Rich Measures wrote:
>
>>>>
>>>One does not have to remember anything if one puts the glitch R in the
>>>positive HV lead.
>>>
>>Except to insulate *both* ends to full HV.
>
>Ian plays the Wild Card.
>
>>>>those
>>>>components are not protected any differently by a glitch resistor in the
>>>>B+ or the B-minus. As Andy said, it's the same surge current flowing
>>>>around the loop, ...
>>>
>>> It can not be the same current if the Glitch R drop is limited to 1.5v
>>>by the glitch diode.
>>
>>You're imagining the diode in the wrong place, shunting the glitch
>>resistor.
>>
>
>
>
>see diagram
There wasn't one... but there is now. I just wasted an hour hacking some
diagrams and text together, and the result is at
http://www.ifwtech.com/g3sek/glitchres.htm
The text is shown below (where <these> are hyperlinks)...
Where's the Glitch Resistor?
This page refers to a present discussion on the AMPS reflector.
The question is whether it makes any difference if you place the surge
limiting resistor ('glitch resistor') in the B+ rail or the B-minus
rail.
The discussion has reached the point where we're desperate for diagrams,
so here they are.
[Fig 1]
The diagram shows meter protection using two diodes in series, as
recommended by <Rich Measures, AG6K>, and used by me on the Triode
Boards. I like this circuit because it lets you tailor the protection
separately for each meter.
When there is a current surge (arc or whatever other reason) the B+
voltage is effectively connected to chassis by a low resistance. There
has to be something to limit the surge of current flowing out of the
smoothing caps in the HV power supply. This is the function of the
glitch resistor.
The current flows in a loop, starting at B+ and going through the arc,
down to chassis, back up through the diodes (avoiding the meters and
cathode circuitry) and returning to B-minus. Until the fault is
interrupted, the positive end of the glitch resistor is at high voltage,
and the negative end is close to chassis potential. This places large
demands on the glitch resistor: for details of suitable resistors, see
Rich's site (<click> and scroll down to Glitch Protection) and/or see
<my site>.
Now what happens if you move the glitch resistor from the B+ rail to the
B-minus rail, as shown below?
[Fig 2]
Actually, there isn't much difference. Exactly the same current flows
around the loop, except that the glitch resistor is in a different
position. Now, the positive (right-hand) end of the glitch resistor is
close to chassis potential, but the entire HV supply has shifted
negative: the arc has 'crowbar-ed' the B+ rail almost down to chassis
potential, and the B-minus rail has gone to a large negative voltage.
The same voltage appears across the glitch resistor as when it's in the
B+ rail, and it gives the same protection to the rest of the components.
The insulation requirements are different, though: in the B+ rail, both
ends of the resistor much obviously be insulated to HV standards; in the
B-minus rail, only the negative (left-hand) end of the resistor needs
to be well insulated, but so also does the negative end of the HV
supply.
>>>---------
>>>In "The Almost Perfect Amplifier" (*QST*/January, 1994), I recommended
>>>using a rheostat to adjust filament V. During the grate parasitics
>>>debate, at least five members of the Rauchian camp argued long and hard
>>>that a rheostat can not be used to control filament V.
>>>
>>Forgive me, but I'm not seeing the relevance of that statement to the
>>present discussion.
>>
>No surprise there. Do you feel that a rheostat can be used to control
>filament V?
>
Sorry, I genuinely can't see the relevance of this question. Unless you
explain, what I feel is that you're hijacking the discussion.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|