I just noticed the thread name changed.
-----------------
| |
V Emitter |
P
_ Plate |
| T1
| |
| |
-o-R1---G---R2-o-
| | |
o o o
V1 G V2
Fig. 1 - Circuit diagram for drop/thread detection
P is a smooth DC power supply floating above R2 and T1 (one technical
difficulty with this approach). The plate current goes through R1 to
ground G. This gives two current proportional voltages V1 and V2
close to ground and thus easily amplified (a major advantage). If
the current is carried by charged droplets then the current signals
will be wildly different in timing. If the current is carried by
conductive filament then the signals should be comparatively flat.
It may be possible, depending on needle choice and humidity, and/or
possibly CO2 concentration, to generate either mode. If the needle
emitter is in conductive filament mode, then it should be possible to
instantly send a HF signal injected by transformer T1, and which is
not received at the same magnitude when there is no filament.
Further, if the plate is bifurcated, (and another current sense
resistor R3 added) and the emitter moved from above one plate to
above the other, then a droplet beam (or dry air dry needle signal)
should transition current from R1 to R3 more gradually and smoothly
than a conductive filament (or set of multiple simultaneous
filaments, which would give a step function).
The emitter and plate can exchange places. Since the power supply is
floated, possibly with an isolation transformer, P and T1 can
exchange places. If P is a small van de Graaf, then T1 has to be
between P and the tip. It is also possible to locate T1 between R1
and the plate.
Some handy pre-amp circuits for anyone interested. (Note: I am not an
electrons guy!)
True full wave signal amp:
+Vdd (Drain)
|
|
|
I----
Gate I
Input---> I----
|
R1
|
o----->Output
|
I----
Gate I
---> I----
| |
| R2
| |
---------|
|
-Vss (Source
Two NTE455 MOSFET's, Dual Gate, N-Ch, TV UHF RF Amp (gate Protected),
gfs - 22,000 micromhos Typ, BVdsx 20 V min, Idss 0.5-8 mA, Pd00 mW
Max, Vgs(off) - G1=2, G2=0.7 V, Ciss 3.5 pF Max, Crss 0.03 pF Max.
Gates tied together. R1 = R2 = 2.2 k. +Vdd and - Vss supplied by
two opposed 9 V batteries. I have never actually tried this.
For a capacitor isolated (and thus differential) signal at about half
the cost (and I've tried this, thanks to John Schnurer):
+Vdd (Drain) (Vdd = +9 V)
|
R2 (22 ohms)
|
|---------------C1---Ground (C1 = 1000 pF)
|
I----
In Gate I NTE455
o--C2-o-> I----
| | (Source)
R3 |
| o------------------->Out
o |
R1 (2.2 k)
|
Ground
To avoid charge build up on the gate R3, as large a resistor as
possible (e.g. 20 Mohm), can shunt to ground. If the signal is too
small this can be temporarily removed while looking at the signal.
Regards,
Horace Heffner
