[AMPS] AL1500

[Tom Rauch]

 Now, to Rich's point (I believe)...the fuse may in fact blow faster in
> this arrangement than in the case of the PA-77 where a sampled
> switching transistor operates a relay to force a standby condition. 
> The relay uses no special acceleration circuit.  So, it would be an
> interesting test to measure the "break" time of the fuse versus a
> transistor/relay combination.

Fuses are notoriously unreliable, if you doubt that test some...or 
look at references. Unreliability is the whole problem with fuses.

A fuse takes many minutes...or can even require hours....to fail at 
125-200% of rating. For light overloads, a fuse is both slow and 
may not be reliable at all for opening and protecting the system. 
Even with large overloads, some fuses can be very slow.

For example, the Littlefuse 3AG Fast Acting 250mA fuse typically 
requires 1 ampere for 20mS to open, 1/2 ampere for .2 seconds, or 
0.4 amperes for 2 seconds. They only guarantee, with a 200% 
overload, that it will open in 5 seconds!

I find it hard to believe people actually think a fuse is faster than an 
electronic circuit, or better in the case of grid protection. The grid 
certainly has much more thermal lag than 20mS, but not enough 
thermal lag to handle 5 seconds! The thing that eats the grid up is 
either manufacturing or material problems in the tube, or lengthy 
periods of improper operation.

While I'm not a big fan of the single transistor threshold detectors, 
the general advantage of electronic overload is it has a very 
predictable threshold, rather than one of time vs amount of 
overload. Electronic systems can be designed to prevent people 
from "slightly hammering" the grid for extended periods of time, 
since that "slight hammering" has the effect of slowly displacing 
the plating from the grid over time.

Now the real laugh in all of this is a certain left-coast person 
suggests using resistors as grid fuses. He actually advocates, or 
has advocated in the past, removing (not augmenting) electronic 
protection systems and replacing them with resistors!



  
73, Tom W8JI
W8JI@contesting.com 

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