MSU HV System Overview

What is it?

The LGD for E852 uses custom high voltage bases to drive Russian-made FEU 84-3 photomultiplier tubes. These bases have a digitally controlled output ranging from 0 to -2000 Volts.

How does it work?

• Each base has as its brain a custom CMOS chip for decoding external communications. External communications consist of an 8-bit address and a 12-bit voltage setting. The bases are mute, there is no communication from bases to the controller hardware.
• The LGD has in excess of 3000 channels. This requires the use of so-called splitting boards to break the LGD up into 15 "groups". These boards each contain a 4-bit address. To address a specific base one specifies the group and the base address within that group. Thus, to avoid confusion each group may have at most one copy of each base address. The splitting boards also handle the distribution of DC power to the bases.

Components of the HV System

• Power comes in from the AGS experimental hall power through a motor-generator (MG) set.
• From the MG set 208V AC is sent into a 5kW uninterruptable power supply (UPS). This UPS can drive the load of the high voltage system for approximately 30 minutes in the event of a power outage. The UPS output voltage is 120V AC. There is a terminal in the LGD hut which may be used for monitoring the UPS status. Also, in the event that the UPS switches from line to battery back-up an audible alarm will sound in the MPS control room.
• Power from the UPS goes through a contactor. This contactor serves as an emergency disconnect for the AC input to the DC supplies that power the LGD. This contactor may be opened by pressing either of the two crash buttons. These buttons lock in the off position until reset with a key (presumably by a person knowledgable in the system). Also in the event of a fire being detected in the LGD hut or the LGD darkroom, this contactor will automatically be opened.
• After the contactor power is distributed to the DC power supplies. All of the supplies have a redundant partner. DC voltages which are required for the opration of the MSU HV system are as follows: -24V, +6, +10 (also known as U-reference), +12, +24, +200. These voltages may be monitored with a voltmeter via the sockets located on the front of the rack containing the DC power supplies. Typically these voltages are set a little higher than the nominal values to compensate for transmission line losses.
• DC power passes through a fuse panel inside the grey box mounted above the UPS. This box also contains diodes to protect the redundant power supplies from back feeding should one of them fail.
• After a magical, mystical trip through conduit the DC power is passed into a grey box on the roof of the darkroom . This box contains fuses and fans the voltages out into three sections of splitters (5 splitters each). Also in this box are transistors which repeat (and amplify) the digital communications lines for input to the splitting boards. Each splitter section has test points for monitoring the voltages for that splitter section.
• From the grey box power and communications go through the splitting boards and are fanned out via ribbon cables to the individual bases.
• Control signals are generated by a CAMAC module located in the CAMAC crate in the rack containing the DC power supplies. Control signals are carried on RG-58 cables to the grey box on the roof of the darkroom.