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?