RS Isolating Transformer

[Skywave]

Michael: re my Post #21. I did state that my drawing was 'representative' - and I then went on to talk about the appropriate selection of secondary fuses.

Al.

[pwdrive]

Hello rob, from the wording in the picture, that is what it appears to be, I might need to be corrected.
To check the actuall secondary connections for the center tap then I would just do resistance checks on each secondary winding to establish which connections are the actuall center taps if they are not marked as such, they usually follow a logical layout on the transformers connector strip, so it's basically identifying the two seperate windings by doing resistance checks, when you find the highest reading between two connections of either of the secondaries then you have found the two maximum secondary voltage connections ie 60v-60v that should equate to 120v output, next establish the center tap for that winding by a resistance check from either of the 60v connections you have just established, the cener tap connection that you will be looking for for that winding will return a resistance reading that is approx. half the resistance of the resistance reading obtained initially across the two connections you first established.
Once that is established then repeat for the other winding. the readings should return ohms not K ohm or M ohm.
I do not know what experience you may have in these matters so if this is all familiar to you please ignore.

Lawrence.

Addition: Al beat me to it, my post should guide you more or less to the same conclusion but using resistance checks as opposed to voltage checks.

[Bushbaby]

Hi Al,
Any chance of another one of terrific diagrams?? (Crawling here!)

Thanks

Rob

[Bushbaby]

Hi Lawrence,
Thanks for your advice - I'm no expert when it comes to transformers hence why are asked for all of your most valuable help.

Regards

Rob

[pwdrive]

No worries I think you are getting there now.

Lawrence.

[Skywave]

Hi Al,
Any chance of another one of terrific diagrams?? (Crawling here!)

Thanks

Rob

Working on it ~ don't g'way now . . . .

Al.

[Skywave]

Rob - drawings as requested.

The 'problem' with your transformer is that it is designed to cater for multiple needs - and that is what is confusing you.

I've shown 3 possible arrangements:

[attachment=3781][attachment=3782][attachment=3783]

However, reading this Thread again, it seems that your prime requirement is for a 110-v / 120-v. source for your American equipment. So on that basis, I would recommend that you configure the secondary windings for that output only. If you subsequently need an independent and isolated 230-v/ 240-v source as well, I'd look for another transformer for that need. Trying to get this transformer to do 'double-duty' (110-v and 240-v) involves too many compromises.

Al.

[Michaelr]

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.

Al , I think it needs pointing out for Bob's sake that a 3amp fuse on the primary would allow the potential for over running the transformer significantly . He states that he wants to run 240volt out at the sametime as having 110volt out. The 2Amp fusing on the output would cause a maximum draw with both secondaries being used of just under 700 watts.

Maybe Bob could confirm he fully understands that the maximum current draw he is restricted to when using all of the windings is limited by the 500 watt rating of the transformer, otherwise it is going to get hot!.

Mike

[Skywave]

I'm no expert when it comes to transformers.
Rob

O.K. ~ what follows may help . . .

Basic L.F. transformer operation is fairly straightforward: as I said above, the particular one you have there is giving you an 'introductory baptism of fire'. :omg:
I assume that you are familiar with the basic fundamental law of volts in / volts out = primary turns / secondary turns. What confuses many people, with a transformer, such as yours, is the issue of 'phase'. IMHO, 'phase' is a word that is probably mis-used and misunderstood more than any other word in the fields of electrics / electronics (with the possible exception of 'decibel' ). The word 'phase' gets banded around as though it is some absolute quantity: it isn't! Like 'voltage', 'phase' is relative to something. Take a simple transformer with one secondary winding: one end of that winding is 180 degrees out-of-phase with the other: think of that as when one end 'pushes' the other end 'pulls'. Then half of one cycle later, what was 'pushing', now 'pulls', and vice-versa. Now with two separate secondary windings, what this means is that when we connect them together, we have to be sure that all the 'pushes' and all the 'pulls' are working together at the same time, if we connect the two windings in parallel. However, if we join the windings the 'other way round', so that when one end of one winding is 'pushing', the corresponding end of the other winding is 'pulling' - so the voltages across the two windings are 180 degrees out of phase . . . then the voltages from the two windings will add up. If we had connected those two windings the 'wrong way round' then the resultant output voltage would be zero, since when one is 'pushing', the other is 'pulling'.

It is usual to denote the 'in-phase' windings on multiple secondaries with a dot - as I have done. In practice, the actual phase relationships between any number of secondary windings can most easily be determined by voltage measurements of those voltages. When I do that, I usually mark all the corresponding 'ends' of windings with a mark of some sort - such as a dob of 'Tippex' - so that I then know for sure when I come to use the transformer.

HTH.

Al.

[Skywave]

Mike - I take your point about the fitment of a 3-amp. fuse in the primary cct. The only reason I specified that rating was on the simple assumption that he would be using a conventional 3-pin 13-amp. mains plug and that a 3-amp. fuse for such a plug is the lowest rating fuse for that plug that is commonly available. So, yes, it would be sensible to fit such a 3-amp. fuse in that plug but also to fit a lower amperage fuse between that plug and the transformer primary. (Rob please note.)

Rob has clearly stated that he's no expert on transformers: none of us want to give him advice that turns out to be potentially dangerous; thanks for your 'input'.

Al.