On Thursday, 21 January 2016 10:42:08 UTC, Dave Plowman (News) wrote:
A fuse characteristic graph will show what size is needed to cope with the
inrush. But the inrush may be unknown to the designer, and the graph not co
nsulted. And even if it is it wouldn't give the right answer, as fuses run
anywhere near their i squared t limit repeatedly fail in time. So ultimatel
y falling back on eperience that says '2A doesn't blow, 1A eventually does'
is good enough, and is likely to be what one engineer went with.
On Thu, 21 Jan 2016 00:34:04 +0000, Dave Plowman (News) wrote:
30VA is 130mA at mains voltage (if the transformer is fully loaded), but
you have 2 inrushes to think about. The first is the magnetising current
of the transformer, the second is the initial charging current of any
reservoir capacitor that you might be using if the transformer is being
used for a DC power supply.
I'd generally use a x2 safety factor for something like this, making the
fuse about 250mA. It would almost certainly need to be a slow-blow, type
You can get more precise, but you need to do a lot more work. :)
On Sat, 23 Jan 2016 14:59:12 -0800, tabbypurr wrote:
Not with a type T fuse. That's the sort of job they are designed for -
time delayed blowing. At 2x rated current you're in the realms of 10s to
blow. At 10x it'll still be around 200ms.
In this particular case, 30VA is a very small toroid so it's inrush won't
be all that big anyway. We don't know what the secondary load is, so the
whole thing is guesswork anyway. :)
On Monday, 25 January 2016 19:53:32 UTC, mick wrote:
Toroids have major inrush current, especially when the core was left magnetised the wrong way at previous switch off. IME time delay fuses don't even have enough delay for EI transformers, they still need to be oversized.
On Tue, 26 Jan 2016 14:43:27 +0000, Dave Plowman (News) wrote:
Hehe... It's only 1A per secondary if the load is purely a resistor. A
30VA toroid is perfectly capable of giving several times that for a short
time without any damage. That's a good thing when you consider that we
tend to rectify the AC and feed a big electrolytic from the half-cycles
to get smooth DC. The initial charging current of that capacitor is
reflected back to the transformer primary and forms part of the inrush,
and hence affects the size of fuse that you need.
Of course, in your case there may be no secondary load when the
transformer is switched on. We don't know that. :)
Toroids are a sod to protect with a fuse. Small ones have a pretty low
ratio of full-load to overload primary current and sometimes you can't
easily find a fuse that is within the right range. They either blow
before full load is reached or they don't protect the transformer against
overheating. The smaller the toroid gets the worse the problem is.
It's not really safe to overrate a primary fuse, even marginally. It
stops giving the protection needed from overload situations - which need
not be dead shorts on the secondary. That's why type T and TT (even
slower than T) fuses are used - they will almost always blow at somewhere
between 1.25x and 2x their rating, but they will take their time about
it. The transformer may get hot, but eventually the fuse will blow. Just
using a higher rated fuse is like replacing it with connecting wire. It
does nothing. The transformer has a maximum primary current that can
flow, based on it's inductance. (That may actually be less than the wire
size will allow!) If the fuse can pass more than that for any length of
time then the transformer will burn out under overload conditions and you
may as well not have bothered with the fuse.
Big toroids can have such a huge inrush that they can take out what looks
like a well over-rated fuse or breaker. They do usually have a much
better primary current ratio though.
On 26/01/16 22:20, email@example.com wrote:
Need to know the primary inductance and the gauge of wire on the primary.
Good rule of thumb is 'two gauges smaller than the primary wire'
I'd probably guess at 500mA though = 10 x 12VA roughly.
The biggest threat to humanity comes from socialism, which has utterly
diverted our attention away from what really matters to our existential
On Tue, 26 Jan 2016 22:51:18 +0000, The Natural Philosopher wrote:
A 500mA type T fuse won't blow at 500mA, it'll probably be between 625mA
and 1A before it takes under 20mins to blow. 12VA is about 50mA at 230V.
I'd be very surprised if the primary could carry 1A for over an hour
without melting (that would mean winding it in something like 20swg wire,
which is pretty thick for a primary winding and may not even fit into a
12VA toroid). It's not the outside of a toroid you need to worry about,
that cools nicely while the primary underneath it gets hotter and hotter.
I'd be surprised if the primary could handle much over 200mA for any
length of time. Are you willing to test that? ;)
Once again, there is load info missing. :) I'd start with a 100mAT or
125mAT for a 12VA.
On Wednesday, 27 January 2016 19:05:06 UTC, mick wrote:
those fuse at what, if we say upto 250mA in 20 minutes that's way beyond th
e thermal ability of a 12VA transformer to handle, it's anything upto 60w.
So those don't offer thermal protection, that has to be done another way. I
normally do it with a secondary fuse. Once you have a secondary fuse you c
an then safely use a primary fuse that's much less fragile than 100mA, and
is thus reliable.
On Wed, 27 Jan 2016 16:42:00 -0800, tabbypurr wrote:
A 100mAT should start to blow at between 125mA and 200mA (this is
variable between manufacturers & types of fuse - it's not gospel). Note
that fuses have an inverse time though, so the higher the current the
faster they blow, but the rate of increase also increases. What you say
is true, a slight overload probably wouldn't be seen and the transformer
might fry, whereas a bigger overload would take the fuse first. A lot
depends on the transformer and load characteristics.
Secondary fusing is very useful, but it's main use is to protect the
transformer against short-circuits or heavy secondary loads such as a
shorted diode in a bridge rectifier. You still can't overrate the primary
fuse as you don't know what sort of current your secondary fuse will have
to take. It will certainly have to withstand charging currents of any
reservoir capacitors but it must be rated to protect the rectifier too.
For that reason it's not usually a type T, as rectifiers can blow faster
than fuses! That makes it an oversized, but fast, fuse. You also have to
be careful of secondary fuses where there are multiple secondaries as
it's very likely that you won't get any primary protection if only one
secondary is overloaded.
For small to medium transformers the very best protection is a
thermistor, preferably buried in the windings, in series with the
primary. It's far better than any fuse.
On Thursday, 28 January 2016 18:32:49 UTC, mick wrote:
200mA is upto 48w in a little 12VA transformer.
I can't make sense of that.
it can't. Trying to protect silicon with a fuse of a few amps is futile.
exactly. It's not type T on cost grounds, pure & simple. It doesn't need to
be a T. Making it time delay doesn't give any added protection to any part
of the system, and it can work without being time delay.
on the contrary the overloaded section will fuse and protect the transforme
r. And when the overload is small, it has far less effect on total transfor
mer dissipation than with a single secondary transformer.
Oh yes. But it costs more.
Trying to protect small mains transformers with fusing is like having one h
and tied behind your back. There is no fuse scheme that confers full protec
tion against damage. But that isn't needed since transformers are very reli
able & robust, and only fire protection is needed. That fusing can protect
I've never forgotten my brother's reaction many years ago when he
accidentally connected a 60mA 32mm fuse across the PSU we used for repairing
car radios - i.e.: 12 volts.
It didn't blow - just glowed orange like a festoon bulb!
(No doubt the wire would have melted sooner or later if he'd left it
connected long enough ...)
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