[Amps] TWT Protection (was fuses)

[Jim Thomson]

Date: Sun, 9 Jan 2011 10:33:52 EST
From: TexasRF at aol.com
Subject: Re: [Amps] TWT Protection (was fuses)
To: gm3sek at ifwtech.co.uk, amps at contesting.com
Message-ID: <6c6e8.2ca3e207.3a5b2f60 at aol.com>
Content-Type: text/plain; charset="US-ASCII"

Hi Ian, it is good to hear from you again and thanks for the  information.
 
As you suggested, I am thinking that a second vacuum relay would be a good  
solution to the problem. If it was wired to shunt a 50 ohm resistance from  
cathode to ground, the 4400vdc supplied through the 500 ohm limiting 
resistance  would be dropped to 400vdc at the cathode while the shutdown process 
happens. 
 
With the nice ham discount at Gigavac it seems quite unwise to not make  
this improvement.
 
Since the added relay can be wired to switch the added resistor to ground,  
a less expensive vacuum relay not rated for hot switching will serve well.

## IMO, the vac relay may well work better in that cro-bar mode...
VS  say trying to open off the B+.   That scheme, where the B+ is opened off
 via  a vac relay, that w8zr  cooked up, and you see in Orr's  last book, [8877amp]
will work..abt twice.   The eng I talked to at Kilovac  said those cu contacts on small
vac relays won't last at all..under fault current conditions, trying to open off the B+
... with the typ 25-50 ohm glitch R.
He did say they would open off several amps of dc current....as in the amp is off resonance,
and the plate current just drifted past it's pre-set threshold.   [ IE: plate over-current 
trip point set for 1100 ma  on a 8877].

##  However, with tungsten contacts designed for hot switching a DC load, and your 500 ohm
glitch R,  your resulting fault current would be very low, and you could easily open off the B+.
You could use both schemes, using 2 x sped up vac relays.  You could also kick the amp into 
standby. 

Jim  VE7RF   




 
An advantage of this method is that the fault sensing current would be  
forced to a higher value to insure a more positive action in the event of a 
less  severe over current event. Such as when things go wrong and the helix 
current  runs high but an actual arc is not happening. This scenario is much  
more likely than an arc.
 
At the price of hv fuses, only two or three events would pay the cost of a  
new protection relay. So, this becomes a no brainer.
 
Thanks again for your input, it was just what I needed to reason through  
the problem at hand!
 
73,
Gerald K5GW