Flow sensor board modification for the Rack Power Safety System. 16-July-1991 REASON FOR MODIFICATION The previous sensor board was designed to operate as a 'passive' module to convert the AC output of the flow sensor to a digital signal indicating whether flow exists or not. Since the design of the sensor board, the manufacturer of the flow sensor has altered their product so that the same rate of flow produces a signal at approximately half the frequency and amplitude of the signal produced by the original one. This signal is not enough to produce a 'no fault' output from the sensor conversion board under average operating conditions (flow of appx. 10 GPM). There have been no solutions found involving simple modification of the existing 'passive' circuit, therefore an active converter will be used in its place. DESCRIPTION The active conversion circuit mounts below a bare sensor circuit board. Voltage to power the circuit is taken from pin 3 (Vcc) and pin 11 (GND) of the sensor cable input (P1). The RPSS board is modified in order to convert sensor line input pin 3 from a no connect to Vcc and pin 11 from a no connect to the power return. 12 conductor cable must be used for the RPSS sensor input lines. Input from the flow sensor is taken from the TY3F connector on the sensor board enclosure. Output to the RPSS is wired to pin 4 and pin 12 of the sensor cable input (P1) on the sensor board. ACTIVE CONVERSION CIRCUIT DESCRIPTION The conversion circuit maintains a saturated transistor output (in the final transistor of comparator U1B) across the RPSS Water Flow Fault sense lines as long as the flow sensor produces an alternating current of peak amplitude greater than about 75 mv and frequency greater than about 5 Hertz. The flow sensor signal voltage peaks at just over 100 mV under normal operating conditions. This signal is compared with a 75 mV DC reference in comparator U1A. When the flow sensor signal is greater than 75 mV (about 20% of the time during average operating conditions) the final transistor of comparator U1A is saturated causing the open collector output to form a path to ground, discharging capacitor C1. The second comparator, U1B, monitors the voltage across C1. If the voltage across C1 becomes greater than 3 Volts (requires appx. 180 msec of charge time), comparator U1B produces a negative result, causing the open collector output to become a high impedance with respect to ground, therefore indicating fault on the RPSS Water Flow Fault sense inputs. The average frequency of the flow sensor signal should be about 20 Hz, causing comparator U1A to discharge C1 every 50 msec. This will keep the voltage across C1 to under about 1 Volt, which is considerably less than the 3 Volts required to cause comparator U1B to indicate a fault. A resistor was added across the input to the conversion circuit to cause the input to comparator A to be pulled low in case the input is disconnected. Bypass capacitors were added both across the power lines where they enter the circuit and across the power pins of the comparator to limit surges and noise. TESTING The active flow sensor conversion circuit was tested while plugged directly into the modified RPSS (s.n. 2) circuit on 24-July-1991 and via 40 foot cable on 26-July-1991. Forced air was used to activate the flow sensor. The fault threshold frequency was found to be 10 Hz corresponding to a flow sensor peak output of about 75 mv. Each of the 13 RPSS input channels was tested and all were found to be functional. FAILURE MODES - Mechanical failure of the flow sensor would result in no signal being generated and would cause the flow sensor conversion circuit to indicate a fault under otherwise normal circuit conditions. - Electrical failure or disconnection of the flow sensor would result in either a short or an open at the flow sensor input to the conversion circuit each of which would cause a fault indication under otherwise normal circuit conditions. - A collector to emitter short in U1A would cause a constant fault indication under otherwise normal circuit conditions. - A collector to emitter short in U2A would cause a constant no fault indication. - Shorting C1 would cause a constant no fault indication under otherwise normal circuit conditions. - Any rate of flow greater than 5 GPM will result in a no fault indication under normal operating conditions. MODIFICATION OF THE RPSS BOARD Using the active flow sensor converter circuit requires a Vcc line to be supplied to each rack from the RPSS board. The sensor cables will be 12 conductor instead of 10 conductor, allowing two unused pins of the DB15 connectors to carry Vcc and GND. Conductor 3 of the sensor cables is converted to Vcc and conductor 11 is converted to power return (GND). LIST OF MODIFICATIONS TO RPSS BOARD - Connect a pico-fuse to pin 3 of J1-J9 - Connect the other end of each of these pico-fuses together to Vcc - Connect a pico-fuse to pin 3 of J10-J13 - Connect the other end of each of these pico-fuses together to Vcc - Connect pin 11 of J1 - J13 to GND FLOW SENSOR CONVERSION BOARD MODIFICATION POINT TO POINT WIRING LIST from to R6a U1-2 R6b GND R1a Vcc R1b U1-3,R2a R2a R1b,u1-3 R2b GND U1-8 C2a,Vcc U1-4 C2b,GND C2a U1-8,Vcc C2b U1-4,GND U1-1 C1a,R3b,U1-5 C1a U1-1,R3b,U1-5 C1b GND R3a Vcc R3b C1a,U1-1,U1-5 R4a Vcc R4b U1-6,R5a R5a R4b,U1-6 R5b GND C3a+ Vcc C3b GND INTERFACING inputs from flow sensor: - from J1-1 (blue wire from TY3f on sensor board enclosure) to R6a,U1-2 - from J1-2 (grey wire from TY3F on sensor board enclosure) to R6b,GND outputs from converter (to RPSS) - from U1-7 to P1-4 on sensor board - from GND to P1-12 on sensor board power (Vcc) from RPSS - from P1-3 on sensor board to Vcc (@ C3a+) - from P1-11 on sensor board to GND (@ C3b) PARTS LIST FOR FLOW SENSOR CONVERTER BOARD MODIFICATION reference component value type R1 resistor 5.1 K R2 . 75 ohm R3 . 200 K R4 . 2.2 K R5 . 3.3 K R6 . 10 K C1 capacitor 1 MFD silver mica C2 . 0.1 MFD C3 . 33 MFD tantalum U1 dual comparator - LM393N