The Galileo Energetic Particles Detector

Galileo EPD Handbook

Chapter 1. Instrument Summary

Hardware Changes (continued)

Convert HV Reference Digital/Analog Conversion

The actual digital to analog conversion is performed on the new high voltage power supply. The 4 bits of data from the microprocessor are latched into a CD4052 by the clock signal generated on the motor controller board.

The 4 bits of digital information will be converted to an analog voltage by a pair of Harris 8:1 analog multiplexers, a resistor divider ladder, and an LM108-based buffer amplifier.  The 5.12 volt reference generated on the bus adapter (for the housekeeping ADC) will also be used here as the reference.

Switch High Voltage Input Power

The spacecraft ±15 volt supplies will be used as the input to the high voltage converter.  This voltage will be switched via the EPD power converter relay (K71) presently switching the ±3 volts used by the TOF.  Rather than using the "TOF ON" command to actuate the relay, however, we will use the command line presently used for "JA00 ON/OFF." This will separate the control functions of the TOF electronics and the high voltage supply, and allow us to continue using the E0 - E3 macro commands without worrying about the HV levels.  The supply will receive power through the one relay (current ratings are more than adequate).

In addition to these measures, we should install a high voltage arming connector on the exterior of the instrument.  The connector will pull down a logic signal to ground (or the 5.12 volt reference on the Harris muxs).  While this will provide additional protection for the HV supplies, it may introduce unwanted thermal leak paths through the blanket.  The placement and/or necessity of this arming connector requires further consideration.

Switch TOF Input Power

The TOF electronics power (±6 volts) will be switched through relay K71, as in the old design.  The dummy load on the ±6 volts will remain in the converter design to help maintain regulation when the TAC board is off.

The ±3 volt dummy load resistors will be removed since the EPD no longer will require significant power on the ±3 volt lines.  The only loads left on these lines will be the EME0263 hybrids on the rate logic (-3 volts) and the motor encoder assembly (+3 volts).  A 4.3 volt zenor will be connected in the power converter across each 3 volt power bus to prevent excessive switching transients from reaching these loads.

High Voltage Output Analog Monitor

The HV supply will produce an analog output voltage which is between 0 and -2.86 volts.  This voltage will be monitored via the -3 volt housekeeping multiplexer input on the calibrator board.  Since very little -3 volts is required in the new design, and the regulation is not critical, the lack of visibility on the -3 volt level should not be a problem.

Polytwist Wire Allocation

There are a number of new signals that must run through the polytwist.  Luckily, a number of spares are available to accommodate this need.  A list of the polytwist lines affected by the proposed design changes is shown in Table 9.

TABLE 7. PROPOSED CHANGES TO PRESENT EPD POLYTWIST WIRING

Note: Twisted pairs are highlighted in grey or yellow.

Next: Telemetry and Operation Changes 

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