[Top] [All Lists]

Re: [Amps] HV Fuses

Subject: Re: [Amps] HV Fuses
Date: Tue, 16 Feb 2010 20:59:21 EST
List-post: <">>
I love this list!  It always gives me new ways to think about a  problem.
Thanks Steve, Larry and Carl for your input.
Steve, if there's room to fuse at the B- side, that's an easy and elegant  
solution. Given the physical layout, it may require complete disassembly of 
the  power supply and moving 150 pounds of power transformer out of the way, 
so it's  somewhat less appealing.
Carl, I don't have any quantitative information on forming and sustaining  
an arc. If the gap is, say, an inch or and inch and a half, is that going to 
be  likely to sustain an arc when the wire melts, assuming worst case, 5KV. 
The way  it's constructed, there's nothing nearby for the wire to short to 
or arc  to.
Larry, thanks, you gave me the most direct answer to my question. I think  
I'm going to do some experimenting with strands removed from power cords, 
but do  the experiments with a current-limited 12V supply.
Steve  Gilbert

In a message dated 2/16/2010 8:24:55 P.M. Eastern Standard Time, writes:

Message:  5
Date: Tue, 16 Feb 2010 16:10:37 -0800
From: "Steve Katz"  <>
Subject: Re: [Amps] HV Fuses
To:  <>, <>
Content-Type:  text/plain;   charset="us-ascii"

Why not fuse the negative  lead instead, which is at potential close to
ground?  If you do this,  you need to be sure the HV ammeter shunt is on
the power supply side of the  fuse and not the "load" side, so that point
remains close to ground even  when the fuse blows....

Or use primary fuses or circuit breakers to  protect against major
faults, and a B+ glitch resistor to protect against  momentary tube

Message: 7
Date: Tue, 16 Feb 2010  17:11:23 -0800
From: Larry <>
Subject: Re: [Amps]  HV Fuses
Message-ID:  <>
Content-Type: text/plain;  charset=ISO-8859-1; format=flowed


#30 is way too big. I  use #40 for 1-1.2 amp. I'd suggest #38 for your 2 
amp supply. The object  of the game is to EASILY fuse the wire, not wire 
your house with  it.

Fine wire sizes can be easily obtained by stripping a piece of  common 
lamp cord. The fine strands are each in the range of 36 to 42  usually. 
Take a sample from all of your discarded appliance cords and save  a few 
feet of several sizes.

Make sure you pull the wire tight and  keep it 2" or less between 
supports. I use some spring clip thingys on  ceramic standoffs so that 
if/when it blows I can just push a new piece of  wire into the clips 
without tools.

If the wire is sized correctly,  it will fuse very fast and protect 
everything. I've been using this system  for many years on "problem" 
tubes that like to arc. Never had any  collateral damage occur due to an 
"event" even when there was no glitch  resistor present.


Larry - W7IUV

Message:  8
Date: Tue, 16 Feb 2010 20:24:22 -0500
From: "Carl"  <>
Subject: Re: [Amps] HV Fuses
To:  <>, <>
Message-ID:  <024501caaf6f$f1491650$6501a8c0@DAVES>
Content-Type: text/plain;  format=flowed; charset="iso-8859-1";

QRO requires some sacrifice Steve, its called  money(-;

The bad news with your idea is that at that voltage you have a  better than 
equal chance to create a nice plasma arc with attendant  serious damage. If 
it was so easy HV fuses wouldnt be used by all  commercial products outside 
of the ham community.

HV fuses show up  at hamfests and fleabay for a few bucks.

The creative should be able to  duplicate a real HV fuse.


----- Original  Message ----- 
From: <>
To:  <>
Sent: Tuesday, February 16, 2010 6:59  PM
Subject: [Amps] HV Fuses

> In recent threads, there have  been a number of comments about high 
> fuses.
> Carl  mentioned sand-filled fuses, something I hadn't heard of.
> I checked  the Mouser catalog for HV fuses, and was stricken with
>  sticker-shock...$38 for a fuse?
> OK, we re-think this one...
> I  know that an old but workable approach is  to use  a thin piece of  
> as a fuse...mount it between a couple of  standoff  insulators, and it 
> should
> function fine, and if current  is  substantially too high, the fuse wire
> blows. How do you  figure the appropriate  size wire to use? And then, 
> do  you
> go about ordering 6" of #30 wire, or  whatever it is? If you  need more
> current capacity, to use the above example, can  I put  two strands of 
> wire
> in parallel, and expect them to  function as a  reliable fuse?  I'm always
> worried about  kinking the wire, or if it's  enameled, sanding off more 
>  than
> just the insulation.
> The plan is to fuse the HV output of  a 5KV 2A supply to a 3X3, which of
> course will only be used at the  legal limit or into a dummy load.
> Steve  Gilbert
>  K1SG
> 508-435-9133
>  FN42fe
>  _______________________________________________
> Amps mailing  list


Amps  mailing  list

End  of Amps Digest, Vol 86, Issue  63

Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>