[Amps] high voltage fuses

[Fuqua, Bill L]

  I would recommend a fuse wire between two post surrounded by grounded conductive tubing.
Why? When the fuse blows the plasma will conduct the rest of the current straight to ground acting
as a self sacrificial crowbar. The metallic tubing would also protect the nearby parts from the plasma 
flame. 
  When I was teenager, I had built an amplifier connected to a power supply with a very high
current capacity. I, being a young kid, did not fuse it on either the primary or HV side except I did 
have an adjustable latching current limiting relay. I turned the shunting resistor to zero resistance so
that even the relay  was bypassed. My pi-wound RF choke melted and flames shot out the top of the amplifier
with a loud "zonnk". And continued until I unplugged the power supply from the wall. 
  
  73
Bill wa4lav

________________________________________
From: Amps [amps-bounces at contesting.com] on behalf of Jim Thomson [jim.thom at telus.net]
Sent: Monday, October 07, 2013 11:49 AM
To: amps at contesting.com
Subject: [Amps]  high voltage fuses

Date: Sun, 6 Oct 2013 10:53:36 -0400
From: "Carl" <km1h at jeremy.mv.com>
To: "Pete Smith N4ZR" <n4zr at contesting.com>, <amps at contesting.com>
Subject: Re: [Amps] high voltage fuses


###  say what ?   If you want any HV fuse to blow asap,  fuse it for no more than 5-10%
higher than the amps steady state, key down current.  Just make sure to put the HV fuse BEFORE the
glitch resistor.   The glitch resistor can go in the RF deck, or in the HV supply.  IMO, for max safety,
put it in the HV supply, or as close as you can to the B+  of the filter cap.

##  A small ga wire, suspended between stand offs  works just fine for a HV fuse, with the provisio
that you 1st slide some heat shrink over the wire, use anything as long as it barely slides over the
fusing wire.  You don’t apply heat and shrink it to the wire either, just slide it over.  Or  use some
88/33 tape, etc,  and wrap the wire. Then when it blows open, you don’t get any vapor deposits.
Those glass BUSS  HV fuses can be refurbished too,  Take the end off, and pour out the sand.
Remove other end, clean it up and solder in a new fuse wire, and pour the sand back in.
Re-assemble the mess.

##  The 5 kv, .75 – 1.0 amp  microwave oven  HV fuses work just fine.
I also install  a 2nd HV fuse, between one leg of the plate xfmr sec and the
FWD... or  FWB..   Either leg, only one fuse required.

##  Drake used a .82 ohm @ 1/2 watt CC  resistor for a HV fuse in the B+ line.
That works too, and blow cleanly in half.

later... Jim   VE7RF







Thats a bit too close for even a 1200W amp. Might be fine for something in
the SB-200/AL-80 size amp.

Size the fuse at 2 to 2.5 times the actual peak current, not steady state
key down.

Carl
KM1H



----- Original Message -----
From: "Pete Smith N4ZR" <n4zr at contesting.com>
To: <amps at contesting.com>
Sent: Sunday, October 06, 2013 10:30 AM
Subject: [Amps] high voltage fuses


> For what it's worth, I see that there are a lot of inexpensive 5KV
> 0.75-1.0 amp glass fuses listed on ebay for microwave oven use.  I assume
> these would also work in a legal ham amp?
>
> --
> 73, Pete N4ZR
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