Ideally, you should power your PMT from a lab-grade, low-noise, high-voltage power supply designed specifically to bias detectors. However, achieving exceptionally low ripple and high stability in a high-voltage power supply is not trivial, making these power supplies pricey. Designs for these low-noise power supplies are secrets closely guarded by companies that specialize in such products, such as Matsusada Precision, EMCO High Voltage, and Hamamatsu Photonics. Fortunately, relatively low-cost, high-voltage power supply modules are sold by these companies as components for larger instruments.
Our home-built PMT power supply is built around the Matsusada JBE-2P high-voltage power supply module. This module sells new for around $200, but similar units can be found on the surplus market for a fraction of this price. The module is powered by 24 VDC (at 300 mA typical), and generates up to 2,000 V at 1 mA as a function of a 0- to 10-V control input. A 10-V reference output is available on the JBE-2P, allowing a 5 kΩ potentiometer to be used for voltage control.
The schematic diagram of Figure 31 shows that 24 V for the JBE-2P are obtained by rectifying the output of a 25.2-V transformer (T1, a RadioShack 273-1366A) and regulating through a LM317 variable linear regulator. A kit comprising a circuit board and components for the regulator circuit is conveniently available from Velleman as their model K1823.
Figure 31 Our variable-output PMT power supply is based around a Matsusada JBE-2P low-ripple, high-stability, high-voltage power module. Output voltage (up to 2 kV) and current (up to 1 mA) are monitored via two LCD panel meters.
A second transformer and regulator board are used to produce an isolated output of +9 VDC to power two 3 1/2-digit panel voltmeters. The digital voltmeters are used to show the voltage and current output from the high-voltage module. We used Lascar EMV1125 LCD panel meters because they need only a 7/32-in. round hole for mounting, but any other 200-mV full-scale panel meter module should work equally well.
An even lower-cost alternative is to build the PMT power supply from a BXA-12579 high-voltage inverter used to drive a cold-cathode fluorescent lamp (CCFL). This under-$20 module produces 1,500 VAC at around 30 kHz from a 12-VDC input.
As shown in the schematic diagram of Figure 32, the BXA-12579 has to be slightly modified for use in this circuit. The 100-μF/25-V capacitor marked “C1” in the BXA-12579 needs to be removed so that the power input to the module can be varied rapidly. In addition, a pin needs to be added to access the transformer’s high-voltage terminal, and the line connecting the low-voltage to the high-voltage ground must be cut.
Figure 32 A low-cost, variable-voltage PMT power supply can be based on a BXA-12579 inverter module that was originally designed as a power supply for CCFLs.
In this circuit, the supply to the BXA-12579 inverter is modulated by op-amp U3. The high-voltage AC output of the inverter is rectified and doubled by D2/D3 and C7/C12. C13/C14 are used to filter the high-voltage output.
The output voltage is regulated to the desired level by feeding op-amp U3 with an attenuated version of the output. This level is compared against the output of voltage control R17. The op-amp attempts to maintain the output constant by controlling the power to the inverter by way of transistor Q1.
This design incorporates a circuit used by Spectrum Techniques to reduce ripple in their PMT power supplies.7 Op-amp U2 is used to neutralize AC signals from the high-voltage output by adding a voltage that is 180° out of phase with any signal that goes through C9. A very stable and clean output can be obtained if the circuit is built within a grounded, metallic enclosure, being careful to prevent coupling of the high-voltage AC output into the rest of the circuit.
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