[Amps] HV Fuses

[Fuqua, Bill L]

I remember taking a lengthy tour of the VOA site just north of Cincinnati when I was a teen. I was impressed by the Crosley transmitters and their tuned plate circuits which had , believe it or not, very long hair pin inductors and a shorting bar they called a sled that moved up and down it.  The next thing that got my attention was the power supply of the new Collins transmitters. They had a pair of metal balls with a sharp spike between them. This spike was on top of what looked like a large ignition coil. one ball went to ground and the other went to a the B+ supply. A current transformer in the current path of the output tube, sensed an arc current and triggered the "ignition coil" and a spark produced a plasma between the balls to discharge the power supply. This was to protect the very expensive 100kW plate dissipation vapor cooled tetrodes.  I have always wanted to make such a device but it is a bit of an over kill for a 1.5kW amplifier. Apparently this would discharge from time to time and the transmitter would recover with no damage.

73
Bill wa4lav

________________________________________
From: amps-bounces at contesting.com [amps-bounces at contesting.com] On Behalf Of Fuqua, Bill L [wlfuqu00 at uky.edu]
Sent: Wednesday, February 17, 2010 6:19 AM
To: Angel Vilaseca; K1SG at aol.com
Cc: amps at contesting.com
Subject: Re: [Amps] HV Fuses

The capacitor across the points was used as part of the ignition system.
You grounded the low side of the primary of the coil with the points, the current was limited by a
ballast resistance which later was built into the coil and when the points opened the energy stored
in the primary inductance went to charging up the capacitor and then you had a damped resonant
circuit. Current oscillated thru the coil and the condenser.  It was never about protecting the points.
In fact, if you removed the condenser you would get a very weak spark.

73
Bill wa4lav

________________________________________
From: amps-bounces at contesting.com [amps-bounces at contesting.com] On Behalf Of Angel Vilaseca [avilaseca at bluewin.ch]
Sent: Wednesday, February 17, 2010 3:09 AM
To: K1SG at aol.com
Cc: amps at contesting.com
Subject: Re: [Amps] HV Fuses

Just another idea...

In the old car ignitions there was a capacitor in parallel between the
points. This was made to avoid an arc between the points when the point
were switching off 12 Volts into the coil primary.

What we are trying to avoid in an HV fuse is also an arc when the
circuit breaks (wnen the fuse melts).

So how about using an HV capacitor in parallel with the fuse? would that
keep an arc from appearing?

Anyone tried this?

Angel Vilaseca HB9SLV





K1SG at aol.com a écrit :
> 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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>
>

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