brian whatcott, <inet@intellisys.net> said... > >almar@tsc1.cps.unizar.es says... >> >> The problem range is 12000V and only some mA. The source is a >> piezoelectric material. I'd like to be able to see the exact form of >> the pulse in an oscilloscope. > > Place a 100 Mohm and a 100 Kohm in series as a potential divider to > ground. The 100M-ohm would have to be a say 15kV version (i.e. Victoreen glass resistor a few inches long, etc.). And it'll be absolutely necessary to parallel the 1000:1 resistive divider with a capacitive divider. Say 5pF from the pizeo and 5000pF to gnd. The 5pF must be good for 15kV - coaxial cylinders with air dielectric. Also a shield or other element will have to be arranged to insure that only the charge through the 5PF reaches the divider output. Not everyone has a 15kV resistor sitting in their parts bins! No problem! Since the time constant of 5pF and 100M-ohms is 0.5ms, which is no doubt longer than a pressure-induced piezo spike, just skip the 15kV resistor entirely! OK, how about 1M-ohm and 5000pF for the bottom end of the attenuator? Now the high-pass time-constant is 5ms - better yet! E.g. 1M-ohm scope input, 6 feet of coax, additional 4700pF to gnd. Connect this to a homemade 5pF with appropriate shielding. Oops - destroy the scope input in case the homemade cap breaks down! OK, how about 10x scope probe (includes 9M-ohms to protect the scope inputs)? Now the bottom of the attenuator consists of the scope probe and 470pF cap to gnd. Connect this to our home-made 4.7pF cap. Time constant still 5ms. Scope probe sees 120V, scope input 12V 1000:1 overall. Calibration? A 10V 10kHz sig gen yields about 10mv - just measure this and determine the actual scale-factor of your instant HV probe! After calibration, change to 4700pF for 50ms time constant (10,000:1 attn, 1.2V scope signal) if better pulse shape range is desired. -- Winfield Hill hill@rowland.org _/_/_/ _/_/_/_/ The Rowland Institute for Science _/ _/ _/_/ _/ Cambridge, MA USA 02142-1297 _/_/_/_/ _/ _/ _/_/_/ _/ _/ _/ _/ _/ "http://www.artofelectronics.com/" _/ _/ _/_/ _/_/_/_/
Date: 25 Oct 1996 12:40:38 GMT
Original Subject: Re: How to measure a pulse?