Thread: Fuses
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Old 19th Dec 2015, 10:33 am   #48
trobbins
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Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Fuses

For the application of general valve equipment such as radios and amplifiers, the general trend is to add an AC fuse(s) on the power transformer secondary for B+ supply. That B+ usually supplies all circuitry, so is often the next best extension beyond just an AC mains side fuse. AC winding protection, compared to B+ DC fusing has benefits.

For most equipment, I can't see a benefit in heater supply fusing, as the heater winding is usually operating at close to rating, and so it would be difficult to generate a substantial multiplier in current above a fuse value that would typically have to be at least 20% above winding rating. In addition, the heater supply does provide quite a substantial turn-on overload that needs to be considered for the AC main side fuse, as well as any secondary side fusing.

PSUD2 is a good tool to simulate the turn-on current surge levels in a B+ supply. For a valve rectifier, that surge is delayed from mains turn-on surge (power transformer magentisation and heater surge current) unless hot-switched. But if an ss rectifier is used, then that turn-on surge is additive to the mains turn on surge.

A NTC part added to the mains supply input circuit is a very common modern addition to equipment. It is easy to retrofit in most vintage equipment, and reliable if properly selected. The NTC is commonly sized to manage the ramp up of a certain capacitor filter supply - so can be very applicable to alleviating turn-on surge current through the mains side fuse relating to PT magnetising current and ss diode supply current, but not for heater supply suppression as that is a much longer time-frame. As such, the NTC can allow a standard fuse, compared to a slow-blow, or a fuse value increment reduction.
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