My isolating transformer unit

[TonyDuell]

Some years ago I was given an RS 200VA isolating transformer. The primary is tapped 0-230-415V, there are 2 secondaries each 55-0-55V.

Today I decided to finally get round to putting it in a box and wiring it up with suitable connectors.

Mains input on an 'IEC' chassis plug (you know what I mean), 1A thermal circuit breaker (I've seen similar breakers, high rating used in 'site transformers' and so far this one has not nuisance-tripped) and a double pole toggle switch (rated for 3A inductive load, so it should be OK).

On the output side, a 4 pole changeover switch. Two poles connect the secondaries in series (220V out) or parallel (110V out), actually nearer 230 and 115V due to the high-ish mains voltage here. The other 2 poles connect the output to either an 'IEC' socket (230V) or a US mains socket (115V). I've connected a neon indicator across each one to indicate which is live.

I could only find a pair of 110V neons in the junk box. No problem, the neon bulb is the same in the 230V version, just a larger series resistor. So a 180k resistor in series with the unit made the 110V one fine on the 230V output.

The reason for the 2 different output connectors is that to over-voltage an American device I both have to mis-set the toggle switch and plug it into the wrong socket (which would mean finding an adapter lead etc). I think that's unlikely to occur.

And now for that old chestnut of output side earthing. Obviously the secondaries are fully floating wrt earth. What I did about the 'earth' pin on the output sockets was wire them together (only one socket will be used at a time after all) and to a 4mm socket on the case. Another 4mm socket (spaced the standard 3/4" away) is linked to mains earth/the casing. So I can jumper them toghether if I want the safety earth on the output linked to mains earth (and I can see times when it is a good idea) or not as I choose. Or put a flying lead into the 'mains earth' 4mm socket to connect to the exposed metalwork of the unit under test.

Maybe not ideal for use by non-technical people but I think it will be fine on my workbench.

[TonyDuell]

I've attached a couple of photos of the thing (the complete unit and the wiring on the back of the panel) along with the circuit diagram just in case anyone is interested.

[Keith956]

Nice one Tony, where did you get the case from? Been looking for something suitable for a 250va transformer I have.

[Diabolical Artificer]

Grand work Tony, neat work. I have an iso tfmr case Keith if you want it, it's in a different style to that one though.

Andy.

[deliverance]

Nice work Tony very neat I also have a 250va transformer that needs a case .

[TonyDuell]

The case was originally from RS, but I don't think they still do them. I've had it for years, it came in a load of 'junk' (useful components) that I was given. RS do still sell plenty of cases that would have done of course.

The case came as a flat pack and was supposed to clip together with the front and back panels held on with screws into caged nuts. But I drilled through the flanges and put nuts and bolts in as well, I didn't trust the clip part.

[dragonser]

Hi,
that is a nice looking case and design.
I wish my metalworking / design skills were as good as yours.....

[TonyDuell]

Most of the metalwork was done by hand (marked it out, centre-punched, drilled). But I did use my limited milling facilities (a vertical slide on the lathe) to make the rectangular holes for the 3 connectors.

Not much design IMHO. Once I'd got the transformer it was just a matter of connecting the input to the mains and doing something with the secondaries. Series/parallel switching (to give suitable voltages for US or European/UK equipment) seemed an obvious idea.

When the current situation has settled down and I can go and get components again I'll get a couple of stud-mounted 4mm plugs and make a shorting link (metal block with 2 tapped holes to take said studs) for the earth sockets.

[Al (astral highway)]

Nice work, Tony!

I love those old RS cases... wish they were still made!

[jimmc101]

Now there's a coincidence, I used the same case for my isolating transformer.
Instrument case Type 1 from 1973 RS catalogue £4.00 each.
Long gone I'm afraid especially at that price.

Jim

[Herald1360]

Originally Posted by jimmc101

Instrument case Type 1 from 1973 RS catalogue £4.00 each.
Long gone I'm afraid especially at that price.

Jim

Something for £50 today would be about the same price in real terms.

[Keith956]

Originally Posted by Diabolical Artificer

Grand work Tony, neat work. I have an iso tfmr case Keith if you want it, it's in a different style to that one though.

Andy.

Interested if it will fit the transformer I have which measures approx 96mm wide x 102mm deep x 108mm high. All the boxes I've found so far tend to be very large to accommodate the height.

Keith