Isolation Transformers

Today lets discus about Isolation Transformers. They’re a must have for troubleshooting unknown or problematic circuits.

What is a Isolation transformer?
A transformer takes in electricity at a higher voltage and lets it run through a coil of typically magnet wire wound around an iron core. Because the current is alternating, the magnetism in the core is also alternating. Transformers have a primary coil and a secondary coil Typically it’s a one to one ratio so the output matches the input voltage. For a example 120volts from the wall socket to the primary and outputs 120volts on the secondary. Same deal for other countries that use different voltage ratings for their AC mains. Also note that Transformers only for for Alternating Current and does not work with Direct Current. It would require rectification of the AC to DC but that’s not the subject today.

Since the Iron core separates the two coils it isolates them from one and another. Older isolation transformers use two poles Live and Neutral. This is what we call a floating source. Modern transformers attend to add a third wire known as the Earth Ground. It’s fine to have however there is a catch and could be costly. I’ll discus this in a bit. Some but few “isolation transformers” will tie the Earth Ground to the Neutral. However this wouldn’t be a true isolation transformer. When adding a Earth Ground to the secondary at all makes the transformer basically non-isolated.

Earth Ground – EG
I’m not totally sure for other countries but in North America the Earth Ground is also connected the Neutral line at the Fuse/Breaker panel and then from there to a solid copper rod embedded into the ground about 8 to 10 feet. Typically the copper rod is next to the building’s power meter. Also the Earth Ground is connected to assortment of things from Cable TV panels, Telephone lines, water pipes and even the rebar in the building’s foundation.

Isolated Earth Ground – IG
The primary reason for the use of isolated grounds (IG) is to provide a noise-free ground return, separate from the equipment grounding (EG) return. The EG circuit includes all of the metal conduit, outlet boxes, and metal enclosures that contain the wiring and must be grounded to provide a safe return path in case of fault currents. The IG provides an insulated, separate ground path for the ground reference in electronic equipment, such as computers, hospital equipment, and audio equipment. IG does not break ground loops, which can damage equipment like computers, printers, etc.. Interconnected computer equipment often benefits from single-point grounding. IG is only used with special equipment that require it. The IG is typically insulated and separate all the way back to the point of earth grounding rod out side of the building. The IG is NOT connected neutral or any other. Due to the installation of a separate, insulated conductor and the associated special outlets required, IG circuits are more expensive to install than standard power circuits.

Until the 1950s, isolated ground domestic mains supplies tended to have no RCD (Residual-current device) or ELCB (Earth Leakage Circuit Breaker), and too high a ground impedance to blow a fuse if a live-to-earth fault occurred. This could leave metalwork in the house live. The use of RCDs (or formerly ELCBs) with such installs solved this problem. Such installs are called EEBAD (Earthed Equipotential Bonding and Automatic Disconnection).

The Dangers
Now what does all this mean? Well this goes into the dangers of a isolation transformer. It’s a catch-22 but if you take care you will be fine.
Some test equipment uses a EG reference. For a prime example a Oscilloscope is at a EG reference on the front panel BNC probe connectors. A lot of people (Mostly people new to electronics) forget that a scope isn’t a multimeter and attend to go probing into stuff.
Now if you have a isolation transformer that uses the earth ground on the secondary and your device under test uses a EG for it’s ground for a example this circuit.


As you can see the EG is connected at the Neutral even though it’s plugged into the isolation transformer. All because the Earth Ground is still included in the secondary side. Now this is how it can be dangerous. If you were to hit the positive rail with the ground clip from the scope probe it will cause a dead short and send voltage/current into the scope and then back out to the EG. Typically this will hurt the scope and maybe yourself. Not so much with low voltage but if there is enough current it can be a hazard waiting to happen. You just have to remember that your scope is at a Neutral potential. When you use that ground clip you’re bringing the circuit to a EG potential.

There are scopes that are isolated, they are typically battery powered or have a differential input. You can use a Differential probe that basically lets you probe around like a multimeter but they are typically used for high voltage and cost a arm and a leg.

Do’s and Don’ts
Another thing to consider as well. Never use a isolated transformer to power test equipment such as a Oscilloscope. If you float a scope you can create a major shock hazard. For a example if you were to use the probe ground clip on something that wasn’t a zero volt ground but say virtual ground of 12volts at 1amp you just injected that voltage and current into the scope’s ground path and since the probe’s ground is earth referenced it is also tied to the chassis of the Scope, you just made the Scope’s chassis live. Between 100 to 200 milliamps is lethal so keep that in mind.
Once again if you were to float a scope and then plug in a communication cable such as USB, RS-232 or GPIB then you just put the scope back to earth ground potential since those connection types are EG referenced.

A isolation transformer is only attended for testing or troubleshooting a device that you are working on. This is known as a Device Under Test.

An ideal isolation transformer would have a option to switch off the Earth Ground on the secondary. Or implement a IG system for your lab/workshop and have a dedicated wall socket for that IG. It doesn’t matter too much for vintage electronics since 90% of them only use the Live and Neutral poles/rails of the mains power. If anything while the DUT is off probe with a multimeter the ground and the chassis. If they’re connected then be cautious when you power it on.

If you buy a isolated transformer then I would suggest probing with a Multimeter the Primary and Secondary Grounds and the Ground and Neutral to see what is what. If the Ground is connected to the secondary Neutral then I would disconnect it. Also add a heavy duty high voltage / current rated switch to the ground on the secondary. This way when you plug in a circuit for the first time you can have the EG enabled so if there is a fault it will have a proper ground path but if you have to probe around with a scope then I would disable it.

While on the subject real quick a Variac is not isolated.


This is a typical Variac circuit. The Neutral is connected straight thru and the Live/Hot side is connected to a Inductor like coil that a carbon track rides on to vary the voltage.


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