[AMRadio] HV Capacitor question


Alan Victor amvictor at ncsu.edu
Mon Jul 17 08:50:55 EDT 2017


I agree, you need to find out the composition of the dielectric. Many of
the titanate variations share some of the properties you mentioned. Alan
W4AMV



On Mon, Jul 17, 2017 at 7:33 AM, Jim Candela <jcandela at prodigy.net> wrote:

>
> 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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