Theory of Operation
B–9
V o l t a g e a n d C u r r e n t M e a s u r e m e n t
The voltage and current of the Electrospray circuit are displayed on
the digital front-panel meters. This section describes how the
voltage and current displays are measured.
Voltage Measurement
The voltage at the output of the high-voltage supply is divided down
through series resistors. Two 10 M
Ω
resistors are used to reduce
the maximum current of the high-voltage supply. If a person comes
into contact with the high-voltage fitting of the Electrospray the
maximum current is 0.25 mA. A further divider of two 50 M
Ω
and
one 100 k
Ω
resistor divides the voltage of the Electrospray down by
a factor of 1000. The sense voltage is picked up at the 100 k
Ω
resistor and buffered through an OP-27 follower. Two IN-457, low
leakage diodes are used as clamping diodes across the op-amp
inputs. In order to reduce the voltage for the
±
2 V voltmeter display,
the voltage is further divided down by a factor of 10. This also
provides the opportunity to balance any divider tolerance on the
high-voltage divider. A capacitor of 0.1 µF on the input of a second
op-amp follower with the resistors provides a filter for any noise.
Current Measurement
To measure the current of the Electrospray, an op-amp operation
as a current-to-voltage converter is used. The current path
measured goes from the positive high-voltage supply side into the
current-to-voltage converter, and then into the analog ground.
From the analog ground, the current path continues through the
grounded electrospray side, through the Electrospray (in the form
of charged droplets), through the liquid in the capillary, through a
20 M
Ω
protection resistor, and into the negative high-voltage
supply side.
S e l e c t e d R e f e r e n c e s
The following list contains papers that are referenced in this
appendix as well as other references that may be interesting to the
reader.
Adachi, M., K. Okuyama and T. Kousaka [1985]
“Electrical Neutralization of Charged Aerosol Particles by Bipolar
Ions.”
Journal of Chemical Engineering
, Japan, 16:229.
