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
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