torroidal transformer vendor

[John Larkin]

I'm not familiar with the specific EN you mentioned. Seems to be rather more
application specific than the ones I'm used to.

Can you explain the 'transformer overload test ' ? Sounds most unusual to me.

I explained it in my post of 2/8/05.

John

 

[Pooh Bear]

No need; I can do the math. More like 50 amps.

Not with a 50VA toroidal transformer.

Note - *a short on the secondary side*


Graham

 

[John Larkin]

Not with a 50VA toroidal transformer.

Note - *a short on the secondary side*

I can *still* do the math!

John

 

[Tony Williams]

Nope - nothing is allowed to smoke and catch fire.

[/QUOTE]

Which is why, presumably, they did the test.

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.

 

[Pooh Bear]

I can *still* do the math!

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

 

[Pooh Bear]

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.

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

 

[Tony Williams]

[Woods-metal links]

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.

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.

 

[John Larkin]

In which case you'll know a 10A primary fuse won't help. Or did you mean 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

 

[Pooh Bear]

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.

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

 

[Pooh Bear]

[Woods-metal links]

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.

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.

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.

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.

Choose your poison !


Graham

 

[richard mullens]

Hi,

Can anybody recommend a good source for a custom torroidal power
transformer? It will be roughly 50 VA; we need prototypes fast, and
maybe 100 per year after that.

There are about 200 suppliers listed in the EEM for torroids, and I
guess we'll spam lots of them with our spec, but I just wondered if
anybody has a vendor they especially like.

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.

 

[John Larkin]

Maybe not what you need, but how about one of those switched mode "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

 

[Pooh Bear]

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,

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

and good old fashioned iron and electrolytics are more economical here.

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

 

[Ken Smith]

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.

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.

 

[Tony Williams]

[I think there is a polyswitch type of device]

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.