Purpose
The Berkeley Zener Regulated Photomultiplier Divider Panel has been
in use for many years. A High Voltage Vernier Control and Monitor
Add-On Unit has been designed at Fermilab recently which provided
features that are not presently available on the original Berkeley
units. First, continuous adjustment of the photomultiplier tube
voltage between the 20 volt increments provided by the Berkeley unit
is now available and second, the photomultiplier tuba voltage is
accurately monitored and available for both computer interfacing and
local checking.
Operation
The Add-On Unit is connected to the 20-channel Berkeley unit by
placing a 25KOhm, single turn, cermet pot in series with each of the
Berkeley unit output wires. Since the photomultiplier tube draws from
1 to 5 ma, the 25K Ohm pot allows the photomultiplier tube voltage to
be adjusted by a minimum of 25 volts. The voltage dropping pots are
at a high voltage and thus insulated shaft extenders are used to
bring the pot; shaft to the front panel for adjustment. Since the
pots do not have the benefit of front panel heat-sinking, they are
sized appropriately for free air operation.
The photomultiplier tube output: is applied to a 10MOhm, 1010:1, three-terminal voltage divider which is trimmed by,a 500Ohm, single turn, cermet pot to provide an exact 1000:1 output for computer and local monitoring. A 560Ohm resistor is connected in parallel with each 500Ohm pot to trim the pot to the desired adjustment range and provide a connection to ground for the divider, in the event that the 500 Ohm pot fails open for some reason. All of the divider outputs are fed to a single 25 pin, rear-panel connector for computer monitoring. A front panel 20 position rotary switch allows any of the channels to be monitored locally in parallel with the computer output.
Performance
Important to the operation of the Add-On Unit is the per- formance of
the 3-terminal resistor divider. A Victoreen-type MOX-2RD was chosen
for this application. According to the factory, published temperature
curves for the 1.5W, MOX-2RD are incorrect (the curves show
temperature to be too high versus power dissipated).. A curve of
resistor divider surface temperature as measured at Fermilab is shown
in Figure 2. At 3.0kV applied to a resistor divider, the temperature
rise of the resistor was 18 C. The factory specifies the voltage
tracking of the dividers as 15-25 ppm/ C. Thus, the self-heating of
the resistor at 3kV, should cause the steady state output voltage to
change by less than .045%. In, tests at the factory on the dividers
to be used in the Add-On Units, the voltage tracking was found to be
less than 14 ppm/ C on a sample of six units. Listed on the next page
are the important, electrical characteristics of the add-on
units.
Voltage Divider Monitor Characteristics
1. Voltage Ratio 1000:1 +/- 1%
2. input Impedance 10 MegOhm +\- 5%
3. Output Impedance lOKOhm +/- 5%
4. Voltage Tracking Temperature
Coefficient 25 ppm/ C maximum
5. Resistor Temperature Coefficient 100 ppm/ C maximum
6. Voltage Coefficient 5 ppm/V maximum
Series Voltage Control Pot Characteristics
1. Potentiometer Resistance 0-25KOhm +/- 10%
2. Load Current 6.3 ma maximum
3. Voltage Resolution and
Resetability (1T, linear) 1.0V minimum
4. Temperature Coefficient (cermet) 100 ppm/ C maximum
5. Isolated Shaft Insulation 5,500V minimum