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I am in the "a little knowledge is a dangerous thing" zone, but at least I am aware of it.

If I take a working (not plugged in) transformer for Halogen lights (240 V mains down to 12 V) and measure the continuity of the 12V side, my multimeter says it is continuous.

But if I measure the continuity of the 240 VAC side, I get that it is not. Huh?

If a transformer is basically two coils inside each other, right? Why isn't there continuity on the mains side?

Here are some more details:

Transformer #1:

ATCO Speedy TED-070 Electronic Safety Isolating Transformer

Continuity across output No continuity across input.

Transformer #2:

OSRAM ET-REDBACK Electronic Transformer Like this one. Same results.

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    \$\begingroup\$ What ohms scale do you have your meter set to? Is there an 'Auto' setting? \$\endgroup\$
    – BobT
    Jan 19, 2018 at 2:54
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    \$\begingroup\$ Can you edit your question & add a photo of this transformer to your question, showing i/p & o/p connections and any manufacturer's markings? (in case that shows relevant new information) I'm wondering if this isn't only a transformer, and is really a small SMPS. \$\endgroup\$
    – SamGibson
    Jan 19, 2018 at 3:02
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    \$\begingroup\$ The continuity function on a DVM reports continuity when it measure less than some low value of resistance - on one of my meters, anything below 35 Ohms is "continuous", while higher resistances show no continuity - but if you look at the actual resistance measurement, you should see some valid reading. I've just measured the primary resistance of a couple of small "wall-wart" transformers, and got 350 - 1000 Ohms - that shouldn't show "continuity" on any meter. \$\endgroup\$ Jan 19, 2018 at 7:26
  • \$\begingroup\$ @PeterBennett: Should that be an answer? \$\endgroup\$ Jan 19, 2018 at 15:26
  • \$\begingroup\$ @BobT: 20K ohms. No auto. \$\endgroup\$ Jan 19, 2018 at 15:34

3 Answers 3

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Thanks for adding the photos & details of the actual "transformers". They have confirmed my suspicion explained in an earlier comment.

If a transformer is basically two coils inside each other, right? Why isn't there continuity on the mains side?

The reason is because, as I mentioned earlier, your photos have confirmed that despite being called transformers, the devices you are using are not simply transformers, in the sense you were expecting and which would perform as expected in the tests you are doing.

I couldn't find the exact datasheet for the first "transformer" you mentioned, the ATCO Speedy TED-070. However the datasheet for the Speedy TE-0070 specifically mentions that it is an electronic halogen transformer, mentions an operating frequency of 55kHz, and features like "Short-circuit shutdown feature with automatic restart". Those details, along with the small size of the device (much smaller than if it just included a transformer), confirm that it is a constant-voltage SMPS designed for halogen lighting.

As already mentioned by Neil_UK, such SMPS would typically have a bridge rectifier immediately at the mains input, so you would not expect to measure continuity (or some relatively low resistance) as you would with a simple mains power transformer's primary side.

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In a standard transformer, you'd expect there to be continuity on both sides.

For years now, things sold as 'transformers' for 12v lighting have been SMPS systems. This is mainly for cost. The old style iron transformer is expensive and heavy. It's cheaper to use some electronics round a small high frequency ferrite transformer to drive low voltage lighting, even at the 50 watt level.

The first thing the input terminals see is a bridge rectifier, so you're not going to see anything at all below 1.4v. Once above that, little more than leakage until, depending on design, the thing decides there's enough input voltage to start turning on.

Something like this is typical, 35-105 VA, constant output voltage, £5.

enter image description here

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The continuity function on a DVM reports continuity when it measure less than some low value of resistance - on one of my meters, anything below 35 Ohms will cause the continuity buzzer to sound, while higher resistances will not - but if you look at the actual resistance measurement, you should see some valid reading.

I've just measured the primary resistance of a couple of small "wall-wart" transformers, and got 350 - 1000 Ohms - I doubt if that resistance would show "continuity" on any meter.

The primary of a transformer will have many turns of fine wire, so will have a much higher resistance than the secondary of a step-down transformer, which will have relatively few turns of larger wire.

As another answer states, many "transformers" these days are actually switch-mode power supplies, and depending on the internal circuit may or may not show resistance on either the primary or secondary side.

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