Re: [Amps] Crowbar components

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

Having been away for a few days, please excuse my replying to several 
points all together.

Rich wrote:
> > > Frank --  Why go to the trouble of interrupting the flow of electric
> > > mains current by special means?  The stored charge in the filter-C is
> > > what can produce enough peak current to cause bent filaments in 
> > > 3-500Zs
> > > or gold to sputter from the grid of 8877s, and this current is already
> > > limited by the glitch resistor -- provided it is able to withstand the
> > > Joules available from the filter-C without going kaput.
> > > >
Depends what you mean by "special means"...

When the peak current through the tube has been limited, by the glitch 
resistor or by a crowbar, you still have to switch off the mains.

A fuse, a circuit breaker, a mechanical relay, a solid-state relay - any 
of them will do, and in this context none of them is particularly 
special.

Frank replied:
> Well, I see the crowbar as a very elegant solution - and I've never 
> done one myself so far. So, technically speaking - I can't really say 
> what the killer argument for a crowbar would be. I see it as a second 
> protection in case the Glitch-R doesn't do his job or something else 
> happens out of the blue...
>
> And, of course, building one and testing it is in itself a new one for 
> me.

* That it reduces the surge current through the tube to almost zero! At 
best, a glitch resistor can only reduce the peak surge to several tens 
of amps.



> > I see it as a second
> > protection in case the Glitch-R doesn't do his job or something else
> > happens out of the blue...

I'm not getting through here...  the crowbar circuit is instead of the 
glitch-R, not as well as.

> Hello, Frank --
> To prove that a crowbar protects the tube adequately, one needs measure 
> the peak fault current,  To do this, connect a 0.1-ohm precision 
> resistor from the neg. HV to ground, and connect an oscilloscope across 
> the resistor to measure the peak V.  When the pos. HV is shorted to 
> ground with a large screwdriver, for a 2, 3-500Z amplifier, the peak V 
> should be less than 20V, which = 200A-pk.   For oxide cathode tubes of 
> similar output capability, the peak current should probably be under 100A.

Tube manufacturers already told us how to test a crowbar circuit. Eimac 
in Bulletin 17, and Seimens in their tube data, both say that if you 
short the HV supply with a piece of very thin wire, the crowbar circuit 
must shut it down without damaging the wire. For example, you must be 
able to short a 3-4kV supply with a piece of 3-amp fuse wire, without 
blowing that fuse!

The important thing about this test is that it limits the maximum ENERGY 
that the crowbar will allow through into an arcing tube. Not current; 
energy.

A scanned version of the original OE5JFL article is now at:
www.ifwtech.co.uk/g3sek/misc/oe5jfl.pdf
It is in German and English, and contains a few more details than 
Hannes's web page.



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