[Amps] HV fuse

[Ian White, G3SEK]

Oleg Skydan wrote:
>OK. Consider I have a 40 uF capacitor in the PS charged to 3000V
>(one that uses in the pulse laser PS). Will it be safe for electronic
>crowbar to discharge it ?
>
Good call - you have to think about the maximum current that the crowbar 
will have to handle. The OE5JFL circuit is quite carefully designed to 
limit that peak current, using a combination of a small inductor to 
limit the rate of rise, and also a 10R sense resistor. (The inductor is 
only 30ft of wire, random-wound on a bobbin, but it makes a very 
significant difference. It also means that the 10R sense resistor 
doesn't have to be built like a conventional glitch resistor.)

Bottom line: if you're going to copy OE5JFL's circuit, copy *all* of it.

http://www.qsl.net/oe5jfl/flashover.htm


>My idea was having a glitch resistor and vacuum relay in series in HV.
>In case of overcurrent the glitch resistor will limit current until relay
>will
>switch off (~10ms). This will prevent discharging PS capacitor through
>glitch resistor and tube.
>
This would work, but the current that you'd need to break may be 40-50A, 
at however many kV. It's always difficult to open a contact reliably 
under these conditions, and much easier to build a crowbar based on 
closing a contact.

>P.S. Fuses in mains is a must in any case !

Well, there certainly has to be *something* to disconnect the mains... 
otherwise the transformer keeps on pushing current through the glitch 
resistor or crowbar! For instant-restart capability, it's nice to use a 
relay, with the fuses only as a backup.



-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek