Rich Measures wrote:
>
>
>>
>>Picking up on one of Rich's remarks:
>>
>>>Henry's glitch-R in the neg. HV
>>>lead is a mistake. A glitch R should be added to the pos. HV circuit.
>>
>>It doesn't really matter which end of the HV circuit the glitch resistor
>>and HV fuse are - unless the designer thinks that putting them in the
>>negative return rail requires less insulation. In fact it requires
>>*more* insulation, because a "crowbar" short from HV+ to chassis will
>>tend to push the return rail down to *minus* the normal HV.
>
>>To avoid breakdown, the negative rail and the negative end of the
>>capacitor stack therefore need to be insulated to the same standard as
>>the positive end.
>>
>? The sticky wicket
You're obviously a bigger cricket fan than I am, Rich!
Where were we...?
>? The sticky wicket with putting the glitch-R in the neg. hv circuitry
>is that the filament transformer/cathode circuitry must be insulated for
>several kV. However, allowing the cathode to rise to several neg. kV
>during a HV+ short to chassis is likely to cause an arc to the
>grounded-grid. Thus, it is my opinion that it matters which end of the
>HV circuit the glitch-R is in.
The cathode doesn't go strongly negative at all - it stays around
chassis potential, much as it would if the glitch resistor is in the
positive lead. The point that dives several kV negative at the start of
the glitch is where the negative (power supply) end of the resistor
connects to the negative end of the smoothing capacitor.
In both cases, the full HV appears across the glitch resistor, as it's
intended to, and the resistor limits the maximum current that can flow.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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