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So I have a transformer that is like 230 || 30 V.
One thing I don't get that on primary side it has the following leads:
0 --- 230V --- 240V

When it can be useful? I mean I can't think of situation when somebody would say "We should wire it to 240V, 230V will be a bad choice".
Isn't it hard to know what exact value particular wall outlet will be outputing? I always though that in one house it can be 230, then you go to the different city and it's 240, not to mention it is never precise, so you can get like 235. Moreover regular customer doesn't know what he has in the outlet.
What is the reason that manufacturer decided to have two options?
I've never seen a 230/240 switch on any device either. And bonus question - if you already decided to make such a primary side - why not make 220V also?

I tried google this and all the answers were like "there is no difference, don't bother", but I suppose manufacturer would not make it such a way if it wouldn't have an application.

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  • \$\begingroup\$ In the US it's not too uncommon to see a motor or transformer with both 208V and 240V inputs, since 208V is used in many commercial/industrial situations. \$\endgroup\$ – Hot Licks Nov 27 '14 at 15:38
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This type of multi-tap transformer primary was common before the days of regulated DC power supplies. The user was supposed to select the appropriate tap according to local mains voltage. Variations from 210V to 250V were common, in European radios, TVs, tape recorders and the like. Old test equipment like scopes also had this type of voltage selection.

I've seen a 1957 valve radio with such a tapped primary, and it was set to the wrong one. The secondary AC voltage was low, and that meant that the rectified DC was low, too. We changed to the correct tap, and the voltages came up to levels much closer to the nominal voltages in the schematic. The radio worked a lot better like that!

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It's precisely because of that inaccuracy in the supply voltage that you may want such a transformer.

By itself it's pretty useless, but with active voltage monitoring, and a method for switching automatically from the 240V to the 230V tap a power supply is able to compensate for brownouts, etc.

The same technique is used in many UPS systems for compensating for brownouts, though they often have more than just one extra tap.

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  • \$\begingroup\$ Sorry, I still don't get it. If for example monitoring system reads 240V and then suddenly it goes down to 230 and the tap is replaced to 230 does it really matter? It will affect secondary voltage for sure, but not much. And if it was 240 and then brownout takes it to 190 or something like that, than 230 tap is not useful either, isn't it? \$\endgroup\$ – ScienceSamovar Nov 27 '14 at 1:25
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    \$\begingroup\$ @ScienceSamovar Yes, it mattered in the days before regulated DC power supplies. This may be an old transformer, or at least an old design. \$\endgroup\$ – John Honniball Nov 27 '14 at 9:21
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This kind of transformer would normally be permanently wired to the correct voltage tap for the target market (country) and the unused tap left open. Adding a tap only costs pennies,and it provides a bit of optimization (especially for something like an audio amplifier that may not be able to meet the advertised performance without the proper tap connection). It could thus run afoul of truth-in-advertising laws in a market with a lower nominal mains voltage, or run a bit hot in a market with a higher nominal mains voltage.

It was not unusual to have mains transformers with taps for 100/120/200/230/240 VAC before switching supplies became common.

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You may have a transformer that was built for use during the time when Europe was standardizing the line voltage.
Up until 1995, various countries in Europe used different line voltages for the outlets. For example, in Germany it was 220V, but in the UK it was 240V.

In 1995 (Mains voltage wikipedia ), they all began switching to 230V ,and were supposed to have completed the switchover by 2008. So, you may have a transformer that was built to be installed into a device and used when the line voltage was 240V, but could then be switched (switch or jumper) to use the lower voltage after the changeover.

Here in Germany in the early 1990's I used a big test rig for two way radios that had a switchable supply voltage - it was adjustable from like 210V up to 250V. I also own an oscilloscope that was built in 1967 that has a jumper on the back to switch between 110V and 220V - with various jumper positions for voltages from 220V up to 250V (or something like that I don't have it memorized.)

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