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transformer

In the image above, one of the leads in the primary plug of the 220v-110v step-down transformer has continuity with all the leads in the secondary outlet lead using a tester (see arrow in blue). I'm not talking about ground, because the ground is connected to the casing, no problem. But I thought the primary and secondary winding of step-down transformers are isolated. Why is continuity test positive? This is tested with transformer off. I plan to connect the neutral/ground to one of the leads of the secondary, but won't this cause a problem? Note it's not an autotransformer.

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    How do you know it's not an autotransformer? – ThreePhaseEel Oct 15 '18 at 1:13
  • imagizer.imageshack.com/v2/640x480q90/923/krenZF.jpg the inside of another with the same one lead of the primary and secondary leads being continuously (tested positive in tester) is shown.. here it is clearly not an autotransformer, isn't.. or can the appearance also be an autotransformer? – Samzun Oct 15 '18 at 1:39
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    Are there any leads connected to the other side of the transformer? – ThreePhaseEel Oct 15 '18 at 1:46
  • I'm not sure. Why.. isn't this normal? Are you not supposed to have one of the leads in the primary connected to all the leads in the secondary inside the unit? And what does it mean if they are connected? what kind of step down transformer is that? – Samzun Oct 15 '18 at 1:51
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This is either an autotransformer, or a regular transformer wired as an autotransformer, not an isolation transformer

The manufacturers of this transformer-unit most likely used an autotransformer, as all they are after is a voltage step-down, not isolation. Even if they did use a two-winding transformer, they have wired it with a common neutral between the primary and secondary sides, which defeats any isolation provided by the transformer itself. This is unlike an isolation transformer, which has no galvanic connection between current-carrying conductors on the primary and secondary sides.

The reason this is done is because it's less expensive (less copper) to make an autotransformer, and also less expensive (less insulation and testing) to make a step-down transformer than an isolation transformer.

  • Why would they wire it with a common neutral between the primary and secondary side? What would be the advantage of this? What components could they be saving instead of making it as a pure isolation step-down transformers? – Samzun Oct 15 '18 at 2:24
  • another odd thing is that the main secondary leads (not the ground) are also shorted or also tested positive in the continuity test.. how could the secondary leads be connected together.. what kind of step down transformer is this where one of the leads of the primary is connected to both leads of the secondary and both leads of the secondary is shorted too? – Samzun Oct 15 '18 at 3:04
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    @Samzun -- that "short" you are seeing is due to the relatively low (single digit ohms to tens of ohms tops) DC resistance of such a transformer's secondary winding – ThreePhaseEel Oct 15 '18 at 3:16
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    @Samzun -- no, it's not an isolation transformer -- there are two connections you're seeing here, one is the intentional connection primary-neutral to secondary-neutral, and the other "short" is simply the secondary winding – ThreePhaseEel Oct 15 '18 at 3:55
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    @Samzun -- what they do is jumper the neutral side of the primary to the neutral side of the secondary – ThreePhaseEel Oct 15 '18 at 4:05

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