[AMRadio] HV Capacitor question

[sr2000 k9axn]

Jim,

Take a look at Notes 1, 2, and 3

https://www.k9axn.com/_mgxroot/page_10835.html

Kindest regards Jim K9AXN



-----Original Message----- 
From: Jim Candela
Sent: Monday, July 17, 2017 6:33 AM
To: AMRadio at mailman.qth.net
Subject: [AMRadio] HV Capacitor question


I have an unusual question.
At my work, we use a electrostatic clamp to secure a silicon wafer while it 
gets an ion implant. The clamp controller supplies +/- 1.44 KV under the 
wafer, and uses a thin insulator to keep the HV current draw zero.
>From each HV output, there are three .01uf  3KV Z5U ceramic capacitors in 
parallel. These caps feed an op-amp oscillator that makes a ~ 20.5 Khz 
square wave. The frequency varies with wafer on or off the HV clamp. The 
presence of a wafer capacitively bridges the two HV electrodes under the 
wafer.
When things are screwed up, the oscillator frequency is not repeatable. 
There could be many reasons, but for the sake of discussion, I've found it 
is usually the .01uf 3KV Z5U capacitors.
If you look up similar capacitors, the Z5U designator for the ceramic 
dielectric is most telling. We might use one in ham radio for a plate bypass 
capacitor, but never in a high Q tuned circuit. The capacitance change with 
temperature is great, and they cannot stand much RF current. These 
capacitors are also piezoelectric, i.e. a sudden change in voltage across 
the capacitor will make an audible click...like a speaker!!
I just learned another characteristic that I've not found anywhere in print. 
If I take a DVM, and clip the leads across the capacitor (used bad ones), I 
will measure a voltage!! Maybe something like 20mv, and varying with 
temperature by just grasping the device with my fingers. Now the kicker, 
wave the device with a heat gun, and as the device warms up, the voltage 
increases upwards rapidly to 1 volt!! A good (used) capacitor, same type, 
will never go beyond 20mv, hot or cold.
Using the same DVM with a capacitance range, the measured value moves a lot 
(> +/- 50%), but which way depends on the test leads (red, black) are 
applied. I dismiss the measurement as invalid due to the voltage generated 
by the heated capacitor messes with the DVM measurement method.

There seems to be something going on with the dielectric, or a mechanism 
from dielectric absorption.

One clue is with resistors. Some resistors such as leaded metal film type 
crimp the resistive element to the leads inside the package. Each crimp 
joins two dissimilar metals making a thermocouple (TC). Heating a TC 
generates a voltage. With the resistor example, two TC's are in series, and 
back to back, like +- in series with -+ such that the voltages offset each 
other so long as each TC is the same temperature.
It would take dozens of TC's in series +-, +-, +-, etc. to make a volt...
What is going on with my "bad" .01uf 3KV Ceramic Z5U capacitors??
JimWd5JKO

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