I'm looking for a brand of Toroid isolation transformer with electrostatic shielding. There may be one brand which has this. Unless there is just none in the whole world? Is it not possible to put electrostatic shielding in any Toroid isolation transformer?

Original message (findings: It's a semi isolation transformer)

I bought a Hammond Isolation Transformer (dual 234v/117v primary and secondary) for isolating test projects from ground. But when I measured it from one of the output to the soil, it measures 112.3 volts. The other lead is near zero. I thought it was supposed to be isolated to ground.


Here's the model.


I chose the m117 model or 500va.

Here's the primary connection


Here's the secondary connection


Note all the 4 windings have no connection to each other initially (tested using continuity meter). Even after connecting them right. There is still no connection between primary and secondary (any of the wires), when not powered.

It's not an autotransformer which has connection between one of the leads of the primary and secondary. Unless a toroid autotransformer has no connections between the primary and secondary too?

So why does one of the leads reads 112.3 volts to the soil? Isn't it an isolation transformer? But the primary and secondary windings are supposed to be separate which is what makes isolation transformer, isn't it. What do you think?


An autotransformer only has one winding. The Hammond 1182 toroid has 4 windings. So how could it still be an autotransformer??

  • 3
    "it measures 112.3 volts". But did you try to measure the resulting current? I bet it will be less than 100uA – Ale..chenski Dec 7 at 7:40
  • 2
    Nothing is perfect in real world :-( – Ale..chenski Dec 7 at 8:00
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    There is no "fully isolated" power sources. In this case there must be some DC coupling between primary and secondary, because it is possible that some high-frequency components of AC could be rectified over parasitic junctions and charge the secondary wires into kV area, which will be really dangerous. So some level of coupling is necessary for safety. And no, 100 uA won't give you any shock, although some tickling sensation is possible if you touch floating winding with your underarm. – Ale..chenski Dec 7 at 8:18
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    "If I accidentally touch the lead..." The Hammond probably sold these transformens to millions of people over nearly 100 years, and no one yet complained about any shock. Why don't you try to measure the actual current, and tell us? – Ale..chenski Dec 7 at 8:24
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    You don't need any "loads", just stick you DMM leads (in AC current mode) between the "semi-isolated" lead and ground. – Ale..chenski Dec 7 at 8:32

First, the 1182 series are not truly "isolation transformers". At least not on the linked datasheet. They are regular transformers with electrically unconnected secondary winding. While the secondary winding is not connected, the area of windings overlap is huge along the toroidal core, and thus there is capacitive coupling, since the whole thing operates on AC.

enter image description here

My take on the difference in readings is that by design, the windings are made in multiple layers, so one end is closer to primary winding, and the capacitive coupling is higher, while the other end is likely on the surface of the transformer, and has higher distance from primary winging.

The value of capacitive coupling could be 1 nF - 2 nF, which is about 3 MOhms, so a high-impedance DMM will see some AC voltage between floating leads and earth ground. And the effective leakage will be about 30 uA, which will be easily grounded once you connect it to your workbench ground.

True "line isolation transformers" usually include a single-layer winding (for toroidal core, and a non-shorted copper foil in rectangular bobbins, as Jeroen3 commented) with only one end exposed, which is connected to enclosure ground. In this case the cross-talk between primary coil and secondary coils is minimized, see Section "Line Transformers"

enter image description here

from main Hammond transformer catalog.

  • 2
    @jtl Yes it still isolates. It just has poor common mode. True safety isolation transformers only have capacitive coupling to earth due to a metal foil between primary and secondary winding. – Jeroen3 Dec 7 at 7:44
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    @Jtl, the 240 model is of toroidal type, so windings still go over each other a lot. Good isolation transformer has coils on opposite sides of magnetic core, where the coupling is much less. Unless you demand coupling data from manufacturer, you shouldn't expect anything. Why a 100 uA leakage is bothering you? – Ale..chenski Dec 7 at 7:53
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    @jtl no. Still a toroidal. I have not seen those as isolation transformer. You need a normal iron core transformer with foil, or two separate coils. – Jeroen3 Dec 7 at 7:54
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    @Jtl, it is getting more and more as "X-Y" problem. You are not telling why "strong magnetic field" is of any importance in "test projects", nor why 100 uA (easly grounded off) is of any concern. – Ale..chenski Dec 7 at 8:11
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    @Jtl, if you have special requirements as low magnetic interference, then encapsulate the entire transformer into a thick custom-made magnetic enclosure made of transformer steel or Nu Metal. No one will do it for you. – Ale..chenski Dec 7 at 8:27

Why do you assume it is not an autotransformer?
The 3rd bullet point of the pdf you linked contradicts what you stated:

Note: Units are designed to have all windings engaged (either series or parallel connected) or connected as an autotransformer.

In an autotransformer primary and secondary are connected.

EDIT: I think Ale..chenski's answer is right.

Now I understand the sentence quoted above, that primary and secondary are isolated but the manufacturer assumes/suggests that the transfmormer will be connected by the user in such a way, that all windings are engaged or as autotransformer.

  • see imageshack.com/a/img923/6457/a4BM5X.gif An autotransformer only has one winding. The Hammond 1182 toroid has 4 windings. So how could it still be an autotransformer?? – Jtl Dec 7 at 6:51
  • 1
    Maybe the diagram shows those 4 windings without the connection between them. – Curd Dec 7 at 6:56
  • Yes. But what does the phase "Note: Units are designed to have all windings engaged (either series or parallel connected) or connected as an autotransformer." mean? How could you have not any series or parallel connection and still have outputs? Any illustration? – Jtl Dec 7 at 7:04
  • 2
    Hi-Pot test of 4,000VAC RMS between primary & secondary is not possible on a autotransformer. – Jeroen3 Dec 7 at 7:41
  • So the 1182 is a semi isolation transformer? Would one get a shock if one accidentally touch one of the secondary output and ground? – Jtl Dec 7 at 8:16

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