[AMPS] glitch resistors

[Ian White, G3SEK]

OZ5IQ wrote:
>
>HI guys
>
>Why are the most of you talking of HV resistors   ????  Be  realistic and think 
>that this is to avoid the current during arc.
>
>I Always put those current limiting devices at the LOW voltage potential.
>
>A few ex. could be : at the center to GND of the HV trafo, 
>                                   In the   - HV lead just between this and the 
>GND.
>
>This is GD construction  design, and you get the exact same kind of protection 
>AND avoid  HV resistors  or HV  fuses.

That's  not true - and it can be dangerous! 

If the resistor is where you describe it, in a +3kV supply, a current
surge from the HV+ rail will push the center-tap towards -3kV with
respect to chassis. The voltage across the resistor is EXACTLY THE SAME
as if it was in the HV+ line. 

Also the center-tap is designed to be grounded (or very close) so the
insulation at that point may not stand 3kV.

There is a range of commercial amplifiers where they do this. The glitch
resistor and a 1-inch mains-type fuse are in the negative lead, just
mounted on the PC board with all the other low-voltage components. The
HV-minus connection is ordinary hookup wire, and it goes back to the
bottom of the capacitor stack through an un-insulated hole in the
chassis. This is all FB when the amp is operating OK, and it saves the
extra cost of mounting a resistor and fuse in the HV+ rail. But what
happens if there's a current surge, I dread to think - it could side-
flash to anywhere!

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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