[Amps] Crowbar components

[R.Measures]

On Jul 22, 2004, at 11:24 AM, Frank Goenninger wrote:

> Hi Rich!
>
> On Thu, 2004-07-22 at 11:40, R.Measures wrote:
>> On Jul 21, 2004, at 8:47 AM, Frank Goenninger wrote:
>>
>>> Hi all on the list:
>>>
>>> There have now been several posts here pointing
>>> to
>>>
>>> http://www.qsl.net/oe5jfl/flashover.htm
>>>
>>> as a suitable crowbar circuit. I want to copy that
>>> one...
>>>
>>> When searching for suitable components I could not
>>> find a suitable opto-coupled Diac as shown in the
>>> mains circuit.
>>>
>>> My HV transformer will draw abt. 21A of current so
>>> I need a Diac able to withstand that or I have to
>>> substitute that part with ??? A diac controlling
>>> a thyristor ? Or ??
>>>
>>> Are there any such diacs or what would a suitable
>>> replacement circuit not altering speed too much
>>> look like?
>>
>> 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.
>>>
>
> 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.

That it's elegant.

> 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...

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.
cheerz
>
> And, of course, building one and testing it is in itself a new one for
> me.
>
> 73, Frank, DG1SBG
>
>
>
>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org