correct? Also on the output (secondary) side of the transformer should the scope ground wire be tied to the secondary neutral wire and NOT the wall ground?

#### dorke

Jun 20, 2015
2,342
What do you mean when you say: "a circuit that is also powered by 120"?
Can you show a circuit diagram of what you need to probe?

That is the most common of equipment and usually there should be no problem as long as your prob ground is connected to the ground signal on the board/equipment you are testing.

Problems(both destructive and security) arises when:
1.You start probing non-isolated mains circuits.
2.High voltage signals.

The proper solution is to use a "High voltage differential probe" -costly!

#### oldfart

Jul 24, 2017
10
I don't have any circuit diagram. It is part of some old machinery that has a circuit for monitoring the movement of the table that are not working. I tried probing around with a multimeter but didn't learn a lot from that. There are rectifiers and capacitors etc. not all located in one place and wanted to look at the waveform to help identify the problem area. So i just wanted to use the scope to probe around and see what I could find. This is the way the machine was when I bought it and don't know any more details. If I can't do that with the scope I may start disconnecting components and testing those individually...or scrap the whole thing in favor of a new circuit. I thought if I isolated the scope I could do that without a problem but if I can't then I will do something different. If I can do that with an isolation transformer then I will do that but if I need to spend \$ for a high voltage differential probe I will most likely just forget it.

#### Minder

Apr 24, 2015
3,366
The equipment I want to test is hardwired and the oscilloscope is plugged into the wall. If I have an isolation transformer that plugs into the wall and then the scope plugged into that is that correct? Also on the output (secondary) side of the transformer should the scope ground wire be tied to the secondary neutral wire and NOT the wall ground?
The secondary is not referenced to ground so no point connecting the scope GND to it.
If you isolate by using the transformer for the scope, the scope is no longer grounded, I prefer to galvanically isolate the circuit I am working on.
M.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Does this device have a transformer in it which is connected to the mains input? If so, and assuming the transformer don't have a huge fault, the circuit is probably already isolated.

Be aware though, that if the designer tied a power supply rail to ground, then you are limited in what you can connect the ground of your scope to.

If you do go down the path of using an isolation transformer, you will need one without a ground connection. Once you're using this, my best advice is to do as much as possible "hands off" because your scope could now be live.

I have a small, battery powered scope that I've used on live equipment.

#### oldfart

Jul 24, 2017
10
Thanks for all your advice. I am going to proceed cautiously and make sure I don't do something to harm my equipment and more importantly ME. After posting the last comment I realized that if I isolate the scope then the scope could be live depending on what I probe (not very crazy about that). I am going to think about it and look things over some more before going the scope route. I will look to see if it already is isolated via transformer and what parts are grounded. Hoping I can make more progress thinking and trying to understand what is there and why ... that should go a long way to diagnosing the problem. And I am very glad I asked about it before finding out the hard way.

#### skenn_ie

Sep 7, 2009
33
As long as you don't connect the probe ground to a point that isn't mains ground, you should be OK, BUT, you may find that some of the points that you want to probe, have a floating (relative to mains ground) reference. If your scope allows you to add/invert inputs from two probes, you can do that to see the difference between two points (DC negative to signal). I have, on occasions, disconnected the scope ground from mains ground, but I can't actually recommend it. If the input to the electronics is through a transformer and rectifier, there is a fair chance that the DC ground is also mains ground.
On older equipment, my first action is to check electrolytic capacitors for visible signs of pressure in the cans. That will suggest either an open circuit, or a short circuit in the capacitor. Result is rectified AC, or much reduced DC.

#### easyrider8

Aug 10, 2016
1
The isolation transformer is always used on the equipment being repaired, not on any associated test equipment

#### Kevin Pearce

Aug 29, 2014
6
skenn_ie has suggested disconnecting the scope earth from mains earth. I have done this, but it is not recommended, especially for a novice. Instead (if you are unsure if your circuit is isolated), use two scope probes, as he suggests. You should connect them to 2 separate input channels, switch one channel to INVERT, and ADD them together (assuming your scope has these features). You should tie both scope grounds together, but do not connect them to anything else.
This arrangement is known as a "differential connection", and removes the necessity for a special probe.

#### Heliman

Feb 4, 2016
65
Ok, so I have an old analog Tektronix oscilloscope that is powered by standard 120v outlet. I want to measure voltages in a circuit that is also powered by 120v. Before I do that, is there any precautions that I need to take to avoid bad things happening? I read a discussion about using an isolating transformer but not sure if that is needed or wanted. Can you please give me some guidance?
A lot of people frown on it but Ive been using scopes for 40 years with no problem. I use a 50 cent ground adapter on my scope so it isn't grounded.

#### oldfart

Jul 24, 2017
10
A lot of people frown on it but Ive been using scopes for 40 years with no problem. I use a 50 cent ground adapter on my scope so it isn't grounded.

But isn't neutral still grounded to earth from the connection at the breaker box? Not sure how that solves the problem of isolation?

#### BobK

Jan 5, 2010
7,682
The isolation comes from the power transformer in the scope. Post transformer, nothing will be connected to the neutral.

Bob

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