Rutgers University Experimental Nuclear Physics

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Low Voltage Power Supplies

Contents

• Introduction
• Power requirements
• Power box details
• Power box connectors
• Low voltage bus mechanics
• Some Chamber 4 bus details
• WARNING

The CEBAF FPP straw chambers are equipped with two types of power supplies. Eight high--current low--voltage power supply boxes provide +/-5V power to amplifier, comparator, and logic chips on the readout cards. These supplies were constructed by the Rutgers University Department of Physics & Astronomy electronics shop. Eight low--current low--voltage Hewlett--Packard supplies supply a positive threshhold level to the comparator chips on the readout cards. All low voltage supplies provide relative voltage levels. Returns for the supplies are tied to the chamber ground. Output cables also provide differential signals. Ground for the chamber is supplied solely by the high voltage supply for the straws.

The readout cards are rated for a maximum current of 0.52 A at +5 V and 1.53 A at -5 V. There is some rate dependence to the current required, and generally the cards draw around 70 % of the rated numbers. The threshold circuit has a 1.5 kohm resistor external to the card, and about 1.51 kohm resistors on the card, leading to a 3.33 mA current per card at 10 V threshhold at the box, or 5 V threshold applied to the card. Thus, the current and power requirements are the following:

The threshold supplies are Hewlett-Packard 6111As, rated for 0 - 20 V and 0 - 1 A. The two 6111As used for each chamber could supply 6 - 8 times more current than needed at 10 V setting, but only 3 - 4 times the current needed at 20 V setting. If trouble develops with a supply, the two threshhold busses could temporarily be jumpered together, consistent with safety standards, to allow operation.

Due the the different power requirements of each chamber, and the cost of high current supplies, details of the high power boxes vary. Each chamber is supplied by two power boxes.

In addition, power is needed for the level shifter card crate. 41 - 43 level shifter cards are needed, depending on how the cabling ultimately is done. Each card requires 0.32 A at +5 V and 0.66 A at -5 V. Total current / power required for 42 cards is 13.4 A / 67 W at +5 V and 27.7 A / 139 W at -5 V.

Individual box details include the following:

The spare box S/N 17 has supplies equal to or greater than the corresponding supplies for all boxes except S/N 11, of chamber 3. In practice, this spare can be used in place of S/N 11. Since the actual current drawn is close to the limits of the supply, this should not be done for long periods.

The power supply box performance has been quickly checked on chamber 3. Voltage on the low voltage busses was found to be stable to a few mV at 5 V. Readout card performance was found to have only insignificant variations for supply voltages from 4.7 to 5.3 V, indicating that low voltage supplies should be sufficient.

To better regulate the voltage, especially under conditions of changing loads, the power supplies have sense leads. In practive, we find the snese leads are best connected to the ends of the power cables, where they hook into the chamber busses. This system will then compensate for voltage drop in the 10 foot long power cables. In general, the power connectors are labelled as +/-5 V and ground/return, whereas the sense leads have a +V / -V label. The +V sense goes to a +5 V lead for a positive supply, but a ground/return lead for a -5V supply. In initial hookups, if there is any question about the hookup of the sense leads, it can be easily checked by a resistance measurement; there is 6 ohms between the sense lead and the corresponding power lead.

The three types of power box output power connectors are shown (outside view) in the following tables.

Note that the original labelling on the outside of some of the boxes reversed the low voltage (+/- 5) and ground/return pins incorrectly. The above chart, and the labelling on the far ends of the low voltage cables that go with each box, are correct to the best of our knowledge.

Also note from the above diagrams that there appear to be 3 -5 V supplies for boxes 13, 14, and 17. This is not the case. The -5 V_a supply connects to both the top and bottom connectors in parallel. The lower cable made up by the e-shop only has pin connections for the -5 V_b supply and the ground shield.

Input power is from standard 110 V wall AC. Nominal power requirements, given on labels on the rear of each box, are quoted as 10 A per power cable, for either 35 or 50 A output supplies. There is one such cable for each power supply in the box, leading to two or three cables per box. (Note: 50 A at 5 V is 250 W output; 10 A at 110 V is 1100 W input.)

Each box is also equipped with a monitor cable output. Shunts convert the current into a voltage that can be monitored. The shunts are usually about 0.7 mV per amp output, although some are 1 V / amp. With a maximum supply output current of 50 A, this leads to a typical current monitor voltage of 20 or 30 mV, with a maximum of 50 mV.

Low Voltage Bus Mechancs

The low voltage busses are connected to cards and power supplies by connectors that are screwed in. The busses are mounted on plastic mounts machined by the Rutgers Physics Department machine shop; the mounts bolt into the I beam frame with 1.5" (?) long 8-32 screws, fastened by nuts (11/32" head nut driver). The busses were drilled and tapped for 4-40 screws for the wires to each card; the screws are about 1/4" (?) long. The power connector on the end of the bus is attached with a 1/4-20 bolt (7/16" head), with the barrel of the bolt sheared to about 1/4" length. The connector is designed for wire sizes from #2 - #14 AWG; the threshhold wires are sufficiently small that they do not fit firmly in this connector, and can pull out. Additional strain relief is needed for them. To prevent accidental shorting together of the busses, all except the ground bus are covered with report cover plastic spines, cut to length to fit between the connectors screwed to the busses.

Some chamber 4 Bus Details

Chamber 4 uses two LV power supply boxes, each containing one +5 and two -5 V supplies. The box attaching to the upper +V corner of the chamber supplies readout stacks V9-17 with the -5 V_A supply, and stacks U1-8 with the -5 V_B supply. All of these stacks share the same +5 V and threshold supplies. The box attaching to the lower +U corner of the chamber supplies readout stacks U16-17 and V3-8 with the -5 V_A supply, and stacks U9-15 and V1-2 with the -5 V_B supply. All of these stacks share a second +5 V and second threshold supply.

WARNING:

Applying the high current low voltage power to the readout cards with polarities reversed may lead to blowing the power capacitors on the readout cards. Thus, it is VERY IMPORTANT to check the power polarities when hooking up power supplies before turning power on. Either disconnect every card from the power busses and check the bus polarities, or disconnect the busses from the power cable, check the cable polarities, and make absolutely sure that you will apply correct polarities when you hook up the power cable.

Revised May 5, 1996 Norma Lucero