From: Paul
Date: Thu, 11 Jan 2001 13:10:15 +0000
Subject: Re: [TSSP] NSVPI - Terminated Coil Results
Terrell W. Fritz wrote: > I measured the junction capacitance of the diode at 20pF. > If the contact capacitance is say only 1pF it would be a 20:1 > attenuator and the diode will not turn on which is what I > suspect will happen. With caps and a diode in series with an EMF source, and assuming the diode turns on, any rectified current will result in the rectified DC potential appearing across the *diode*, with the stored charge limited to what the diode can carry. The addition of a DC current path allows the charges to redistribute so that all the rectified voltage ends up on the desired storage cap. Without a DC path, you won't see anything across the storage cap. Thinking about it, a suitable resistance will have to be large enough so that the division ratio between it and the contact capacitances is not far below unity. On the order of 10Meg for 2pF contacts at 150kHz, so I guess the use of the choke is out. The probe would have to remain in contact for the appropriate RC time constant with the storage capacitor. I hand't thought to consider the diode capacitance, but I can see that would be a problem too unless a low cap diode could be used. I suspect this whole approach would upset the nice absolute accuracy of the conductive contact method, but it might provide a simple proportional detector suitable for folk to find their maximum gradient points. Cheers, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.