Repair and Restoration => Electronics => : TC Chris September 11, 2019, 06:44:09 PM

: Power transformer fuse
: TC Chris September 11, 2019, 06:44:09 PM
I'll answer my own question about a power transformer fuse, discussed under another topic recently. While filing away some info I had printed out, I found the file on that subject.  One guy said he used #47 lamps on the PT center tap.  "This acts as a fuse with a 150 MA trigger."  I suppose you could consult tables of lamp currents to select another value. Another guy worried about using fuses rated for 125 VAC at higher voltages because when they blow, the vaporized metal can coat the fuse inside and conduct.  He said that HV fuses are longer & larger.  He uses an inch of #30 wire ( a single strand from an 18 gauge line cord is #34) across a terminal strip, or larger sizes for more current.  Interesting ideas.  These were both ham radio guys discussing their receivers.

Chris Campbell
: Re: Power transformer fuse
: electra225 September 11, 2019, 09:38:06 PM
All a fuse is would be a piece of wire that melts when it get too hot by carrying more current than it can handle.  Most of them are inside glass tubes, but what you are talking about should work.  Seems like a lot more trouble than just using a commercially available fuse, but to each his own, whatever works, etc...... ;) :)

I'd say any of the above options are preferable to using no fuse at all.   :) :)
: Re: Power transformer fuse
: TC Chris September 11, 2019, 10:14:34 PM
The point of one of the suggestions was that the 125 VAC glass fuse could conduct via vaporized metal deposits on the glass, when used in a high voltage application.  The wire would just fizz into the atmosphere.

Chris Campbell
: Re: Power transformer fuse
: ed from Baltimore September 20, 2019, 10:09:28 PM
              I think a fuse rated for AC is different than the same fuse rated for DC, even for the same voltages and currents. When a fuse used for DC blows, he arc formed can keep alive even as the distance between the open metal wires gets bigger and bigger as the arc vaporizes the fuse element. The current never stops like it does in an AC circuit (120 times a second, as the current stops and reverses along with the voltage). 
              So if the DC power source interrupted can supply more power to the arc than is dissipated by the arc, it has no reason to extinguish. A conducting arc has very little voltage across it because it is ionized air which is a low resistance. The arc could become the full length of  the fuse, even if the distance is so great that the arc could never get started if it didn't start out as a small gap first.
            When a fuse in the DC circuit of a power supply blows and a DC arc starts it might draw a heavy enough current to make smoke and even a small flame, but not enough to blow a slow-blow fuse instantly, just enough to blow it eventually. Meanwhile a lot of smoke and flame occurs.
          I have seen industrial type equipment where an overcurrent, even a small amount over,  will trigger an SCR or a shorting relay to force the AC primary circuit breaker to trigger fast, like in a circuit breaker box in house wiring. 
          I think that if you want more than a single fuse on the transformer primary, you can always put DC automotive fuses in series with the output tube cathodes instead of in the audio transformer plate circuit. An inductor such an audio transformer primary winding with DC on it is actually the best way to keep an arc going instead of extinguishing it. Like the ignition coil in a car engine or a welder.