Rich Measures wrote:
>
>>
>>Andy Cook, G4PIQ wrote:
>>>
>>>Rich wrote
>>>> The glitch R needs to be in the positive HV lead.
>>>
>>>Piece of education needed for me here. The glitch resistor is there to
>>>limit fault current in event of a flash-over. The only source of these big
>>>currents is from the filter capacitor and the only route for the current
>>>back to that cap is via the 25R resistor. So why does the resistor need to
>>>be in the +ve line.
>>>
>>You're right, it doesn't necessarily.
>>
>>There are just two things to take care of, if the glitch resistor is in
>>the negative lead:
>>
>>1. To be sure that the resistor is the *only* path back to the HV-minus
>>point (and is not bypassed by the meter protection diodes). This
>>requires some thought and drawing-out the simplified DC circuit to make
>>sure it's correct.
>>
>>2. To be sure that the insulation around the negative end of the
>>capacitor stack will withstand voltages spiking to MINUS HV. This also
>>applies to the negative end of the glitch resistor and any connecting
>>wiring.
>>
>in other words, can the fil. transformer CT handle a several kV spike
>during a hv arc.? Putting the glitch R in the pos. lead eliminates this
>problem. On a Henry 3KA, 2K4,et cetera, the 7.5-ohm glitch resistor in
>the neg. circuitry is virtually certain to burn out during a glitch.
In this example, you cannot separate the problems caused by having too
small a glitch resistor from those that *might* be caused by putting it
in the B-minus return.
How big is the resistor (wattage and physical length)? Does it "burn
out" by going open-circuit, or by arcing across its length?
If they used a higher-value resistor (as Eimac recommend) and made it
big enough to hold up until the breakers operate, what would happen
then? Too many unknowns for anyone to be certain.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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