Actually the "crowbars" in large transmitters are there to save the "tube
innards" (grids, cathode) in case of
internal arcs which are common occurrence. The sense circuit is connected in
the plate return, and the ignitron diverts the the major part of the fault
current in the order of a few hundred microseconds.
Grid overcurrents due to other causes are usually sensed by more conventional
overload relay circuits
which have time constants of some tens of milliseconds, in order not to trip on
modulation peaks.
Standard procedure for checking the crowbar circuit in 100 kW Telefunken
transmitters (which, as a whole, were quite tempermental creatures) was to
short circuit the 14 kV 15 A rectifier using a 0,3 mm silver wire, which should
not burn off or melt. This obviously required some courage by the maintenance
personnel.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "Jim Tonne" <tonne@comcast.net>
To: "Tom Rauch" <w8ji@contesting.com>; "Rick Stealey" <rstealey@hotmail.com>;
<amps@contesting.com>
Sent: Tuesday, October 02, 2007 5:33 PM
Subject: Re: [Amps] 8877 Grid Trip timing
>
> Tom wrote (in part):
>
> > My homebrew amps trip in less than 2mS of excessive grid
> > current. They also fault if the reflected power is more than
> > 20% of forward power.
>
> Excellent!
>
> And in the high power shortwave transmitter world those
> transmitters use crowbars and trip even faster than that.
> Those big tubes are pricey and even a millisecond grid fault
> can melt it and cost a fortune.
>
> - JimT WB6BLD
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
> --
> No virus found in this incoming message.
> Checked by AVG Free Edition.
> Version: 7.5.488 / Virus Database: 269.13.39/1044 - Release Date: 2007-10-02
> 11:10
>
>
--
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.5.488 / Virus Database: 269.13.39/1044 - Release Date: 2007-10-02
11:10
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|