RS Isolating Transformer

[Skywave]

Just as an aside on the issue of 'mains transformers running hot', I fell foul of that in my last employment. It was soon after I had started there, so I wasn't too familiar with the 'environment'. There were a couple of twin 13-amp. sockets on my bench. I was testing two 1-kW. VHF RF amplifiers. Having satisfied myself that all was in order, I ran each up to its full rated output capacity into RF dummy loads. About five minutes later, there was an almighty bang! - accompanied by a shower of sparks from under the bench :omg: and the room filled with smoke. :s.
When I re-entered the room (later - when I could see where I was going), an investigation revealed that someone had fitted a 500-VA isolation transformer between the incoming 230-v. mains supply and those two twin sockets - with a 13-amp. fuse feeding the transformer primary. The transformer was bolted to the underside of the bench-top, so a quick glance underneath (for any reason) would not have made its existence that obvious.
You'd have thought that that someone would have labelled those twin sockets accordingly - no? Not to mention that 13-amp. fuse!

Al.

[Bushbaby]

Hi Trev et al,
Many thanks for the diagrams which are most helpful.

I think that its unlikely that I shall be using the transformer for 240v and 110v at the same time. I wanted the facility to use either 240v or 110v sockets as needed. I intend to use this with 3amp fuse on the input side and to use switched fused 3amp sockets on the secondary side for both the 240v and 110V outputs.

In 2005 I had my house rewired and all of the circuits are on individual mini RCDs within the consumer unit. I believe these are rated at 30mA trip. I was intending to put this transformer to an MK 10mA trip socket as I was given two of these when we had the lab I worked in rebuilt many years ago. These are quite old but have never been used from new so assume that they will be safe to use.

Thanks again

Rob

[Mark Hennessy]

I think that worrying about the fuse rating is moot here - unless you've found a fuse that lasts forever at 2A and blows straight away at 2.001A. Such a fuse would never survive the inrush current anyway.

The best way to protect that transformer is to add a thermal breaker. These open after a short delay as something inside needs to warm up. But, the element warms up faster than the transformer windings, and that's the key point. For an example, see here: http://www.markhennessy.co.uk/isolating/

It's not obvious from the pictures, but the IEC mains inlet is fused. I can't remember the rating, but it could be 5A or 6.3A - I could take a look if anyone needs to know. The transformers are 500VA each, and the thermal breakers are nominally 2A. These transformers/thermal breakers had done some 20 years in a radio studio before I rescued them, so I guess the arrangement works

Al, you're dead right to say what you've said about phase, and you've covered a lot very comprehensively. But in my humble opinion, I felt that it was worth expanding slightly on how to ensure the phase is definitely correct before the two secondaries are connected together in parallel - it's also worth stating what the outcome of getting this wrong might be (not wanting to scare you Rob!).

Rob, if you have access to a 'scope, this might well be very easy to you. But, as you raised the question, I'll assume that it's worth spelling this out - but apologise in advance if it's stating the obvious. Also, it's worth stating to anyone else who might be in a similar position.

If you connect them in parallel, as per Al's third drawing, but get the phase wrong, there will be a "fight". With luck, the primary fuse will fail. But it could be much worse...

One approach to determine phase without using an oscilloscope is to wire the secondary windings in series. If you get 240V that's great - add the dots as shown is Al's first diagram, then proceed to diagram 3. But, if you get 0V, you know that you've wired them out of phase - but luckily, with a series circuit, the windings will dodge each other rather than fight it out

And, even when you're sure you've got everything right, can I suggest that this really is a good use for a lamp limiter. If the lamp remains on at full brightness, you've probably got a phase error on the secondary side.

Hope this helps,

Mark

[Michaelr]

I am not worrying about the fuse rating that is why I am always talking power .
If you can draw more than it is rated for you will have a problem sooner or later

Mike

[Skywave]

Thank you Mark for your valued contribution. I wrote my little missive in a bit of a rush. And yes, the important thing to do with any transformer with multiple secondaries that you intend to join in some configuration is to determine the phase relationships amongst those windings first. In the absence of a 'scope, an a.c. voltmeter is ideal. The important point is this: it is only safe to determine what those phase relationships are when there is zero - or very little - current flowing from those windings. For the transformer in question in this Thread, even if the two windings are rated as having equal voltages and current ratings, it is still worthwhile checking that the voltages from each winding are equal and that the currents that can be safely drawn are equal too.

Picking up on the point Mark raised about windings in parallel that are connected the wrong way round, phase-wise, what will happen in that situation is that each winding will effectively present a virtual short-circuit to the other winding. Hence, the resultant current will be huge - and hopefully, the resultant primary current will also be large enough to blow the fuse in the primary. Hence, determine phase relationships with no secondary currents first.

Al.

[Bushbaby]

Hi Mark,
I've 4 'scopes that I can use - what do I need to do please?

Thanks

Rob

[pwdrive]

I would like to add that if it were mine I would build it in a suitable safe enclosure, if it is connected to the mains via a standard 13 amp plug or hardwired into a switch fused outlet plate then those two methods usually means the minimum standard fuse is the 3 amp type, this is fine to cover a short in the connecting cable and any associated primary switch in the constructed unit, but from that point on a primary fuse or thermal trip should be fitted that is rated to blow or trip at or just above the transformers maximum wattage/power rating, unless the transformer is subjected to exceptionaly high voltage surges (insulation breakdown) the biggest enemy the transformer has is heat due to prolonged excessive power disipation. I would also fit a double pole switch In the live and neutral supply to the transformer within the constructed unit as it might be quicker to reach in an emergency. I would also fuse the secondary outlets to a rating that equals the maximum secondary current that the transformer is designed to deliver on whatever secondary voltage taps are used in relation to its power rating, I say this because when transformers fail it is normally a relatively expensive business to replace them unless you are lucky enough to source a cheap replacement, also in the interests of fire prevention and general safety in the workshop/house.

Some might agree or disagree or want to correct or add but the above is what imediately springs to mind, any comments welcome.

Lawrence.

[Mark Hennessy]

Hi Mark,
I've 4 'scopes that I can use - what do I need to do please?

Excellent - nearly as many as me

Using a pair of X10 probes, after first verifying that they are OK at 120VAC, connect each one to each secondary. Make sure you are triggering from one channel only (some 'scopes offer an "alt-trig" facility) - or perhaps trigger from Line. Observe the two 50Hz waveforms - the phase relationship should immediately become apparent.

Puts safety hat on - as posts can be quoted in isolation (excuse the pun!): Of course, before doing this, you will have ensured that the secondaries are genuinely isolated, as detailed previously in this thread. If in doubt, power the transformer via an RCD, which I think you'll be doing anyway.

Good luck,

Mark

[Mark Hennessy]

I would like to add that if it were mine I would build it in a suitable safe enclosure

Snipped for brevity...

Would agree with all of that, Lawrence

Mark

[Bushbaby]

Hi Mark,
Am I correct in thinking that I need to connect the secondaries in phase and not out of phase?

Thanks

Rob