[Amps] Fuses

[Carl]

----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Sunday, January 09, 2011 10:24 AM
Subject: [Amps] Fuses


>
> Can you two get back to earth and keep it somewhat relative to ham amps in
> the under 20KW range (-;
>
> Carl
> KM1H
>
>
>
> ##  the basic concepts are still the same.. 'slightly'  larger scale. 
> Same problems too....
> huge back EMF, when opening off a fully loaded plate xfmr..esp if on 
> rtty/fm.
>
> ##  Now Rich Measures, and other's will say the HV fuse in the B+ line is 
> not required..and
> that the 'follow on'  current can easily be shut down with the 
> fuses/breaker  in the 240 vac line,
> [or some kind of current sense lead scheme that open's off the main 
> contactor].
>
> ##  If a HV fuse is used in the B+ line..just prior to the 50 ohm glitch 
> R...  and a B+  to chassis
> arc occurs  [or anode to a  chassis grnded grid]  the 50 ohm glitch R 
> ...LIMITS the fault current
> to a safe value.   The HV fuse  INTERRUPTS  the fault current..a split 
> second later.  The 240 vac
> breaker/ fuses in the primary of the plate xfmr will never open.   Ok, now 
> you don't have the back
> EMF problem any more.    I also use a 2nd HV  fuse in one leg of the plate 
> xfmr sec..[ either leg].
> Same deal, if the HV-AC fuse blows open,  no back emf on the 240 vac 
> primary side of plate xfmr,
> FWB is still protected, and so is the plate xfmr.   I  still use big, 
> fused mov's directly across the pri
> of  the plate xfmr.. and also from each side of the plate xfmr hot leg to 
> neutral...also fused.  And
> more movs  directly across all contacts  on the contactor.
>
> later... Jim  VE7RF


But not everyone runs a 20kw amp Jim, with a string of high 4 digit caps in 
the PS. Those amps sure do require special failure consideration.

Mere mortals are more interested in keeping their AL-811 to the typical 2x 
3CX800 or 4CX800 amp or HB equivalent from self destructing without 
resorting to voodoo science.

Its the zillion Heath SB series of amps that often need the help since there 
are so many of them on the 3rd or higher owner that have been modded beyond 
comprehension based on all the pure nonsence on a few web sites.

Carl
KM1H