[Amps] HV Fuse

[Vic Rosenthal]

It would seem to me that if the fuses had slightly different resistances then one would blow first, followed by all the others. 
Interesting question: which would go first, the highest or the lowest R?
The current would divide in inverse proportion to the R, so the lower-resistance one would get more current. But the one with the highest resistance would get hotter for a given current.
I think the answer is that the lower one would go first. The heat is proportional to I**2 * R.

Victor 4X6GP 

> On 20 Sep 2019, at 22:38, Paul Christensen <w9ac at arrl.net> wrote:
> 
> I don’t think fuses in parallel are a good idea :-). But probably you weren’t serious 
> 
> Vic, I am serious!  Actually, I would use them in parallel to determine the overcurrent point then try and fine a single fuse of adequate size.  That said, I have seen as many as three used in parallel in another amp design.
> 
> Paul, W9AC
> 
> 
> . I don’t know what the present rectifier modules are,  but rather than trying to protect them with a fuse, I would just make up a few strings of 6A10s, which are basically indestructible. Then you size the glitch resistor so it won’t blow up before the primary fuse blows in the event of a flashover. A nice 50 watt vitreous enameled one of 30-50 ohms should work.
> By the way, I don’t know what the tubes are in that amp, but I presume one or more triodes. A fuse in the plate circuit of a tetrode will cause the screen to ignite like someone hit it with the heat ray in War of the Worlds, unless there is a screen trip circuit.
> 
> Victor 4X6GP 
> 
>> On 20 Sep 2019, at 22:04, Paul Christensen <w9ac at arrl.net> wrote:
>> 
>> Yesterday, I lost all four HV bridge rectifier modules in my Alpha PA-70V
>> vapor-cooled amp.   After replacement, HV returned but whatever caused the
>> fault event also took out a 2N3053 switching transistor that controls 
>> a 8.2V Zener bias diode from the EBS circuit.  I used the opportunity 
>> to change it to a more robust transistor in a TO-220 package that 
>> effectively doubles Ic and Vceo.  The transistor is already configured as a "discrete Darlington"
>> with a 2N4401 ahead of it so the shock didn't get very far on the EBS PC
>> board.    
>> 
>> Going back through the list archives, I see mention of using a HV fuse 
>> just ahead of an amp's glitch resistor.  VE7RF recommended a second HV 
>> fuse placed between one leg of the HV transformer secondary at the 
>> C-input filter cap.  Sounds like a nice way to potentially save the HV bridge diodes.
>> 
>> I have sized the glitch fuse adequately and will use a HV microwave 
>> oven type as discussed in the archives.  However, because of some 
>> unknown variables, it's a bit tricky to calculate the HV transformer's 
>> secondary fuse.  I can use PSUD II software but primary and secondary 
>> HV transformer ESR are unknown variables.
>> 
>> I'm also a bit skeptical that a fuse on the HV secondary will work as 
>> intended due to the extreme initial C charging current (35 uF/5KV 
>> filter cap), even though step-start will limit the initial inrush and 
>> that too factors into the equation.  I could just keep stacking 900 mA 
>> HV fuses in parallel until they don't blow but that isn't an 
>> intelligent approach to a solution.  Any guidance here?
>> 
>> I don't see sand-filled ceramic HV fuses in this category and I dread 
>> the thought of cleaning up glass shrapnel in an amplifier.  I'm 
>> thinking that Teflon heat-shrink tubing placed only over the glass 
>> portion should keep the shattering damage under control.
>> 
>> Paul, W9AC
>> 
>> 
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