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Old 14th March 2013, 04:42 PM   #1
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Default fuse calculation?

I have a pair of LM3886 monoblocks in a single enclosure with a single mains inlet. I know that the mains inlet should really be fused, but how do I work out what is a suitable value?

Each amp has a 120VA toroid & 2x 8300uF filter caps, so the total inrush will be for 2x 120VA toroids & 32,800uF of caps.

I'm guessing that even if I push them to their full power (which I never will) the current draw from the mains will likely be less than 1A but obviously the inrush when I turn them on will be a lot more. Would something like a 1.5A timed fuse be enough? Is there a proper way to calculate what I should use?
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Old 14th March 2013, 05:21 PM   #2
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If Vmains = 230VAC is applied to the primary of a 120 VA transformer, the max permitted primary current is 0.522 amps RMS. So you'd probably use an 0.6 ampere, time delay / slow blow fuse in each primary circuit.

FYI, I just bought a Triad Magnetics transformer, 80 VA, whose datasheet says:
  • Output voltage: 18.0V @ 4.4A
  • Recommended fuse** 250V @ 5A, Littelfuse p/n 313 5.0 HXP, slow blow
  • ** Fuse must be used on secondary as conditions of acceptability for UL Class2/3 operation
The manufacturer's datasheet of the fuse, shows plots of current-versus-time-to-blow, which can help you decide whether your inrush current might blow the fuse during the first two cycles (40 milliseconds) of the 50 HZ mains wave: http://www.mouser.com/ds/2/240/313P_315P-60249.pdf

Last edited by Mark Johnson; 14th March 2013 at 05:28 PM.
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Old 15th March 2013, 12:12 PM   #3
AndrewT is offline AndrewT  Scotland
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fuse each transformer separately.
Use a T500mA in the primary of each transformer.

Close rated fuse = VA / Vac = 120/240 =
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Old 15th March 2013, 01:32 PM   #4
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Quote:
Originally Posted by AndrewT View Post
fuse each transformer separately.
Use a T500mA in the primary of each transformer.

Close rated fuse = VA / Vac = 120/240 =
1x fuse per primary seems like the best approach. Now to decide whether I can be bothered to dismantle the amp enough that I can drill & mount panel fuse holders or whether I just splice in-line holders...
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Old 15th March 2013, 01:33 PM   #5
AndrewT is offline AndrewT  Scotland
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Now that you have decided to adopt close rated fusing, you will need a soft start.
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Old 15th March 2013, 01:47 PM   #6
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Quote:
Originally Posted by AndrewT View Post
Now that you have decided to adopt close rated fusing, you will need a soft start.
Oh. Can you explain a bit? Soft start circuits would probably double the number of components in the amp!
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