How hot should a transformer get?

[Adam Lipscombe]

Apologies if this is not strictly a DIY question. I cannot think of better forum to ask.

I recently bought a cheap and cheerful LCD TV for our camper van. It can run off

12 volt DC or 240 AC, with a supplied transformer. The transformer looks like the type you get with a laptop - a separate oblong black box.

When working from the mains the transformer gets hot - very hot. Last night we watched 2 hours of telly and when time came to put it all away I could hardly pick the transformer up. It was almost too hot to hold.

I am worried about a potential fire risk. Is this normal? How hot do they normally get?

Thanks - Adam

[Andy Hall]

It shouldn't get to that kind of temperature. I would return it. If the replacement is as hot, then select another product.

[Dave Liquorice]

Agreed. Are there any ratings on the "transformer"(*) and on the TV? What was the power source when you were using it 12v or 240v? It's not 100% clear if the "transformer" is needed for 12v operation.

(*) I have "transformer" in quotes as quite often these days they are really small switched mode power supplys rather than a lump of iron and a couple of coils. Normally easy to tell as iron is quite heavy, circutry not. I'd be surprised if a SMPSU would run down to 12v though most have a lower voltage limit somewhere above 50v.

[Arfa Daily]

Almost all are switchers, and it's not that uncommon for them to run very hot - especially if they are not particularly well ventillated. It's why they fail so regularly. Just to be on the safe side, it's probably worth checking back with the shop where it was bought, but you are unlikely to find that it really is a problem. There is very little that could cause a switcher to run hotter than it was designed to. Any faults that switchers develop, tend to kill them dead, or stop them starting up in the first place. Most are pretty well protected against excess current draw, and if it is the right power supply for the TV, then that should not be an issue, anyway. Check that it is CE approved, and by all means check with the shop, but you will probably find that it's 'normal', if not desirable ...

Arfa

[Adam Lipscombe]

Hi

Sorry my mistake: Its is a small power supply rather than a transformer. Its only for 240v AC operation - there is a separate cigar lighter type supply for 12 volt.

Adam

Arfa Daily wrote:

[Arfa Daily]

I expected it was, so all I said previously, still stands

Arfa

[Dave Plowman (News)]

Eh? Both my current phone and the last one had SMPS. The current one tiny and weighs less than a normal plugtop.

[meow2222]

e supply for 12 volt.

Yes. If it runs that hot it wont last too well. But as long as it lasts 1 yr you probably have no comeback. If its an expensive brand name, you would. If you want it to last longer you could stick it in a metal biscuit tin with a small fan.

NT

[Steve Lupton]

Input not output :) Most that I have come across seem to stop working at around 80V

[Dave Plowman (News)]

Ah. But there's no reason why an SMPS can't work from any voltage - I've got a handy little circuit that produces 12 volts with an input of 3 - 30 volts.

[meow2222]

Germanium switchers can run off a single solar cell

NT

[The Natural Philosopher]

Sadly they can get that hot. Cheapo tranformers are run at pretty low efficiency: most iron based ones will start to be bad news around

75-80C..thats well above the too hot to touch level.

[Andrew Gabriel]

It's a question of the design criteria. At the lower voltage input, the current drawn will be higher which will be a higher average current in the switching transformer (or inductor, depending on switcher design). The windings have to be designed to take this.

This issue became aparent in electronic control gear for fluorescent lamps. These ballasts are all rated for both AC and DC supplies, so they can be used on batteries for emergency lighting. As the batteries die and the voltage drops, the ballast would increase the current to the point where the transformer overheated and burned out. Nowadays, ballasts always have under-voltage protection to prevent this, but early ones didn't.