Fuse Value

[blindsjc]

For a 30watt rms amplifier with a transformer like 2,5 amperes and
12-0-12volts secondary, what is the correct fuse value and type on 110volts?
and...theres difference when on 220volts?

Thanks
Ronaldo

[teemuk]

I1/I2 = U2/U1 = N2/N1,

where 1 is the primary side and 2 the secondary side. I=current, U=voltage, N=turns in winding. If inrush current is not a problem then that should be about it.

Usually due to latter the current is momentarily higher than calculated. My method is testing various slow blow fuses starting from the derived value. I increase the fuse rating until it stops blowing during power off/power on. This method is of course for mains side fuse - secondary fuse should not be troubled by the inrush if it's in a right place.

[blindsjc]

Sorry,
I think I didnt get it...
U2/U1... so 25/220 = 0.11 ... then the fuse is 0.11 amps?

[joecool85]

It can be taken a little simpler than that.  Basically you have your max rating on the chip (30w RMS), normally peak is double that (it's good to find out for sure though) but lets assume it is.  So 60w PEAK, and then you want to multiply by 1.5 or so for inefficiencys to make sure you won't blow your fuse under regular use.  So, that'd be 90watts you are dealing with.  To get amps you divide watts/volts.  So on 110v you'd want a 0.81amp fuse (1 amp would be fine obviously) and get a slow-blow fuse, its great for this type of application. 

This has always worked great for me.

[blindsjc]

Thanks a lot Joe,
this is clear and really helps me.