Have a 240 volt to 100 volt transformer, whose switch on the front 'gave-up'. I've taken the switch off and now have three wire ends a,b and c that are now disconnected.
Please See link for diagram.
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have some terminal blocks ready to go, but since it was a while ago that i disconnected the switch I'm not sure now how to reconnect the ends a,b and c so I can use the transformer minus the switch. (which i will replace at a later date).
Thanks for any advice on how to connect these wires.
There is a blob drawn on the incoming 240V mains, to the left of and below the circle with the 'c' in it. The line then forks with one going to the fuse and the other to the right hand end of the transformer. So presumably this fork represents the point to which the mains cable sheath has been stripped back, and then the brown goes to the fuse and the blue to the transformer.
Wire C then comes from the fuse, and I reckon the switch would have been a simple on/off switch to connect C to B when on. This would have applied
240V between the leftmost and rightmost connections to the transformer.
Unless the OP has "lost" a 4th connection, this must be a non-isolating transformer (i.e. an autotransformer). There will be 100V between Left and Middle, and 140V between Middle and Right.
The wire marked A is not needed. Perhaps it was connected to an indicator lamp inside the old switch (which would have to have been a 100V lamp) to show that the output sockets are live. It's confusing that the OP labelled the plugs as "incoming", since although the plugs "go in" to the sockets, they are delivering outgoing power to the 100V appliances.
So, use one terminal block to connect C to B, and a second terminal block to "connect" A to nothing (just to park it for safety).
Shouldn't be. The only time the transformer is centre-tapped to earth is when it's a 240-110 isolation transforme for on-site use. Those are
5 wires, because the 110V output is centre-tapped and earthed (to limit the max potential above earth), but also they're an isolation transformer.
If it's 3 wires, it's an auto-transformer. If it's an American transformer, then &deity; help you. Their electrics are just _nasty_. I saw a new Taig milling machine recently, supplied with a transformer so bad that both transformer and motor have since gone skipwards.
On what basis do you suggest he should decide which two of the three wires going into the transformer are "the mains side"?
What if he doesn't have a 12V or 24V transformer?
It's unlikely that the setup is any different from the one I described earlier, i.e. that the "mains side" of the transformer is between the left (same as wire B) and right (already connected to mains neutral) connections at the transformer, and that the "100V side" is between the left (B) and middle (A) wires. An autotransformer doesn't really have "sides" as such.
But if he has a multimeter with a range for measuring lowish resistances, he could confirm this off line (i.e. with the mains disconnected). The resistance between Left and Right should be bigger than that between Left and Middle and also than that between Middle and Right. It may be that the LM winding uses slightly thicker wire than the MR winding, and so it will not necessarily be the case that the resistance ratios LM:MR:LR are equal to 100:140:240.
One thing that strikes me as being slightly odd about the way this box is wired is that neither of the two pins of each 100V outlet are at the neutral potential of the incoming mains, rather one is at live and the other at roughly "half-live". If this is as undesirable as it seems, it could be fixed by reversing live and neutral on the mains input, i.e. by having brown=live go to Right and blue=neutral ending up at Left (wire B).
But then the fuse would be in the neutral line instead of the live, which is a bad idea because if it blows, then both pins on both outlets will be live. The whole thing seems not particularly well designed.
Are we sure that the OP has his diagram right in this one? Photos would in this instance be more helpful. It seems as if the OP is a bit confused as to what went where in the past anyway.
Easier to take it from scratch now.
Take blue core of mains cable straight to one end of the transformer wiring and thence to one terminal on each of the sockets. Take brown core of incoming mains cable to the fuse and thence to the other end of the transformer winding. Take middle tapping of transformer winding to the other terminal of each of the sockets.
The chances are that the transformer tapping is at the mid point, but a quick check with the multimeter will probably show if it's not. The neutral wire should obviously be at the end with the lower resistance between it and the "central" tapping and the live wire through the fuse should be at the end with the higher resistance between it and the "central" tapping.
Here is a photograph which i hope will make things clearer. Esentially all three loose wire ends (two of them shown with solder on the ends and the third one which I have now put the connector on) all were originally soldered to the switch (now missing, but where the hole in the case is). The switch had three terminals.
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for easy reference here is the link to my original drawing.
Why did you not show a picture of the whole thing including the other side of the transformer where the mains are connected? Or do you like to watch people fumbling around with insufficient evidence to comment?
this still does not add up. if it is an auto transformer and the wires at the top are the only wires from the winding , the left and right ones would be connected to two of the wires in the flex with the knot in it. It is a pity we can not see where the wires in the flex with the knot end up.
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