RS Isolating Transformer

[Skywave]

Rob - The drawing is only 'representative': it gives the general idea. Note the earthing connections.
Fuses: if the primary is wired to a fused 230-v. plug, I'd put a 3A fuse in that plug. As for the secondaries, not so easy to specify. Obviously, there is a upper limit of fuse current-rating so that the transformer secondaries are adequately protected. On the other hand, there is minimum size of fuse current-rating, which depends on the load: 1-amp would seem a sensible figure.

Al.

[Paula A]

tx is marked 2A max per secondary winding.. I would just check the 0V tag on each secondary is actually just a CT and not connected to frame/core.

working on basic principles (step down/up etc) 3A on the mains side is fine.. allows a (zero loss assumed) a nice 6A available out the other side

I also was taught to work "off ground" in the workshop.. test gear all properly earthed and chassis of whatever on test also to real earth.. power to UUT through isolating tx. Just makes it safe in respect of anything you might grab by accident... and as always the old roadies "left hand rule".. stick it in your back pocket if unsure.

[Skywave]

I would just check the 0V tag on each secondary is actually just a CT and not connected to frame/core.

Perhaps I'm being a bit thick here, but I don't quite understand that.

Your "CT" could mean "current transformer" or "centre tap": most likely the latter. Indeed, there is the possibility that the secondaries might just be one secondary with a centre tap - which, in turn, may be connected to the core - but I think that that is very unlikely. And that, in turn, means my drawing needs a slight modification.

Al.

[Bushbaby]

Hi Al,
I might be a bit thick here too - how can I tell if this is indeed one or two secondaries??

Thanks

Rob

[Paula A]

it is two secondaries.. it's rated as such. ct in my books is centre tap. somebody at "t'other place" got a nice 4x120v 2A secondaries fully cased all sockets included tx from me in January.

[pwdrive]

Hello all, looking at the photographs it looks like two times 60-0-60 secondaries which I presume could be parallel or series connected or used indipendantly. Am I on track?

Lawrence.

[Bushbaby]

Hi Paula and all,
I'll rephrase my origianl question - how do I tell if it is two secondaries on a centre tap?

Thanks

Rob

[Skywave]

Ah-Ha! All is now clear! Y'know, when I read this Thread on its first appearance, I didn't see a photo of the transformer in question. Perhaps I'd overdone it on the 'Glenfiddich' at that moment: that sorta thing happens to me this time of year :s

So . . . my sincere apologies to you, Paula - your post indeed makes sense!

O.K., now that's been said & dealt with ~ I'm familiar with that transformer.
There are two secondaries, each configured as 60v - 55v - 0v - 55v - 60v. So each winding gives 120-v max. Simply connect the two windings in phase-additive to produce 240-v; from that commoning connection to either of the end connections of the two secondaries will produce 120-v.: my submitted drawing still applies.
There are no connections between any of the secondary windings and the core - not if the transformer is any good that is. Obviously, a quick check with an ohm-meter first would be a good idea.

Al.

[Michaelr]

Would I be able to connect both secondaries for 240v and then operate one set of secondaries for 110v ??

Rob

Yes; as per the drawing below:



Note that the dots on the two secondary windings denote that these two ends are in phase.

Al.

Al Sorry to throw a spanner in the works but technically as you have drawn the circuit you could run just under 600watts without blowing fuses , the transformer is rated at 500 watts.That transformer could get very hot at full output on the 240v and 110v at the sametime.

Personally I would just use as two seperate secondaries ( or parallel them) or one single 240 volt secondary

Mike

[Skywave]

Hi Paula and all,
I'll rephrase my original question - how do I tell if it is two secondaries on a centre tap?

Thanks

Rob

First measure the resistance between the two secondaries: should be infinite ohms. Then measure the resistance between primary and each secondary: again, should be infinite ohms. Next, measure the resistance between primary, each secondary and core in turn: 3 measurements in all: each should be infinite ohms. If any of these measurements are not infinite ohms, you have a suspect transformer.
Then power-up the primary and measure the voltages on the secondary windings (on no-load).

Al.