POWER SUPPLY

This chapter describes the power supply that supplies the +5, +/- 15 and 32 volt DC voltages. It is custom designed to supply the voltages and currents needed to operate the controller in a quad readout configuration with four analog boards. It is a linear supply optimized for efficiency, size and reliability. At currents typical of a four readout system the efficiency was measured to be 54%. It is available in either 240 or 115 VAC input voltage and 50 or 60 Hz configurations so it can be used internationally. It is housed in a robust cast aluminum box painted with acrylic enamel that measures 23 x 20 x 11 cm (9 x 8 x 4.5 in) with the output connector protruding by 2 cm on one end. It weighs a hefty 6 kg. (13 lb.), is dust tight, though not quite watertight. A printed circuit board of Revision 1B contains a diode rectifier, regulator circuits and filter capacitors. Four large electrolytic capacitors and the toroidal transformer reside in the aluminum box along with an AC input module, a reset switch and an output connector. A large push-button switch is mounted next to the output connector that is wired to the power control board on the back end of the controller to reset it and the utility board. The utility board in turn normally resets the timing board with a software command during its booting sequence by setting the backplane line SYSRST low. This switch is more accessible than the switches on the power control board or the utility board and is designed for observers to access should the system 'hang'.

A toroidal transformer is chosen to minimize stray magnetic fields and to raise power supply efficiency. Some of the power supplies have two primary windings to allow operation from 240 VAC. The schematic shows the connection for 115 VAC operation, where the two primary windings are operated in parallel and the 115 VAC taps are chosen. For 240 VAC operation the windings would be operated in series, with the black and violet wires (both 0 VAC) connected to the AC power source, and the red (115 VAC) and violet (125 VAC) wires connected together. The primary winding has three taps to allow the user to operate over a range of input AC voltages. The AC voltage needs to be high enough for the regulators to operate and low enough to keep the power supply efficiency high. The power supply is jumpered to the 115 VAC rms tap (red/blue wires); the two other taps are at +/-10% from this, at 103.5 V (brown/green wires) or 126.5 V (orange/violet wires). If the AC voltage differs significantly from the nominal 115 volts another tap will need to be selected by the user by changing the jumpering on a terminal block mounted to on the wall of the case. Either an rms meter can be used to measure the AC voltage, or the outputs of the power supply can be monitored to determine if regulation has failed. Typical AC rms voltages at which regulation fails for the 115 VAC tap are listed in the Table below for each of the four power supply sections. The +32V section drops off the earliest as the voltage is lowered, so should be monitored first when setting up the supply. Most reliable power supply operation in areas of low quality power is achieved by jumpering to the 103.5 VAC tap.

Schottky diodes are chosen for the three low voltage (+5, +/- 15V) full wave rectifiers because of their fast recovery times and their low forward voltage drops in order to maintain high power supply efficiency. The high voltage section (+32 volts) uses a standard bridge rectifier and high voltage adjustable regulator, with efficiency not a concern because of the low current draw. Each low voltage power supply section uses a low drop-out solid state regulator with a voltage adjustment potentiometer targeted to adjust the output voltages by approx. 10%. The Table lists the approximate adjustment range for the four supply outputs. There is some significant voltage drop in the cable running to the controller box due to ohmic losses, especially on the +5V line. The four pots are adjusted to supply the voltages at the controller listed in the Table with the cable length specified by the user. If a significantly different length cable is chosen re-adjustment of the pots may be desirable, especially of the +5V supply. Improper adjustment may be manifested by a failure of the utility board DSP to pass the hardware test consisting of 'TDL' (Test Data Link) commands issued by the host computer.

The large filter capacitors (C1, C3, C5 and C7) were given large capacitances to minimize ripple and smooth out AC voltage drops. Computer grade electrolytics were chosen for reliability and long life. The returns for the four voltages are returned to the power supply on a single wire in the power supply cable. These returns are not connected to the case ground of the power supply box since the ground reference is at the front end of the analog boards. The only case ground is from the case to the CORCOM entry module that connects it to the ground wire of the AC plug. The power supply cable to the controller has a shield that is grounded only at the power supply through the MS-type connector S1.

The maximum currents listed in the table are approximations for room temperature in the absence of external cooling of the power supply box. The currents are limited primarily by dissipation of heat produced by the transformer and regulators. The power supplies are all operated for several hours after assembly while driving large load resistors that dissipate an aggregate of 48 watts. This is slightly higher than the 44 watts drawn by a quad readout system

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