Electronics > Beginners
Is my Isolation Transformer not isolated?
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AngraMelo:
Hello everyone,
I just bought an isolation transformer. It should be noted that I live in Brazil, isolation transformers are not a thing (meaning, no one has one, everybody just take the risk of working without one). Here in my city it is 220VAC with 2 phases, We do not have a neutral, just phase, phase and earth.
On the primary it has 2 input voltages, 110V and 220V, and a ground wire

On the secondary it has 2 wires for 220V only.

The seller claims it is a Galvanic Isolated transformer but I have my doubts.
For the following experiments and measuraments I did NOT connect the earth wire from the primary to the outlet earth.

I got a voltage from the secondary against the outlet earth, I connected a light bulb to those 2 connections and it did NOT light up. After, I measured using a Fluke 117 with a LoZ option and I got 0 volts AC on both wires of the secondary in relationship to earth outlet. Also...and this is dumb...I touched the secondary wires (one at a time) and I do not get a shock no matter what.

Now the tricky situation, when measuring resistance from the primary to the secondary I always get a reading, it starts around 500K and goes up gradually. It does not matter which cables Im measuring, I always get a resistance reading. How in the world is that isolated?
But if it was not isolated, the LoZ function would have accused a voltage right? Or the light bulb would have lit up, right?
This is driving me nuts, I watched a ton of videos but I cant figure out what is going on.

Please help me guys.

I have a limited knowledge but I want to make sure I have the real deal.
helius:

--- Quote from: AngraMelo on May 04, 2018, 11:58:09 pm ---Now the tricky situation, when measuring resistance from the primary to the secondary I always get a reading, it starts around 500K and goes up gradually. It does not matter which cables Im measuring, I always get a resistance reading. How in the world is that isolated?
--- End quote ---

Even with galvanic isolation, there is capacitance and mutual inductance between the windings. The mutual inductance is what makes it a transformer and is absolutely necessary for it to work, and the capacitance is unavoidable given the constraints of transformer assemblies (the primary is wound over the secondary unless you use a toroidal design). An ohms reading that starts high and increases is caused by the capacitance (just like when you connect DC across a capacitor, current flows briefly and then stops). It sounds like your transformer is correctly isolated.
Electro Detective:
If the Fluke 117 on Low-Z reads zero, it's as isolated as it needs to be  :-+ 

Some photos please if possible

and welcome to the forum  :clap:
AngraMelo:
I did not know that!
thank you very much both of you!
Electro Detective:
Also make sure the polarity is correct for input and output

You do not want up to 440 volts appearing between both input and output 'Live' connections

If you get that, then either reverse the output wires, or the input wires, depending on what is connected to the chassis or transformer body. We need to see photos to make a proper judgement on this before you try that   

i.e. there should be ONLY 220 volt or zero to a few volts in any way you test the input and output connections with the meter

Beware if you do these tests using Low-Z you will trip your GFCI or RCD, so watch out for that or use just the AC meter setting,
but that will give you ghost voltages (perhaps 60v + 160v) , which may not be a problem as you are watching out for anything much higher than 220 volts


Hope it made sense and probe carefully    :scared:

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