Gerald wrote:
>All this talk about HV fuses got me wondering how quickly a fuse will blow.
> After some time googling, I realize this simple question is not so simple
>after all.
>
>So, is there a good reference that will answer the question as related to
>our typical need to protect amplifier hv components and tubes?
>
>I have a specific application for protecting a non-obtainable twt if
>damaged. This particular tube is supplied with 4400vdc at 400 mA
>through a 500
>ohm limiting resistor. The resistor is purposely made this high to limit
>fault current to a value that will not kill the tube in case of an internal
>arc. This technique works ONLY if the fault current is interrupted very
>quickly, like 2 or 3 milliseconds.
>
>So, I am thinking that a 1 A fuse will survive turn on surges ok but will
>it open in 2 or milliseconds with 8.8 A fault current?
>
>The existing protection circuit uses a simple current sensing circuit in
>the ground side of the hv supply that operates a small latching relay that in
> turn opens the control circuit to a vacuum relay in the hv lead. This
>scheme works reliably but there is no way the two relays can kill the high
>voltage in 2 or 3 milliseconds.
>
>Any information, comments or suggestions are greatly appreciated!
>
The usual technique is to apply a crowbar short across the valuable
tube, while other, slower, circuits are shutting down the rest of the
power supply.
The voltage is a bit high for a thyristor crowbar, so how about using
another vacuum relay, triggered directly from the fault sensing circuit
for maximum speed? With a voltage boost circuit this relay should close
in well under 2ms, protecting the TWT while the existing slower relays
are shutting down the PSU.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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