Transformer Question

My read on this is that the transformer converts input to 1/2 voltage on output. This means that connecting 240 on the input side will result in 120 on the output side (similarly 480 ==> 240).

You need a 1:1 transformer not a 2:1 as described here.

Dan

Note: If the transformer you have has multiple windings or taps, it may be usable as a 1:1 transformer. How many connections are there?

In case I was not clear.

Primary is labeled 240 or 480 v Secondary is labeled 240 or 120

What I want to do is connect 120 volts to the primary winding intended for 240.

Then use the secondary winding intended to be for 240. This would be

1:1, except I'm running 120 volts into windings made for 240volts.

You will probably have to reduce the current drawn as the size of the wire will be smaller than if it was for 120 volts.

It would be OK but you'd better put the 2 240 lugs in parallel on each side.

This is needed for having the maximum available power.

Huh? You lost me. Yes, I know what the word parallel means, but what do mean?

On the primary side, I use the 240 lugs, and connect them to a power cord.

On the secondary side I connect the 240 lugs to an outlet.

That's really all there is to it, (not including any fuses or switches I might include).

--

As far as this project, I have abandoned it, due to learning from the replies, that the output current would be halved, from 750va to 375va. I need a supply which outputs at least 500va.

I have an isolation transformer with a 100va output. It works ok for the "American Five" (five tube) hot chassis radios, but I want something with more current for larger amperage needs.

I saw this transformer on ebay and thought it would work. I did not know that the current would be halved, which is why I asked abotu this, before buying it. I'll have to just keep watching ebay for something more suitable that is affordable. My other thought is to wire a 240v outlet to my bench and use some of the 240 to 120 v transformers on ebay that sell for small dollars compared to the 120 to 120 transformers. But the problem with that, is that I often work on stuff away from my bench, and then I would not be able to use it.

I dont need it tomorrow, so I'll just keep watching for something more affordable that will give me the power I need. Either way, I'm sure I'll end up buying a transformer and making my own isolation device. The commercial ones are very costly, especially the ones that output above

250va. I already have plenty of power cords, outlets and enclosures to make one. And wiring it up is about as simple as any project can be.

Thanks to all who helped!

Just unlink the two winding by cutting at the middle point which is the common one (it makes 4 separate windins).

Then put them in parallel at primary and secondary (and in phase !).

Philips used to do this all the time.

** OK - so 240 & 240 plus 120 & 120.

** Nah, that makes a 240 to 120 step down.

But only if the parallel pairs of windings have the *exact* same number of turns.

If not, you get major overheating.

.... Phil

Exact !! But for me it was obvious

Well...not completely exact :

What counts is the magnetic flux seen by the 2 windings ; Since they may not be at the very same place is might not imply the same number of turns.

Generally, these winding are executed (manually or automatically) by the "2-wire in hand" method.

The 750VA rating applies to the part at it's label voltages. If there is a dual primary - two windings which, connected in series, will support 480V and which, connected in parallel will support 240V, then a primary current of 1.5A is expected in each winding under rated operation.

Reducing the voltage does not change the current capability of these windings. If you halve the applied voltage, you halve the throughput power rating, in this case, to 375VA.

A less efficient winding method, with a single tapped winding, will use different gauge wire for the 240V section of the winding.

The same considerations apply to the secondaries.

Wired as 240in to 240out will produce a 1:1 isolation transformer.

RL

Windings with the same label voltage will have the same turns. This is checked automatically/electronically during fab. Unbalanced turn coils are discarded/scrapped.

RL