[Dave Plowman (News)]

I've just been repairing a linear PSU where the toroidal transformer had
failed due to a faulty 0.1µF disc capacitor across one of the rectifier
diodes which had gone short circuit. The insulation on one of the low volt
outputs from the transformer had melted - so it was obviously drawing
excessive current before it failed.

It's a 28 volt regulated 1 amp PS made by a long defunct pro audio
company. The regulator is an LM317T, and it has an adequate heatsink and
has survived. The transformer a 15 + 15v 30 VA. No LT fuses, only mains
(240v, 250mA standard type.) which had blown.

Now toroidal transformers have a high 'startup' current, so I'd guess an
anti-surge would be better at perhaps 125 mA?

Is there a 'rule of thumb' for mains fuses for such devices? And this
isn't the first disc ceramic I've come across which has failed short
circuit. Better type?

 

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Dave Plowman (News) wrote:

Is there a 'rule of thumb' for mains fuses for such devices? And this
isn't the first disc ceramic I've come across which has failed short
circuit. Better type?

Hi,

To fuse a transformer winding I would:

1. Check manufacturers current rating on the winding.
2. If unavailable, determine the wire guage and work the amperage
3. To allow for inrush, I would measure it and pick the right curve on
the fuse. If that is impratical, a thermal type resettable protector
might be an idea (integrates the current pulse over time).

You may check http://www.belfuse.com/SignalTransformer/ to see if
recommendations for fusing their products are provided. Even though
your transformer may be different, the considerations and methodology
should be similar.

Regards,
Jim

 

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It's a 28 volt regulated 1 amp PS made by a long defunct pro audio
company.

Is it built into something or can random external equipment be connected
to it? If the latter, an output fuse might be a good idea.

No LT fuses, only mains (240v, 250mA standard type.) which had blown.

My Tek 465M (and I assume other similar Tek scopes) have an interesting
approach. There is a regular mains fuse, then the transformer. On the
secondary, there is the winding, bridge rectifier, and main filter cap -
then another fuse before the regulator. I think that's kind of an
interesting place for a fuse.

Now toroidal transformers have a high 'startup' current, so I'd guess an
anti-surge would be better at perhaps 125 mA? Is there a 'rule of thumb'
for mains fuses for such devices?

AoE 2nd suggests using "a slow-blow fuse with rating 50%-100% greater
than worst-case current drain of the instrument." I'm pretty sure that
"anti-surge" is rightpondian for "slow-blow". At 30 VA and 240 V, this
would suggest a fuse in the range of 188 to 250 mA.

And this isn't the first disc ceramic I've come across which has failed
short circuit. Better type?

Maybe Mylar or polystyrene? I'm not sure how much better they might be,
but those seem to be the other common types in similar capacitance and
voltage ranges to small disc ceramics.

Matt Roberds

 

[Dave Plowman (News)]

Is it built into something or can random external equipment be connected
to it? If the latter, an output fuse might be a good idea.

It is internal, but looks like they might have used it on other products.

My Tek 465M (and I assume other similar Tek scopes) have an interesting
approach. There is a regular mains fuse, then the transformer. On the
secondary, there is the winding, bridge rectifier, and main filter cap -
then another fuse before the regulator. I think that's kind of an
interesting place for a fuse.

Yup. I'd have thought before the rectifier would make more sense - since
as I've found failure there can take out the expensive bit - the
transformer. Although it's possible the correct mains fuse would have
failed first.

AoE 2nd suggests using "a slow-blow fuse with rating 50%-100% greater
than worst-case current drain of the instrument." I'm pretty sure that
"anti-surge" is rightpondian for "slow-blow". At 30 VA and 240 V, this
would suggest a fuse in the range of 188 to 250 mA.

Great. Thanks. Just what I was looking for. Couldn't find this in the
couple of text books I looked at.

Yes, IIRC anti-surge and slow-blow are the same. And even time delay?

Maybe Mylar or polystyrene? I'm not sure how much better they might be,
but those seem to be the other common types in similar capacitance and
voltage ranges to small disc ceramics.

OK. Strangely, the PCB hole spacing looked more for a Mylar than a disc.

Thanks once more.

 

[[email protected]]

Great. Thanks. Just what I was looking for. Couldn't find this in the
couple of text books I looked at.

Just to be clear, AoE == "Art of Electronics", 2nd edition.
http://www.artofelectronics.com/ It has the usual textbook stuff but
also lots of practical advice like the above. There is rumored to be a
3rd edition in the works; you can harass one of the authors (Mr. Hill)
about it on sci.electronics.design .

Yes, IIRC anti-surge and slow-blow are the same. And even time delay?

I'm less sure of "time delay" being the same thing. To me, a "time
delay" fuse is a big (>10 A for me, >5 A for you) mains fuse that gets
used on things like motors and air conditioners that draw high initial
current. It's the same idea as a small "slow-blow" fuse in electronics,
but I think the delay time is longer. All fuses have curves that show
how long they take to blow at what percent of their ratings; these are
usually in the manufacturer's catalogs (notably: Bussman and Littelfuse).

Matt Roberds

 

[Dave Plowman (News)]

I'm less sure of "time delay" being the same thing. To me, a "time
delay" fuse is a big (>10 A for me, >5 A for you) mains fuse that gets
used on things like motors and air conditioners that draw high initial
current.

In the UK, time delay is the name given to the sizes we're talking about
too.

<http://www.maplin.co.uk/search.aspx?MenuNo=405&MenuName=GlassFuses&FromMenu=y&doy=22m9&worldid=3>