John Anderson, <megavolt@io.com> said... > >Everyone keeps mentioning frequency. When you say at "high frequencies" >the probe will not be accurate, do you mean hz, khz, or mhz range? You can calculate this for yourself. The parasitic lead-to-lead capacitance of a typical small resistor is 0.05 to 0.2pF. The capacitance from the _middle_ of the resistor and from any connection node between series resistors, to ground, may range from 1pF to 5pF or more depending upon your choice of a sheilding scheme. Longer glass resistors intended for high voltages have lower lead-to-lead capacitance, but higher distributed parallel capacitance. As a worst case, imagine a 1000M-ohm probe made with a 2-inch long resistor. To start, place the capacitance to ground from the midpoint. If you assume 5pF of parallel capacitance, you'll see you're in trouble even at 60Hz! HV probes with usable high frequency response may have cleverly arranged sheilds which can be connected to ground, the attenuated signal output, and the input. Some shields may overlap and may be adjustable, to provide an adjustable capacitive divider that can swamp the remaining effects of the resistor's capacitance. These probes can be very accurate at DC and high frequencies, but the skill of the designer is tested in the transition between resistive and capacitive divider action. Poorly designed probes can suffer from mid- or high-frequency errors and from severe pickup sensitivity to nearby rapidly-changing voltages. For example, I've seen probes that provide an output even when the measuring tip is grounded! -- Winfield Hill hill@rowland.org The Rowland Institute for Science 100 Edwin Land Blvd. Cambridge, MA USA 02142-1297
Date: 2 Oct 1996 10:33:37 GMT
Original Subject: Re: Can I build a HV probe?