Low Voltage Power supply for QWAD
We are using Agilent 3633A Low Voltage linear power supply (User Guide). We are using this unit in the mode which has 8V maximum voltage output.
We are using two units, one being used for giving -5V and GND and the second unit configured to give GND and +5V. The PS(power supply) unit giving +5V sends typically a current of 1.56A, whereas the unit giving -5V has a typical current of 0.36A. The load of this PS (QWAD boards) have a very low output impedance (~3.2 ohm).
We are using a 5 conductor, 120 feet of cable of 12 AWG (resistance: 0.00187 ohm/foot) between the PS and the QWADs. Another 20 conductor, 120 feet cable of 22 AWG (resistance: 0.0190 ohm/foot) is used for sensing the voltage at the load. The power supply senses the "actual voltage" at the load, using the sense wires and (if needed) increases the voltage output to compensate for the voltage drop along the wire, so as to eventually maintain the desired voltage of 5.0V at the load. Near the QWAD boards we have a terminal block (data sheet) which is rated for wires from 10-24 AWG for sending out this voltage to all 8 QWAD boards plugged to the vacuum - can.
The Agilent power-supply allows 3 settings to be saved in the non-volatile memory of the PS. The current saved settings support:
- Single QWAD
- Double QWAD
- 6 QWADs.
Each of the above saved settings make sure that the Over-voltage and Over-current protection are appropriate for the number of boards that is intended to be used. Further details of the current settings can be seen in the next sub-topic.
In order to invoke any of the stored settings, we should
- have the 'Output Off' on both the (+5V & -5V) power supplies.
- have the molex connectors (carrying the output of the PS) connected to the required no.of QWAD's
- If we have (say) 6 QWAD's connected to the PS, then we have to press 'RECALL' on both the PS.
- with 'RECALL' being displayed on the screen of both the PS, turn the knob to get '3' (selecting Saved Setting # 3)
- having the screen display "RECALL 3" on the screen of both PS, press 'RECALL' button again on both the supplies. This should be done simultaneously on both supplies. (typically we use both hands to ensure that we press the RECALL of both PS at the same time)
- OVP( Over Voltage Protection): 5.6 V per QWAD board (5.0 V needed)
- OCP (Over Current Protection) in +ve supply unit: 1.8A per board (1.56 A needed)
- OCP (Over Current Protection) in -ve supply unit: 0.4A per board (0.36 A needed)
We are using the back panel to connect the cables to the PS. Agilent suggests that only Either the front panel or the back panel be used for connections. The PS unit on the left is being used for -5V output while the one on right is used for +5V output. Color Code: On the LV cable which is 12 AWG,
red : +5V green : GND yellow: -5V black : GND orange: unused.
For the sense-cable, we are using 2 wires of a 20 conductor cable where we are connecting white wire to the positive voltage output of each individual unit.