Isolation Transformer or Variac

Thread Starter

John Czerwinski

Joined Jun 19, 2017
I'm building my test/repair bench (I'm a hobbyist). I've been reading and watching videos on needing a isolation transformer to protect test equipment (oscilloscope) , and the tester (me). I'm wanting to know that best route to take. Should I get a single output isolation transformer or a Variac that has isolation built in (i've provided some links below)?

Also, I've read that you want to put the UUD (Unit Under Test) on the isolation transformer and NOT your oscilloscope. If anyone can direct me to references to this fact, I'd appreciate it.

B&K Precision 1604A Single Output Isolation Transformer

Beleeb 20Amp Variable Transformer Auto AC Voltage Regulator Power Supply, 2000VA Max, 0~130 Volt Output


Joined Jul 10, 2017
If your oscilloscope is powered from the mains supply, it is isolated from that supply but the frame is connected to the supply earth ground. This is to protect you so do not disconnect the ground wire in the power plug.
If you are going to be making measurements with the scope on live circuits at supply voltages, you will need to isolate the supply to that equipment so that you don't short live wires to ground through the scope probe common connection. Otherwise, you really don't need an isolation transformer.
Note: If you scope has two channels (most do nowadays) you can make safe live voltage waveform measurements by using one channel for the measurement and the other for the live reference, with the probe commons connected to ground. Selecting the "A-B" mode, or by inverting the polarity on the B channel, you will make a safe differential measurement between the two.


Joined Oct 7, 2019
Variable Transformer
Most of those have no isolation. (some do)
I use them because I want to test where the power supply stops working. Will the supply work at 90V or will it start up at 90V? Dose the power supply hold the voltage constant from 90 to 240V input? I think most people will not need the variable voltage.
1604A Single Output Isolation Transformer
Looks good to me.

What I use for isolation (mostly) has 120/220 switches so I can test a power supply at 120 or 220 with a switch flip. Most people just need a 1:1 transformer. (isolation only) I spend much time looking at the hot side of power supplies and really want the isolation.

isolation transformer and NOT your oscilloscope.
I know people that isolate the scope. What happens is the scope (case, all metal) is at power line voltage or maybe 260VDC or what ever you are looking at. Do you really want the scope to be sitting at 220vac. Some scopes have a wire over to the PC to record readings. That will kill the PC.

Isolate Unit Under Test and keep your tools at zero volts.

Have many scopes. One is battery powered scope that has no metal showing. The case is two layers of plastic to keep you safe. All switches are plastic. The connection to the PC is a light pipe. The scope is designed to be 2000 volts off ground and not kill the operator. Do not try this with a plug in the wall scope that is not designed for it.


Joined Mar 19, 2019
Variacs are NOT isolation transformers, they are autotransformers. Also on Amazon you will find a Jameco Isolation Transformer which states 4A in the ad but is actually 7A in the fine print and in reality. Well made and a good value. However, the Isolated sockets actually are tied to the wall socket ground and will need to be cut to be a true Isolation Transformer. Simply open the case and cut the ground wire connecting the isolated sockets to the unisolated sockets. The isolated sockets are not grounded to the case so they will be truly isolated when the ground is cut. /


Joined Oct 7, 2019
Simply open the case and cut the ground wire connecting the isolated sockets to the unisolated sockets.
Maybe we don't see "isolated" in the same way. I normally leave ground connected to ground. What I want to do is get disconnected from the power line. I have some little boxes with a micro inside that plug into the power line. The micro lives on the neutral wire or the hot wire depending on how the box was plugged in. This is safe in a plastic box but we open up the box and make changes. The micro is actually 5V off neutral which is not exactly 0V or it is 5V off the line voltage. (who knows which) With a isolation transformer, Green wire ground is ground, but there is no longer neutral and hot. Any where I connect the scope ground wire becomes "0V". It is now save to connect the scope.


Joined Mar 19, 2019
What I want to do is get disconnected from the power line.
The socket ground is tied to neutral at the panel so the only way to truly isolate it is to cut the panel ground connection for the isolated power sockets. You could then install an isolated earth ground for the isolated sockets if desired but must insure that it does not have connection through the chassis to the panel ground. Because of my radios and antennas, I have an isolated earth ground at my bench but have not connected it to the isolated sockets as yet.