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Old 2nd February 2011, 05:18 PM   #11
fhs is offline fhs  Germany
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Hi Erwin,

my only reason for using an NTC is to limit the inrush current so that I can use an appropriately rated fuse. Again, you can use a resistor to perfom the same function. Either one should be short-circuited by a relay after a little while.

Slow-start circuits are a different ball of wax: I do use them for my DC filament power supplies, hopefully increasing tube lifetime. I also delay the application of HV to the tubes. In your initial question, you mentioned a 50000 F capacitor -- it's obvious that we are not talking tubes here. I do not have any experience with solid-state amps and vacate the turf for people more knowledgeable than me. I cannot really comment on the effect of slow-start circuits on solid-state audio equipment.

All the best,

Fred
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Old 3rd February 2011, 08:29 AM   #12
AndrewT is offline AndrewT  Scotland
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Quote:
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The fuse for 220Vac supply is 3 * 75 / 220
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No (not considering the <1 efficiency of your transformer): 75W at 230V = 75/230A <0.5A; 75W at 115V = 75/115A < 0.7A. OK, you have some leeway here and may use a fuse with a somewhat higher rating.
the formula has nothing to do with efficiency.
It's standard for transformers and motors.
You state "somewhat higher rating".

That "somewhat" equals 3.

As I went on to say, if you want instead to use close rating then a soft start of some type must be used to allow the lower fuse rating to survive repeated start ups.
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