Setting up isolation transformer

[soldar]

I have an isolation transformer which I use when I want to isolate a circuit from the mains and also to reduce from 230 to 125V.

On the input side it has 220 and 380 V and on the output side it has 6.3, 24, 26 and 220V.

All are kind of nominal because with no load it outputs higher voltage and the voltage drops with load. To compensate I can connect to different taps in the secondary. But, no matter, this is not what my question is about.

I have had this transformer for many years now and it just sits therein the open. It has nut and bolt terminals and I have installed solder terminals and I just solder to each as needed which is inconvenient and messy.

This is a shoddy setup but I have had it like that for many years now. One drawback and danger is that the terminals are out in the open and anything falling on them could shortcircuit them.

So I am thinking of a better setup and I am looking for ideas and experiences.

I am thinking of an enclosed box which entirely covers and protects at least all the primary side. A fuse, a selector to choose 380/220/none and a 100W incandescent light-bulb in series which can be shorted. Probably also a neon light to show when the primary is connected. This seems pretty straightforward to me. The one thing I am thinking is have the 100W incandescent light totally outside and separate from the enclosure and set it up so I can plug it in and it is inserted in series. Or just a switch that shorts the bulb but I want to have it be very clear when it is in series and when it is shorted because I often have to check this. I want it to be very apparent.

Up to here the primary side.

On the secondary I am just inclined to put some post or plug or anything simple because I might want to connect to any of them.

I'd like to hear any ideas, experiences, etc.

[Roehrenonkel]

Hi soldar,
 
if the secondary windings are isolated you can switch them together
In-phase or Out-of-phase to raise or lower the total secondry voltage.
 
Just an idea.

Good luck

[m k]

If you can verify that windings have equal wires then fine, but 6.3V is an old vacuum tube heater voltage standard, so maybe its winding is less powerful.

[CaptDon]

His secondary windings are not separate as shown in the schematic. Banana jacks and plugs would work great for your secondary connections.

[Zero999]

I would recommend two, make before break, four position rotary switches for the secondary connection.

[soldar]

I would recommend two, make before break, four position rotary switches for the secondary connection.

Make before break would be shorting secondary windings so better not.

I am considering two rotary switches or banana jacks and plugs. In principle I find bananas simpler and more flexible except that I would want the type that are well isolated with sleeves so that there is less danger of touching anything. In that sense rotary switches would be better. But banana connectors offer more flexibility and I could even use more than two outputs simultaneously. Or maybe the spring loaded type like used in speakers.

Regarding the primary side I am inclined to go as per the attached diagram. For the incandescent lamp I might just provide a connector where I can plug it in, rather than include it in the device itself. Until now it was just hanging on the wall and I think I like to keep it that way.

As I say, I have been using this transformer for many years now and I know it well. What I am looking for is advice on how to mount it so I can use it more conveniently rather than soldering, etc. Now I have to double check to see if the incandescent lamp is shorted or not. I want a switch which will provide clear visible indication.

I want to get it right the first time because I want to make a metal box and I *hate* this kind of work. I do not really have good tools to work with sheet metal and it is a lot of work. I tend to go with wood just for ease of build but I do not like that it does not dissipate heat as well and things like switches are better mounted on sheet metal. Commercial enclosures are too expensive and never the right dimensions.

[Zero999]

I would recommend two, make before break, four position rotary switches for the secondary connection.

Make before break would be shorting secondary windings so better not.

Yes. I meant break before make.

Two, four-way rotary switches will be every bit as flexible as banana plugs. The commons connected to the outputs and each way, connected to a transformer tap.

[soldar]

Yes. I meant break before make.

Two, four-way rotary switches will be every bit as flexible as banana plugs. The commons connected to the outputs and each way, connected to a transformer tap.

Many years ago I used to have a tube tester with that kind of arrangement where I could select the filament voltage with two rotary switches.

In this case it would work but with the disadvantage that the rotary switches are more expensive and more prone to malfunction. And with bananas I could use more than a single voltage at a time which may be useful once in a while.

I guess I could do both. Rotary switches AND banana plugs.

My experience buying switches in eBay is not good. Much crap. Any recommendations?

[Zero999]

Yes, banana plugs would be more flexible. It didn't occur to me, you might want to use more than one tap simultaneously. The downside is the circuit will be disconnected for much longer, changing voltages, compared to a switch.

I've never bought rotary switches off eBay. It's always something I get from a proper distributor.

[Ian.M]

Do you *need* the flexibility?    Five shrouded banana jacks, a Schuko socket across the whole winding, and a NEMA 5-15 between the 230V and 24V taps to give 120V with the voltage selector switch at '380' should cover all likely uses.

I would want to avoid rotary switches unless you leave a 'dead' position between 36V and 220V due to the risk of flashover switching them under load.

Also, give it two externally accessible bulb sockets to give more flexibility tuning the limit current to the expected load when using it as a 'dim bulb' tester, and also because the transformer magnetizing current may 'pre-load' the bulb enough to significantly reduce its trip current.

The secondary taps should be appropriately individually fused, as a primary side fuse capable of delivering the transformer's rated VA for 220V input will be ineffective when it is in a step down configuration.

[soldar]

Thanks. My ideas are beginning to take shape.

I think I will leave out rotary switches. Even though I could have both, rotary switches and banana plugs, I find they are too expensive and not worth the expense.

Also, I have found I have a box full of old style, 4 mm round pin, 19 mm centers, panel outlets so I am going to try using those. As each one has two holes I will mount them vertically so each transformer tap is connected to a pair and that way I can connect something else like a voltmeter or whatever. I just have to make sure I label everything correctly and understand what goes where.

A Schuko socket across the whole winding might be useful but they are so bulky I would rather leave it out and do external connections as needed.

It is a good idea to have a couple of base sockets where I can externally connect the incandescent lightbulbs. I think I will do that.

I do not think individual fuses on each tap is really necessary. It seems more complicated and trouble than it is worth. The main use of the lower voltages is not to get a low voltage output but rather to subtract from the isolated 220/125 V output which is too high with no load or light load. This is not a transformer to be used in a permanent installation but rather it is going to be used at the workbench and always supervised. It just seems to me that having a fuse for each tap is a bit much. I suppose I could do it... I'll think about it. As I say, this is to be used on the workbench. I think just using my common sense, like I have been doing all these years, should suffice.

Another idea that I have is that I have a box full of old VU panel instruments and I could use on on the front panel as a volt meter. this seems like a good idea.  Leave two leads hanging out and connect them to whatever output taps I want to measure. Or that could be done with cheap rotary switches as they would carry very low current.

I have to think of all this in the context of what housing I will be using. This is becoming the biggest hurdle now. I would want a metal enclosure but I prefer to work with wood. I need to think about this.

[soldar]

Some more ideas. For the incandescent light(s) I am thinking of installing two sockets in parallel, one in the back panel and one in the front panel. That will provide flexibility in connecting more or less.

Also, I will install a 220/125 V output socket in parallel with the primary. When the primary is connected to the 380 then the secondary is 125 but I can also use the primary as autotransformer down to 125. Not that it will get much use so I will place that in the back.

[Ian.M]

Check your math!  230V*220/380=133V, a bit high for the nom. 115V US mains.  Also its compounded by 50Hz vs 60Hs as at 50Hz, transformers designed for 60Hz start saturating at a 17% lower voltage. 

Using the 220 and 24 taps in this step-down configuration will give 119V, or 220 and 36 for 111V, so if you are going to use a universal mains socket (horrible things) you will need a tap selector switch for it, possibly ganged to the input tap selector switch.   Personally, as panel mount NEMA 5-15 sockets are fairly compact I'd simply fit both it and Schuko and avoid the need for an output tap selector switch.   If you need to deal with other 230V mains plugs (e.g. BS1363), stack a travel adapter (single type, not universal) in the Schuko socket.

[soldar]

Check your math!  230V*220/380=133V, a bit high for the nom. 115V US mains.  Also its compounded by 50Hz vs 60Hs as at 50Hz, transformers designed for 60Hz start saturating at a 17% lower voltage. 

Using the 220 and 24 taps in this step-down configuration will give 119V, or 220 and 36 for 111V, so if you are going to use a universal mains socket (horrible things) you will need a tap selector switch for it, possibly ganged to the input tap selector switch.   Personally, as panel mount NEMA 5-15 sockets are fairly compact I'd simply fit both it and Schuko and avoid the need for an output tap selector switch.   If you need to deal with other 230V mains plugs (e.g. BS1363), stack a travel adapter (single type, not universal) in the Schuko socket.

Transformer is 50Hz on a 50Hz supply. No problem there.

American nominal voltage is 120V and any device designed for 120 will handle 10% more without problem.

Even though nominally our supply is 230, my panel volt meter pretty much always shows around 220V, which is within tolerance. The fact that they changed the nominal voltage does not mean they changed the actual supply.

I would have to measure actual output voltage under load but it will be lower than theoretical calculation due to voltage drop in windings. Taking variables and tolerances into account I would say the chances of the output voltage being over 130 are very very low. And even a bit over that would not be a problem. I would begin to be concerned and careful at over 140V.

I do not see a problem.

[Ian.M]

My point was that any US device containing a line frequency transformer cant tolerate high line voltage as well at at 50Hz, so its better to be slightly low rather than slightly high.  Also there's your transformer regulation factor to consider.  5% to 10% higher voltages are likely at low load currents as the nominal output voltages will be specified at full rated load current.  The marked voltages will allow for voltage drops in windings as extra secondary turns are added to compensate (unless you swap primary and secondary, in which case the compesation acts against you and its much worse).

[soldar]

Yes, well, as I say, I have been using this transformer for almost twenty years now without problem. I know how to check voltages etc in each specific situation. That is not the problem I am trying to solve. I am trying to figure out an efficient way to house it and connect it without having it open on the bench and soldering to tab terminals. Sometimes I have to wait for the soldering iron to heat up and I would rather have a more practical and efficient setup. That's all.   

[Ian.M]

So put seven banana sockets on it, five for the secondary taps and two for the universal mains socket.  Then you can patch it however you need it without waiting for the soldering iron.  If you *really* want to avoid soldering mains stuff, treat yourself to a Cliff Quicktest!

I'd put a Schuko plug on the Quicktest rather than hard-wiring it to your isolation transformer.