EG&G ORTEC 459 Manual de funcionamiento y servicio - Página 7
Navegue en línea o descargue pdf Manual de funcionamiento y servicio para Sensores de seguridad EG&G ORTEC 459. EG&G ORTEC 459 10 páginas. 5 kv detector bias supply
Imprimir páginaadjusted to the required output voltage level while the
power switch is turned off, and then power can be applied
by simply turning on the power with the high-voltage
switch.
4.3.
LOADING EFFECTS
The actual output voltage depends on the current that is
drawn from the 459 by its external circuit. To determine
the actual voltage at the "0-5 KV" connector, consider the
amount of voltage drop in the 2-MJ2 series output
resistance, which will be a function of the output current.
The output voltage can be calculated with the formula
V. = \/h
- 1(2 X IQS),
where
= output voltage,
*^diai ~ setting indicated by 5-turn control,
I = output current in amperes.
If the output current is expressed in microamperes, the
formula is simplified to
= ^dial - 2/.
For example, if / = 50 /tA and the
setting is at
maximum for 5000 V,
= 5000 - 2(50) = 4900 V.
The actual output voltage at the "0-500 V" connector can
be found in a similar manner by considering the approxi
mately 700 kS7 output resistance. The formula will then be
^o = \^dlal-/(7X105),
where the definitions are the same as before.
To find the actual voltage applied to a detector, the
resistance between the 459 output and the detector itself
must be identified. Since the bias voltage is usually
connected through a load resistor in the pream.plifier and
then to the detector, the resistance can usually be identified
from the
preamplifier schematic. The actual detector
voltage can then be found with the formula
where
voltage, 1^
V„ is the 459 output
is the detector voltage,
is the detector current, and R is the series
resistance between the 459 and the detector.
5. CIRCUIT DESCRIPTION
The 459 uses a dc-to-dc converter to charge a Cockcroft-
Walton' multiplier circuit. The primary of the transformer
is driven from an astable multivibrator operating at approxi
mately 20 kHz. Transistors Q1 and Q2 form the multi
vibrator circuit, while Q3 through Q6 serve as drivers and
switching transistors for transformer T1.
The output voltage is adjusted by controlling the voltage
applied to the primary of the transformer. Resistor R23
controls the primary voltage through transistors Q7 and Q8.
The circuitry in the transformer secondary consists of a
7-stage Cockcroft-Walton multiplier circuit. Polarity selec
tion is made with a plug-in board that completes the
necessary circuits for either polarity by its orientation on
the main printed circuit. In schematic 459-0101-S1 at the
back of this manual the alternate circuit connections are
shown as sections of a double-throw switch, S2. For
polarity reversal the input and output terminals of the
Cockcroft-Walton circuit are interchanged. The 0—500 V is
' Everhart and Lorrain, "The Cockcroft-Walton Voltage Multiplying
Circuit," Rev. Sci. Instr. 24(3), 221 (1953).
taken from the first stage of the multiplier circuit so that
this output will always have the same polarity as the 0—5
kV output.
The output voltage can be adjusted with trim potenti
ometer R22. This adjustment should be made with an
insulated screwdriver through one of the holes in the top
cover. The calibration of the front panel meter is made with
trim
potentiometer
R40, and this adjustment is also
avai lable through one of the holes in the top cover. The
meter is intended as an approximate indication of the
output voltage, and also shows the polarity of the output.
If the remote shutdown circuit is used, it wil l clamp the
output voltage through both of the 459 output circuits to
zero when the external circuit grounds the center contact
of the BNC connector with lOOOO impedance or less. This
turns on both Q10 and Q11. With Q10 turned on, Q7 and
Q8 cannot furnish any reference drive to the primary of
transformer T1 and high voltage cannot be generated. With
Q11 turned on, Q12 is turned off and the front panel HV
On indicator will not light.