[Amps] Grid fuses (was: Life and gain of 3-500Z)

[Bill Turner]

On Thu, 20 Jul 2006 06:08:44 -0400, you wrote:

>Bill,
>
>What is the typical reliable opening current and opening 
>TIME of a fuse compared to the safe holding current? Have 
>you ever looked at it?

A typical fast blow type will open in a few seconds with 2x overload
and the opening time decreases exponentially with higher overloads. 


>What is the reliable opening point (where it goes 
>near-infinite resistance) of a 1/4 watt resistor compared to 
>safe working current? Have you ever looked at that?

I have no idea. I would not use a 1/4 watt resistor as a fuse. 


>How many designs do you see that use resistors as fuses to 
>protect components?

If you count B+ glitch resistors, most if not all amps use them, or
should use them.


>Under what conditions would the resistor open?

Properly sized, it will do the job.


>Under what condition would an electronic circuit open? How 
>long would it take?

Ahhhh.. the real question. From long experience in the real world, I
have a healthy distrust of using a solid state device to open and stay
open in case of a major malfunction. When SS devices fail
catastrophically they normally short, a behavior just the opposite of
a fuse, and that is my principal objection. If a designer can come up
with a way of circumventing that issue, I'm all ears.

Bill, W6WRT