[Amps] M.O.V.'s

[Ian White, G3SEK]

Hal wrote:
>
>Some time ago, mention was made about using an MOV near the power 
>amplifier tube as protection against transient surges, perhaps in the 
>filament area.
>
>Looking at Mouser's MOV selection, the reader is presented with a 
>plethora of selections, each one having a 1MHz. capacitance listing.
>
>My question now for the reflector is would the inherent capacitance in 
>an MOV mounted near the final and connected to the wiring going into 
>the tube have an effect on the circuit because of the device's inherent 
>capacitance, or should a Metal Oxide Varistor not be used at all, with 
>a better choice being perhaps a gas-filled device?
>
At voltages below breakdown, MOVs look like capacitors of a few thousand 
pF so they can be used wherever a real capacitor would be OK. In 
practical terms, you can connect an MOV across almost any existing 
bypass capacitor.

A good example is an MOV connected across the screen bypass capacitor of 
a tetrode, to protect that capacitor from voltage breakdown (which would 
probably cost you a whole new socket).

Spark gaps are only a few pF, so they can be connected directly into 
most RF circuits, for example across the transmission line.

Different devices, different uses. MOVs start to protect at only a 
little more than their maximum rated operating voltage. Spark gaps may 
protect better, but only when they've fired - and that requires a much 
larger impulse voltage than the rated operating voltage.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek