[Amps] HV Fuses

Carl km1h at jeremy.mv.com
Wed Feb 17 07:27:36 PST 2010 

----- Original Message ----- 
From: <K1SG at aol.com>
To: <amps at contesting.com>
Sent: Tuesday, February 16, 2010 8:59 PM
Subject: Re: [Amps] HV Fuses


>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.

A plasma does not act the same as a standard HV arc.
Once the air is ionized it will maintain the arc until it finally burns down 
the wire or its drippings will develop a new path to anything even remotely 
resembling a conductive path.

I take HV seriously and prefer to not go by the "well it worked for me" 
crowd.
Bottom line, its poor engineering.

Carl



> 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
> K1SG
> K1SG at AOL.com
> 508-435-9133
> FN42fe
>
>
> In a message dated 2/16/2010 8:24:55 P.M. Eastern Standard Time,
> amps-request at contesting.com writes:
>
> Message:  5
> Date: Tue, 16 Feb 2010 16:10:37 -0800
> From: "Steve Katz"  <stevek at jmr.com>
> Subject: Re: [Amps] HV Fuses
> To:  <K1SG at aol.com>, <amps at contesting.com>
> Message-ID:
> <31A81EFBA7CCA948882648CB542A30F46575BE at jmr-exch01.ad.jmr.com>
> 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
> arcs...?
>
>
>
> Message: 7
> Date: Tue, 16 Feb 2010  17:11:23 -0800
> From: Larry <larry at w7iuv.com>
> Subject: Re: [Amps]  HV Fuses
> To: amps at contesting.com, K1SG at aol.com
> Message-ID:  <4B7B423B.6030804 at w7iuv.com>
> Content-Type: text/plain;  charset=ISO-8859-1; format=flowed
>
> Steve,
>
> #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.
>
> 73,
>
> Larry - W7IUV
>
>
>
>
> Message:  8
> Date: Tue, 16 Feb 2010 20:24:22 -0500
> From: "Carl"  <km1h at jeremy.mv.com>
> Subject: Re: [Amps] HV Fuses
> To:  <K1SG at aol.com>, <amps at contesting.com>
> Message-ID:  <024501caaf6f$f1491650$6501a8c0 at DAVES>
> Content-Type: text/plain;  format=flowed; charset="iso-8859-1";
> reply-type=original
>
> 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.
>
> Carl
> KM1H
>
>
>
> ----- Original  Message ----- 
> From: <K1SG at aol.com>
> To:  <amps at contesting.com>
> Sent: Tuesday, February 16, 2010 6:59  PM
> Subject: [Amps] HV Fuses
>
>
>> In recent threads, there have  been a number of comments about high
> voltage
>> 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
> wire
>> 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,
> how
>> 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
> #30
>> 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
>> K1SG at AOL.com
>> 508-435-9133
>>  FN42fe
>>
>>
>>  _______________________________________________
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>> Amps at contesting.com
>>  http://lists.contesting.com/mailman/listinfo/amps
>
>
>
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