120V / 240V Transformer question

[made2hack]

Hello,

So, I've got a 120V/24V 60VA transformer that I took out of my Hakko fx888d North American version. I replaced it with a 240V/24V 60VA transformer here in the EU.

My question is, what can I now do with this 120V Transformer?

Let us first assume the following:
Turns Primary = Tp , Turns Secondary = Ts, Voltage Primary = Vp, Voltage Seconday = Vs

Let's also assume that Tp = 120 turns, or 1V/Turn. Thus, we know Vp = 120, Vs = 24, Tp = 120
And, Ts = (Vs x Tp) / Vp
Plugging for Ts = (24x120) / 120, so Turns Secondary = 24 Turns.

Now, if I am now connecting the transformer to 240VAC (220-240), does the following occur?

Vs = (Ts x Tp) / Vp, or Vs = (24x120) / 240 =  Voltage Primary = 12 V?  Will I get 12V out of the secondary, ALL THINGS BEING EQUAL, if I simply plug in the 120VAC transformer to 240VAC?

Does it essentially divide the secondary voltage by 1/2? This is also a center tapped transformer on the secondary, so in essence it is as 2x12. Will I now get a 2x6V?

What happens to the power rating? This is a 60VA transformer, or 120V @ 0.5A primary and 24V @ 2.5A on the secondary.

If I plugin 240V, obviously the primary side has wires that are thicker than needed, since I am now only putting through 0.25A. But the secondary, technically has "thinner" wires designed for the 24V. Am I now running 5A through the secondary?

Will this mean that, ALL THINGS BEING EQUAL, that I have reduced the VA by half? To keep things safe?

Hope I explained things clearly.

[Zepnat]

You still need to maintain that assumed '1v per turn' on the primary. Double the primary turns to cope with double the voltage or else it will release the magic smoke verry quickly.

[SeanB]

Simplest is to take it, hit with a hammer to loosen the varnish and remove the laminates, then unwind the copper. Take all down to scrap and you will get around $6 for the lot.

Running a 120V transformer on 220V is a very quick way to have a room full of nasty smelling black smoke and a tripped mains breaker. The other way round it will work, but not with a higher voltage than the designed spec.

I have a very nice variac that has a 5A rating, but unfortunately it is designed for 115VAC 400Hz operation. However it has the right ratio such that it works very well with a supply of 12VAC 50Hz, and I use it as a variable AC supply capable of delivering up to 12VAC at up to 5A. I used it a lot as a room lamp dimmer with a 12V 50W halogen lamp, and it was variable from barely glowing red to bright.

[edy]

If you have 120 V transforming to 24 V, wouldn't doubling the input also double the output? So you would get 240 V going to 48 V? Just intuitively without doing any math... Never mind the current, which again intuitively should also increase given the same load because you have double the potential difference across it.... and higher current will heat/melt the wires?

[miguelvp]

Edy is right it won't half the voltage it will double it.
Vp/Vs = Np/Ns

Np/Ns (turn ratio) in your case:
120/24 = 5.

Moving the turn ratio to the left (divide) and the Voltage for the secondary to the right (multiply)
Vp/5 = Vs

So:
240V/5 = 48V

But like the others comments mention, you can't current limit your mains so it will get really hot really fast dissolving the varnish and melting the wire, shorting and probably catching on fire.

Reversing it might have some use if you ever have some AC power from 0 to 24V that you need to multiply by 5.

Then there is the whole magnetic saturation the core can handle to take into account.

[mij59]

The core of the transformer will be driven in to saturation (search for bh curve), inductance will decrease, and current will increase a lot.
Even with transformers rated for 220V  and connected to  240V  this can be a problem.

[sunnyhighway]

My question is, what can I now do with this 120V Transformer?

Not much I'm afraid.

But if you are a stickler for money, just rip out the copper wires, scrape off the varnish and use it for etch repairs.
You can use the core as a paperweight.