torroidal transformer vendor

A short on the secondary side will happily draw maybe 5A from the a.c. supply and the fuse will never blow.

Just try it and see.

A colleague of mine was once convinced that he could protect a toroidal transformer of around 30VA with a 1A fuse on the primary. I warned him but he insisted. We put a short on the secondary and we rapidly had one transformer with nicely melted insulation after a few secs ! Didn't wait to see the final outcome.

Nope - nothing is allowed to smoke and catch fire.

Graham

Not with a 50VA toroidal transformer.

Note - *a short on the secondary side*

Graham

Some 30VA transformers we buy for a particular application have a Woods-metal fusible link inside them, in series with the 240V primary. If the transformer gets too hot the link opens and the mains is disconnected, permanently.

Standard practice AFAIK on devices with small transformers, such a swall warts.

supply

In which case you'll know a 10A primary fuse won't help. Or did you mean secondary ? Indeed why not fuse your secondary side ( if not already ? ).

Graham

supply

secondary ?

I said primary because I meant primary. If you continually assume that I'm stupid, which is what you seem to continually assume, there's no point in dialog.

John

It certainly is - cheap and reliable - no bother about the fusing characteristics since it monitors the actual transformer temp - and that's the issue.

I was considering suggesting something similar - but for a serious piece of kit you'd want an auto-resetting thermal switch.

I've used these myself in equipment where user abuse has been considered to be a potential hazard.

Graham

Since this is a last-ditch safety measure it could be argued that a permanent cutoff is the safest, in that it forces the equipment to be examined by a qualified person.

However, I think there is a polyswitch type of device that opens at a set temperature, but which then only requires a trickle current to keep it open. Removal of the supply is required to re-set it.

"transformers" used for powering 12V lamps from the mains.

The output of my box is going straight into an NMR probe inside a megabuck superconductive magnet, where they're looking for some very small RF signals; they even cool the pickup coils and preamps to 40K to reduce the noise floor. Switchers aren't allowed anywhere near this stuff.

Besides, I have a high ratio of peak to average power output, and good old fashioned iron and electrolytics are more economical here.

John

supply

secondary ?

Maybe I've misunderstood the equipment.

You were enquiring about a 50VA toroid. I assume that's what you want to protect ?

I can assure you that a 10A fuse on the primary side won't do the job. I know a man who spends plenty of fun hours creating shorts on the secondary side to find the appropriate fusing value for CE compliance. Heck, I even do it myself sometimes !

Graham

In many instances that would be excellent advice.

I believe that this product has some unusual arduous test imposed on it. I'm unfamiliar with the specific EN related to here but John Woodgate ( who should know ) has already mentioned that it's a matter of some debate amongst the 'experts' apparently.

An auto re-setting switch is fine where the 'overload' is user removable by whatever means.

Choose your poison ! :-)

Graham

Maybe not what you need, but how about one of those switched mode "transformers" used for powering 12V lamps from the mains.

In our local DIY shop (B&Q) they have 70W and up to 200W from between £10 & £20.

"transformers" used for powering 12V lamps from the mains.

Not unlike the audio amplifiers I design ( in the kW region ). Interesting stuff

- lol.

If you don't care about weight, yes it's almost certain that it's more economical to use a line frequency solution. Simple too and robust.

Having said that, I've been spending *much* time recently looking at switching supplies since ppl don't like lugging around heavy ampliifers.

Not an issue if it's permanently located of course.

Graham

In article , Tony Williams wrote: [...]

I think the thing you are thinking about is actually ceramic. The polyswitch devices don't seem to be any good at higher voltages. They have the advantage of going open slowly enough that there is no inductive spike to worry about.

Perhaps a PTC type of device, with a sharp response at the required temperature. The sort of thing that is used to protect motor windings.