Looking For A EI Decent Transformer

You must fit a primary fuse.
That could be inside your BS1363 plug top.
But it is preferable to have a separate primary fuse dedicated to the transformer.

Toroidy makes superb transformers.
Send him an Email.
There are many UK stockists
Farnell, RSonline, Rapidonline, etc.

I have many I bought from them when non rohs stock was being disposed of at massive discounts.
I have many that I bought secondhand through Ebay.
But do check that they are really working. Build and use a Mains Bulb Tester.
Check there is no shorted turn. Check voltage is correct. If you have the resources check the no load primary current.

30VA is very small for a tube/valve power supply.
It needs to drive the heater and the audio circuits.
 

infinia

Member
2005-05-15 9:51 am
SoCal
12.6 VAC is the standard perhaps 6.3 +6.3 (it's based on the old tube filament standards)
should be thousands of used floating around. Radio Shack and Maplins used to hang em on a hook. they were decent full housing gold in colour
Typically in commercial applications the thin primary winding of a tiny power transformer acts as a good slow blow fuse, so no fuse is fitted, as always check your countries rules. I reckon it gets difficult buying a such small value fuse to offer protection of the transformer 20VA/230V ~0.1A
 
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tazzo

Member
2012-03-22 1:13 am
I can't see how the 12-0-12Vac transformer can be used to power a Tube/Valve Buffer.

I don't know,it is a chinese one from eBay that I modded, I am told that 30VA toroidal transformer will work on it ok, but I have decided to go for EI core. This is a link to it below, do you think it will be ok or should I go up to 40 or 50VA?

http://www.ebay.co.uk/itm/Assembled...138315?hash=item419811334b:g:p2cAAOSwl9BWJt-K

it says Quote "12VAC 500mA, do not have to worry about looking for high voltage power"

I know how to wire a primary fuse, is a secondary fuse the same principal except it is fitted between the transformer and PCB on one of the secondary windings?
 
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Also would I need a secondary fuse for such a small circuit?

Finally can the amp rating of ~ 0.1A be a larger number so a fuse is easier to locate
the fuse rating for transformers and motors is
3*VA/Vac
eg.
30VA 230Vac transformer requires a fuse of 3*30/230 = 0.39A
One would normally use a T500mA fuse.
You may find that an F800mA fuse has similar protection for a 30VA transformer.

T & F rated fuses in the standard 5mm diam by 20mm long glass package are available from around 50mA upwards.

The BS1362 fuses for BS1363 plug tops are high rupture capability (ceramic and probably sand filled, not glass) and as far as I have seen and bought are available in 1A, 2A, 3A, 5A, 7A, 10A and 13A
 

tazzo

Member
2012-03-22 1:13 am
Thanks for all the help, much appreciated. I am also looking at 40VA transformers, I assume the formula for a primary fuse will be 3*40VA/230V, however I have read elsewhere that it can be just 40VA/230V (what am I missing)?

Anyway, for 40VA will be 0.52A,

The transformer has to be at least 500mA.

If the fuse size required is 0.52A for a 40VA transformer, can I use a 0.6A fuse? How much bigger can the fuse be is it 125%?

Finally, am I correct is saying T600mA simpy means 0.6A?

Thanks
 
Close rated fusing is given by VA / Vac
Normal fusing is given by 3*VA/Vac (this is usually required for transformers and motors where the starting current is liable to blow a close rated fuse. You may find that 2times, or 2.5times, also works for reliable starting over many years.)

Close rated fusing requires current limiting to prevent the fuse blowing at switch ON.
Very small transformer have very high primary resistance and this acts as an inherent current limiter.
Medium and large transformers require some form of soft start to limit the current sufficiently to prevent nuisance blowing of the close rated fuse.
T600mA simpy means 0.6A?
Almost
T600mA = T0.6A
F600mA = F0.6A
m is just the 10^-3 multiplier (milli)

is 600mA available?

I have not seen a fuse marked xAT, the T is always at the front on all the T rated fuses I have, from various manufacturers, i.e. TxA
 
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In any case 30 or 40VA is way over circuit needs.

12VAC @ 500mA means 6VA , anything else is (ample) safety margin.

Fuse rating is basically VA/mains_voltage , multiplied by a safety margin to avoid, as mentioned above, "nuisance" fuse blowing.
In such a case I'd be happy with 1.5X ; agree that on motors and such heavy loads up to 3X might be needed.

Fuse choice often is not meant to "protect the device" but bo protect home wiring, avoid fires, etc. , so current rating might not be chosen by device consumption which might be very low and ask for impossibly small fuses (just try to find a fuse for a 6VA 230V transformer which is what that device asks for) but simply way less than what the outlet can safely supply.

As in:

* actual device needs: (6VA/230V)*1.5= 0.04A <-- try getting one ;)

* wall socket capacity : 10/15A?
* practical fuse choice: say 0.5A
will blow if transformer overheats and shorts, hopefully before it catches fire ; alternatively will blow before home main fuse (or tripping circuit breaker) to avoid you cursing and fumbling in the dark.

Now if you want to protect the device, given it's rated 500mA you may fit a *secondary* fuse rated around 600mA to 750mA , with 1A being still somewhat reasonable.

Remember that fuses *already* have some extra margin built-in so even a 500mA fuse should be enough.

As a side note, I somewhat doubt the "improvements" justifying the use of an "only 12V" powered "tube preamp" , suspecting it more of a glorified pilot lamp than anything else.
 

infinia

Member
2005-05-15 9:51 am
SoCal
Indeed.
A close rated primary fuse will usually blow when you abuse the secondary/ies in any way.
nope not for this size, if u use a teeny tiny fuse then you are just protecting the transformer not providing safety.
A tiny transformer like this will never blow a reasonable sized fuse anyway. ive done the tests for UL, a shorted turn or secondary will just reach some high temp and after a long time the primary winding might give out. ( don't believe me ? look inside any small wall wart, see there is no replaceable fuse.) Contrary to popular belief, a melted/deformed plastic housing is not necessary to fail a inspection, flame and/or smoke is.
I doubt this is the only transformer in his chassis so no additional fuse needed. Just make sure all primary hookup wire gauge exceeds / matches the fuse already installed.
 
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