On a sunny day (Sat, 22 Aug 2009 13:53:40 -0700) it happened Salmon Egg wrote in :
In article
>,
> whit3rd wrote:
>
>> >> 100mV, but that's probably unrealistic. Maybe
>>
>> >
>> > Perhaps this is grasping at straws here, but I'm recalling the first
>> > electroscope, which was a gold-leaf electroscope where the charge
>> > attracted or repelled the metal. So perhaps a tiny version of that
>> > could work at lower voltages
>>
>>
>> Yes, the mechanism of a capacitive voltmeter is presumably applicable
>> here;
>> the capacitance of the leaf is greater when it spreads, and other
>> geometries
>> (like rotating vanes) can be used to get better gain. It's not
>> impossible
>> that a vacuum-insulated variable capacitance element can be made that
>> responds well enough to close a contact (the hard part then is making
>> the contacts not weld together).
>>
>> You don't generally see these with sub-100V ranges, but it IS a good
>> idea.
>
>I am certain that the potential change required for an electroscope to
>work is many times the one or two volts the OP desired.
>
>When you say the capacitance is greater when the leaves spread, you have
>to talk about which capacitance. The self capacitance to infinity
>increases. The mutual capacitance between the leaves decreases because
>the gap increases. Think of each leaf as a separate plate. They just
>happen to be electrically connected in the electroscope design,
>
>Bill
For voltages < 20 V (gate breakdown) what is wrong with a MOSFET source follower folowed by some opamp comparator? Gate current is practilally zero. Crossposts snipped.