HV Board Circuit

The proposed circuit for the HV board is shown in Figure 1. The HV is distributed to the tubes through current-limiting resistors. A series resistor (as shown in Fig 1(a)) matched to the characteristic impedance of the MDT (about 380 ohms) minimises reflections. The terminator is series-connected to isolate the MDT from the from PC board capacitance to minimize noise. An alternative circuit (shown in Fig 1(b)) does provide correct termination but increases crosstalk and noise, and is therefore not preferred. The series scheme does constrain the fuse rating somewhat, as the fuse must be blown through the termination resistor.

HV current is limited by a 1M$\Omega$ resistor in series with the HV supply. The power rating (1W) of this resistor is chosen to permit (briefly) operation with up to 1mA of supply current with a shorted wire, or continuous operation provided that the current is limited to 0.7mA at the supply.

The fuse is currently anticipated to be a standard 20mm x 5mm fast-acting type, chosen with a rating sufficiently large that it cannot be blown by current from the HV supply if a discharge or spark occurs in a tube or on the PC board. A separate high-current pulsing procedure is used to blow, selectively, only fuses connecte to tubes with wires shorted to ground.

The exact fuse rating will be determined after further analysis and testing. A higher rating is suggested by a desire to avoid accidental blowing of fuses, and a lower rating is suggested by desire to minimise the amplitude of the pulses required to blow fuses deliberately so as to avoid undue stress on the front-end electronics.

The diode is a high-voltage diode, tentatively a Philips BYX90 type, with basic specifications given below for reference.

The capacitor is a high-voltage (5kV or 6kV rating) through-hole disc type. Special attention must be given to the choice of dielectric material, as the capacitance of some types is severely voltage-dependent. A final selection has not been made.

 

Maximum Reverse Voltage (working) 6kV

I_F(ave) Maximum Average Forward Current 550mA

I_FRM Maximum Repetitive Forward Current 5A

I_FSM Maximum One-shot Forward Current 20A

$$\mu$$ I_R Reverse Leakage Current

V_F Forward Voltage @ 2A 14.5V

  

Figure 1: HV Board Schematic