Thread: Fuses
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Old 23rd Dec 2015, 10:57 am   #51
turretslug
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Default Re: Fuses

Quote:
Originally Posted by trobbins View Post
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.
One of my motives for using modern inrush limiters (but in a somewhat higher cold resistance than usually employed for SMPSUs), besides giving an easier time to DH rectifiers, dial lamps and elderly and possibly marginal switches) is that a tightly-rated quick-blow fuse can be employed, rather than the vagaries and inertia of delay types. I know that there's an argument against tightening the safety of primary side fitment over original construction lest it end up being frowned on in court but much original construction was minimal, lousy, hasty, over-thrifted and based on dubious premise and marginal or non-existent legislation anyway. I'd rather head off the problem in the first place, rather than sit back to the consequence of fire or exposing an innocent party to hazard, all the while smugly muttering, "I never touched it, guv". (But I knew that it was inadequate....)

The familiar centre-tap fuse for HT secondaries always struck me as a minimum-hassle after-thought really- it probably stopped a few roasting smoothing/decoupling resistors setting fire to waxies, but as Tanuki points out, it doesn't guard against possible rectifier failure modes. To fuse both secondary limbs would imply using 1.25" types, i.e. considerable HV withstand capability, and that's getting both bulky and (from the accountant's viewpoint) pricey, though it is possible in some stuff. When I joined the world of electronics maintenance, it wasn't unusual for individual ICs to be fed through flame-proof fusible resistors as it wasn't unkown for them to go internal short- and some PSUs had impressive current capability. So, when overhauling valve kit, I prefer to use power film resistors in place of small wirewounds- I don't mind a puff of smoke and a bit of repair work over possible more serious and unheralded consequences.
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