[sdiy] Power supply fuse question (really basic).

[Oren Leavitt]

Hi Nils,

What I would do, given the transformer is rated at 12VA (Volt*Amps aka watts), is determine the primary current at max load (12VA / 220V = about 55 mA) and use the nearest standard fuse that is greater than 55ma.
The transformer will draw a surge of current much greater than 55mA when the power supply is switched on as capacitors charge up, so you'll need to factor in some headroom for that.
A 1/16 A or 1/8 A 'slow-blow' type of fuse, or a 1/4 A standard fuse will probably work fine.

If your secondary does 2*(some voltage greater than 15)@1A, it seems to me like the VA rating would have to be much larger than 12...Your +-15V output can't be more than a few hundred mA @ 12VA.

If you have an AC ammeter handy, measure the actual primary current with a full load on the +-15V outputs and see what you're really working with.

My two amps,
Oren

-----Original Message-----
From: nils pipenbrinck <np at inverse-entertainment.de>
Sent: May 7, 2004 1:00 PM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] Power supply fuse question (really basic).

:: sorry for the double post, but I accidently sent the message before I 
was finished writing it :(

I ressurected an old 2*15V power supply, and I wonder how I should 
dimension the fuse. The secondary does 2*1A (through a 7815/7915 
combination). The transformator itself is rated for 12 VA at 220 Volts.

Right now I just use a 1.6A fuse, but I'm sure that one is somewhat 
overdimensioned : )

Thanks,
  Nils