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.
Best Answer
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!