Do I need a step-down converter?

T K

Hi all,

I recently bought a heated cat bed in the US. It's 120V, 4W, and it's intended to be left on all the time.

For the last few days I've been using a simple plug adapter and I've had no issues. I realised today there's a potential problem with the voltage difference, so I've turned it off for now. Can anyone tell me if it's okay to continue using the normal adapter, or if I should be buying something like this? Or is there any way I could just change the plug?

Thanks for your help!

Cerco

If it is. 110v only, then you need a step down transformer. Otherwise you could have a catastrophe.
Check if it is specified for 110 only as many devices nowadays can handle both 110 and 220/230v.

eeguy

Doubling the voltage = doubling the wattage, so your cat's bed is now a 230V, 8W.

Which means he'll be getting a bit sweaty.

Bruthal

eeguy wrote: »

Doubling the voltage = doubling the wattage, so your cat's bed is now a 230V, 8W.

Which means he'll be getting a bit sweaty.

It does in fact quadruple the wattage with a voltage double.

T K

Thanks all. When I had it plugged in, it wasn't hotter than I would expect. I found the packaging and it does say for use with 110-120V AC only. I've emailed the manufacturer to ask too. The cat is sad so I'm hoping they reply soon.

If I do get a step-down transformer, is that suitable to leave plugged in all the time? Will it make a noise?

deandean

Surprised it dodn't go bang when you plugged it in!
Yea you need a voltage converter.
Here's a handy one, in stock in Radionics:

http://ie.rs-online.com/web/p/plug-in-power-supply/4693959/

T K

Thanks - I've read that these aren't suitable for continuous use though?

Bruthal

deandean wrote: »

Surprised it dodn't go bang when you plugged it in!
Yea you need a voltage converter.
Here's a handy one, in stock in Radionics:

http://ie.rs-online.com/web/p/plug-in-power-supply/4693959/

With it being just 4 watts, it would be 16 watts or so on 230v. Still only 0.07 amps. So it might last a fair while on 230vv if its simply a heating wire, rather than an instant bang.

Still needs the transformer though.

Bruthal

Bruthal wrote: »

It does in fact quadruple the wattage with a voltage double.

Please explain.

Cerco

Sir Liamalot wrote: »

Please explain.

I=V/R. 2I = 2V/R (resistive load, doubling the voltage, doubles the current)

Fixed resistive load.

P= VI . When voltage doubled, P = 2V.2I = 4VI

QED

Bruthal

Sir Liamalot wrote: »

Please explain.

If you apply double voltage to a fixed resistance, you have double current, therefore 4 times the wattage.

That`s how we have IxIxR = watts. VxI = watts. V = IxR. So IxIxR = watts. So double I = 4xwatts.

But anyway.........

230v Kettle. 2kw. Ohms = around 26.5 ohms.

So the kettle element is 26.5 ohms

connect to 230v
230/26.5 = 8.68 amps = 1996.4 watts (with rough rounding)

connect to 115v
115v/26 = 4.33 amps = 497 watts. (with rough rounding)

The thought occurred to me the other day. Not that I have any good reason to do it but can you power the "output" of a yellow block site transformer to get 460V from the "input"?

Bruthal

Sir Liamalot wrote: »

The thought occurred to me the other day. Not that I have any good reason to do it but can you power the "output" of a yellow block site transformer to get 460V from the "input"?

Tramsformers can be reversed alright. And if done with the yellow 110v one, will produce 460v alright.

I wouldnt think the primary winding would stand up to the 230v applied to it for too long though.

More realistic is connecting 110v to the output side, and getting 230v from the (normally) input side, probably with the centre tap earth disconnected.

Bruthal

Sir Liamalot wrote: »

Insulation breakdown dya reckon? I thought the primary winding (in reverse operation) would be a higher gauge to cater for the higher current at lower voltage?

It is a higher gauge indeed, which means connecting double its intended voltage might cause overheating even when unloaded. But with the iron core and no load, it might be ok.

There has to be some point where reversing a transformer will have too high a voltage for the secondary to be used as primary.

For example, a 230 to 12v bell traffo, wouldnt be expected to function as a 230 to 2700v transformer, as the 12v side wouldnt take the 230v for long.

But a 110v winding might take the 230v alright when the transformer is unloaded. But its lower resistance (heavier gauge) than the 230v side, might cause it problems with 230v applied to it.

An interesting question for sure.

Bruthal

Hehe, busted post deletion, fair play.

Bruthal wrote: »

There has to be some point where reversing a transformer will have too high a voltage for the secondary to be used as primary.

That'd be core saturation point methinks.

Bruthal

Sir Liamalot wrote: »

That'd be core saturation point methinks.

Yea and it takes a higher primary voltage to cause it, when the transformer is unloaded. But is say there is also a resistive element where overheating will occur, even if the core is large enough to avoid saturation from too high a voltage.

You will have to get a transformer and try it out. Connect 2 100w 230v bulbs in series on the 460v side.

Measure the no load primary, and primary with the 200 watts connected.

If I ever feel the need I'll let you know how it works out.
Probably will require an inrush limit of some kind.
I know you can overwind variacs by wiring the neutral to the centre tap.

Bruthal

It should work OK I'd say. Don't forget to disconnect the centre tap:)

T K

Thanks all. The supplier said it won't work with 230V. I think my best option to replace the internal heating pad with a three-pin plug version. In the mean time, it's back to the low-tech hot water bottle for the cat.

Bruthal

Bruthal wrote: »

It should work OK I'd say. Don't forget to disconnect the centre tap:)

...which begs the next question. If I wire phase to phase and neutral to centre tap. Do I get a 1:1 isolation transformer?

Bruthal

Sir Liamalot wrote: »

...which begs the next question. If I wire phase to phase and neutral to centre tap. Do I get a 1:1 isolation transformer?

Interesting alright.

If you connect phase to one end if winding and neutral to centre tap, you are connecting 230v to a 55v designed winding. Might be heading for 800v secondary then. Might be heading to saturation city there even while unloaded.

If you connect phase to both ends, and neutral to centre tap, I'd guess there would not be many volts on secondary side with primary halves opposing each other.

Only the 110v side is centre tapped and earthed on them transformers.

Bruthal

Bruthal wrote: »

Only the 110v side is centre tapped and earthed on them transformers.

Ah shame, I could use a 1.2kW isolation transformer.

Bruthal

Sir Liamalot wrote: »

Ah shame, I could use a 1.2kW isolation transformer.

You could use 2 of them for that, although probably impractical.

Yeah thought of that already. Thx.
They're not renown for their efficiency, I might buy a toroid but I'm looking for a workaround at the moment unless I find one free to good home.

Sorry 'bout the thread hijixing TK.