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
K1SG
K1SG@AOL.com
508-435-9133
FN42fe
In a message dated 2/16/2010 8:24:55 P.M. Eastern Standard Time,
amps-request@contesting.com writes:
Message: 5
Date: Tue, 16 Feb 2010 16:10:37 -0800
From: "Steve Katz" <stevek@jmr.com>
Subject: Re: [Amps] HV Fuses
To: <K1SG@aol.com>, <amps@contesting.com>
Message-ID:
<31A81EFBA7CCA948882648CB542A30F46575BE@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@w7iuv.com>
Subject: Re: [Amps] HV Fuses
To: amps@contesting.com, K1SG@aol.com
Message-ID: <4B7B423B.6030804@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@jeremy.mv.com>
Subject: Re: [Amps] HV Fuses
To: <K1SG@aol.com>, <amps@contesting.com>
Message-ID: <024501caaf6f$f1491650$6501a8c0@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@aol.com>
To: <amps@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@AOL.com
> 508-435-9133
> FN42fe
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
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