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.
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
In a message dated 1/8/2011 8:42:20 A.M. Central Standard Time,
gm3sek@ifwtech.co.uk writes:
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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