Can I use a step-up transformer as step-up?

[cviller]

Hey

I have some electrical devices bought in the US that needs 115VAC <150W and I live in a 230VAC country.
I borrowed some old large (2kg+) step-up transformers bought in Brazil 15-20 years ago. They are wired as step-up, but on the front it says

entrada: 220 (input?)
saida: 110 (output?)
Ordem: 105

Without connecting anything but my voltmeter I got ~440VAC, and of course I didn't plug anything in.

Do you guys think it is possible to use the primary windings as secondaries to get 115VAC without burning down the house?

[richie00boy]

Yes they should work, but the issue may be that the secondary is not insulated well enough to be used as a mains connected winding, at least as far as regulations etc go.

[cviller]

Yes that was also what I was concerned about. I think I'll insert a fuse as protection if the primary (old secondary) windings melt together.

[richie00boy]

They won't melt together so a fuse would not do a lot, it's creepage and breakthrough putting mains where it shouldn't be that is the problem.

[Tube_Dude]

Originally posted by cviller

I borrowed some old large (2kg+) step-up transformers bought in Brazil 15-20 years ago. They are wired as step-up, but on the front it says

entrada: 220 (input?)
saida: 110 (output?)
Ordem: 105

The transformers are step-down transformers!

220 entrada (means input in Portuguese )

110 saida (output in Portuguese )

They are ideal for your purpose , that is connecting 115 volts AC devices in a 230 volts mains...

[kevinkr]

Definitely designed as stepdown transformers, the fact that they survived operation as a step indicates that the insulation was more than adequate. Currently they are hooked up backwards to operate as step-up transformers and you will have to rewire them the other way round.

One thought - do these have separate windings (isolation trans) or is it an autotransformer. You can check by measuring dcr between any primary and secondary terminal, if you have continuity you have an autotransformer.

If you have an isolation transformer as opposed to an auto transformer I recommend grounding one side of the secondary and treating that as neutral, the other side of course being line.

If you have an autotransformer it's a little trickier as you want to want to identify the cold/common end of the winding and make this neutral on both the 230V side and the 120V load side. Do not ground it in this case! (If you have a good meter these will be the two connections with the lowest dcr.)

Both above cases will minimize the potential difference between neutral and the chassis which in many devices made for the north american market is important for hum considerations, and to a lesser extent safety. (Neutral is usually not switched on 120V appliances, and the neutral(cold) end is usually (but not always) closest to the power transformer core if used.

One other thought is that at 2kg, these are pretty small transformers and I suspect they are good for about 100VA max (105?) - so load currents should be no more than 0.8A or so.. Big in conjunction with one of these would be >10kg..

I would recommend at least 500VA for a 150W load for good regulation. Growing up I had a 1KVA to power small tube hifi system which used about 250W total. It ran pretty cool to the touch. Burned up autotransformers were a common experience in my world..

Most supply houses here and in Europe can readily provide a suitable step down autotransformer. What are you trying to run?