AU626945B2 - Improvements relating to a solid state power supply/ temperature control circuit - Google Patents

Description

f~ COMMONWEALTH OF AUSTAZ 9 4 PATENTS ACT 1 952-696 COMPLETE SPECIFICATION (OR IGINAL) Class Int.

Application Number: Lodged: Form Class a It 4 t C Complete Specification Lodged: at t IAccepted: It" Published: Priority: Related Art 4 'Name of Applicant: a 4 a 9 4 1 4 ,4 .Aildress of Applicant 4 I 6 441 Address for Service DAVEY PRODUCTS PTY. LTD, 2-22 HARGREAVES STREET, HUNTINGDALE VIC 3166 BRUCE ROBERTSON WATERMARK PATENT TRADEMARK ATTORNEYS.

290 Burwood Roa.d, Hawthorn, Victoria 3122, Australia.

Complete Specification for the invention entitled: IMPROVEMENTS RELATING TO A SOLID STATE POWER SUPPLY/ TEMPERATURE CONTROL CIRCUIT Trhe following statement is a full description of this invention, Including the best method of performing it knu;wrto:-u *1.

-2- IMPROVEMENTS RELATING TO A SOUD STATE POWER SUPPLY/TEMPERATURE CONTROL CIRCUIT The present invention relates to a power supply and temperature control circuit which may have a number of applications including operation and control of a fluid flow sensor and electric motor switch. There are many situations where maintenance of fluid flow particularly in the electric water pumping applications in which the present invention may be useful.

For example, in water pressure systems where frequent cycling of the pump shot Id be avoided or where blockage to water flow has occurred.

1 0 The invention may be used for sensing water demand related to volume of flow at any given ',ime; in pressure boosting applications cr maintenance of water levels.

The invention will be described in general terms with reference to the provision of a solid state power supply and temperature control circuit and in specific terms in reference to use in a fluid flow sensor. However, the skilled addressee will appreciate 1 5 that the circuit would be applicable to varied uses where a non-isolated AC/DC power supply with low quiescent power demand is desirable. The circuitry is simple in construction and may be applicable to AC or DC circuits of any type with simple changes.

A liquid/fluid flow sensing apparatus including a temperature controller for connection between a mains supply and a DC load, the DC load including a heating 20 impedance in series with a switch means controlling current flow in said impedance, said switch means being operable under the influence of a temperature sensing means such that in a heating mode, current flows through said heating impedance and in a non-heating quiescent made, a reduced current flows to maintain a substantially constant output voltage across the impedance irrespective of the switch mode, 25 wherein the switch means is a thyristor under the influence of a comparator such that in the non-heating quiescent mode the output of the comparator is zero and in the heating mode the thyristor is permanently triggered by said comparator under the 'influence of said temperature sensing means until a predetermined temperature is reached, whereupon the comparator output reverts to zero and the circuit to non-heating 3 0 quiescent mode, the heating impedance is exposed to liquid/fluid flow and connected to a bridge circuit, which depending upon balance of temperatures in a no-flow/flow situation switches the heater to said heating or non-heating mode.

i: -3- In this mode, the triac will generally only be fired for occasional single mains AC cycles when the output voltage applied preferably across a capacitor drops to a predetermined value.

The circuit described above has specific application to temperature control in a liquid/fluid flow sensing apparatus in which the heating element is exposed to the liquidifluid flow and connected in a bridge circuit, which depending upon balance of temperat,,res in a flow/no-flow situation will switch the heater to said heating or non-heating mode.

Gf.eneral Description oLEI.ow.ensor 1 0 This description refers to a method of flow detection and control with an On/Off pump, primarily intended for liquids (although the methodology would be equally applicable to gases with little modification), which may be used to maintain continuing liquid pressure under conditions of varying demand as found in many applications. The embodiment described may be manufactured consistently, at low 1 5 cost and proves very reliable.

In the embodiment described, liquid demand is intially detected by the Spressure drop detected in a pressure reservoir mounted close to the supply pump.

(In turn, the pressure reservoir is connected to the liquid discharge supply lines).

44 i r t o 1 0 hsdscito(efr oamehdo lo eetonadcnro iha OnOfpmpiail4neddfrlq4d>atog h mtoooywudb equaly pplcabe t gass wth itte mdifiatin),whih my beuse tomaitai 4 When a pressure drop is detected, the pump motor will be turned on and, as well, a simple temperature controller turned on which attempts to set a cemperature gradient along a member in contact with the (flowing) liquid. As long as liquid is flowing past the member, the thermal gradient cannot be adequately established and this can be readily sensed by an intermediately placed temperature sensor or thermistor. When the flow ceases, the temperature gradient can be established and the temperature rises at the intermediate thermistor. This indicates that the flow has ceased and the pump motor will be turned off.

When this principle is used in conjunction with a nearby liquid pressure tank, reservoir or header tank, and in conjunction with a pressure switch, a particularly 0.4 15 suitable means is established for the convenient and tI!" normally accepted dispensing or supply of liquid, for example domestic water in remote areas not serviced by a pressurised town water supply. This configuration prevents untoward pump cycling in considerably varying flow rate 20 conditions and is found to allow larger tolerances and simplicity in the pressure switches than were previously "necessitated.

The invention will be described in greater detail with reference to the annexed circuit diagrams in which Figure 1 shows the components of the power supply circuit; and Figure 2 shows the overall flow sensor controller 4 4 t 9' circuit in association with the power supply circuit.

Figure 3 shows the temperature controller for use in association with the power supply circuit.

Referring to Figure 1, the supply is generally obtained from AC mains supply but the device is applicable to AC circuits of any type.

A heating impedance RIl is connected to supply and under the control of switching thyristor Ql which drives the f output voltage limited to D3 connected across a capacitor Cl and diode D4. Diode D4 may not be required depending on the 1 r 5 selection of Q1. The circuit is equivalent to a voltage dropper network with the addition of thyristor Q1 allowing the control of current flow and a secondary supply voltage limiter D3.

Thus, R11 can be identified as a dropper impedance and the switch, thyristor Q1. Thus, a half-wave rectified current passes through R11 with D3 limiting the output supply voltage with D4 and Cl providing rectification and filtering.

With reference to Figure 2, the comparator IC1/A controls the operation of the power supply in two modes; S, where the pin 7 is at zero volts, and (ii) where the voltage at pin 7 rises to switch Ql to be permanently triggered.

.o 15 In mode requiring an output voltage of say 9 volts, D2 has 10 volts applied to it. This voltage will **o drop if the voltage across capacitor Cl falls requiring R2 to fire thyristor Ql, and therefore current for say one half a cycle will drive through thyristor Q1. The current will 20 charge Cl and then be directed through diode D3 to "dump" to 4 the excess current. With the increase in output voltage, tha thyristor Q1 will cease conducting. In this mode of operation the primary (input) current wili approximately Sequal the output current being merely sufficient to maintain charge on capacitor Cl which in the practical case illustrated will be approximately 0.1 milliamps to 8 milliamps m i In the mode (ii) of operation where the triac Q1 is permanently triggered, a half-wave rectified current will flow through dropping resistance or load impedance Rll to a current determined by the impedance and supply voltage. In the particular implementation of this circuit, the current flor.- is approximately 30 milliamps Control of the thyristor Ql is via diode D3, D2, and resistor R2 and the comparator output of IC1/B.

Resistor R2 provides the trigger current to the thyristor gate Q1 which provides "start-up" to an otherwise inert circuit.

i il 'ii ft,^- -6 If the comparator logics output is at zero volts, the thyristor operates in mode (i) for example, when the capacitor voltage on C1 drops, the combination of D2, D3 and R2 provides gate current until the C1 voltage rises again.

If the comparator logic output is high, that is the open collector goes to about volts, then resistor R2 provides continuous gate current, to keep the thyristor fully on which means that the thyristor is operating in mode The control is forced to logic low by diode D1, when the sensor is inactive. Diode D1 upsets the sensor bridge to force comparator IC1/A to a low output.

Thermal control is provided by thermistors R12 and R13 and a resistor R3 and 1 0 the comparator IC1/B. The two resistors R1 and R3 in combination with the two thermistors R13 and R12 are used to form a bridge circuit, with the comparator connected across the bridge. The bridge will balance when there is an exact temperature differential, say about 200C. between the thermistors. The comparator implements a "bang-bang" controller to switch the heater between modes and (ii) as the bridge 1 5 passes its balance point.

Specific description of Flow Sensor A thermal flow sensing method is used to turn off a supply pump in conjunction with a pressure drop actuation system including pressure switch P1. Three NTC (Negative Temperature Coefficient in other configurations PTC or both NTC and PTC, Positive Temperature Coefficient) thermistors and a heater resistor (with the other critical temperature sensing bridge components) are suitably located on a ceramic, thick film hybrid, electronic circuit substrate assembly or member. This substrate material forms the temperature and temperature gradient sensed member. Importantly, the heater resistor also provides the necessary impedance to reduce the inconveniently largo mains electric supply voltage to be accommodated by simple, low cost, electronic circuit parts used in the rest of the embodiment described.

2. The ceramic substrate member very practically allows the necessary and mandatory electrical isolation

C

c 7 required for safety consistent with the required thermal diffusitivity requirements.

3. The heater resistor is placed close to one thermistor which, in conjunction with a furthest thermistor, is used to attempt to set the temperature at the heater thermistor about 20 0 C. warmer than the notionally ambient liquid temperature. This establishes a temperature gradient across the substrate between these two thermistors. An approximately centrally located third thermistor is then able to conveniently sense the temperature gradient. These components are printed or otherwise processed onto the non-liquid side of the hybrid substrate member with other bridge balance resistors. These compo-nents may be conventionally, easily and cheaply trimmed in manufacture to allow a very low cost, physically and electrically functional bridge assembly.

Detailed Circuit Description Reference is made in this description to particular circuit schematic components as shown in an embodiment of the invention.

A bistable latch is formed by the logic gates of IC2/A and IC2/B and determines the on/off state of the pump motor. The latch and associated circuitry is configured such that the pump motor will be turned on only by a drop in pressure, stay on for a minimum time, as described, and will normally be turned off by a subsequent cessation of flow.

The temperature controller and power supply are conveniently combined. A silicon controlled rectifier (SCR) Q1 is used to control current through the heater (and mains power limiting) resistor Rll. Ql is controlled in two modes, Mode 1 and Mode 2.

In Mode 1, the SCR is only intermittently triggered maintaining a minimum voltage by intermittently injecting charge into the power supply filter capacitor Cl. Mode 1 is the condition of no flow where litle power is required to 1 maintain the temperature gradient across the substrate, nonetheless, the electronics must still be consistently 8powered. Indeed in the embodiment described, the temperature control function is turned off when the pump motor is turned off and Mode 1 provides only a simple power supply function. (If diode D1 were removed, the temperature controller could be continuously energised for alternative applications reliant on flow sensing only.) In Mode 2, the SCR may be triggered continually and the voltage control at +V is provided by the action of the zener diode D3 dumping excess current not required by the rest of the electronic circuitry. This enables adequate triggering current for turning on the pump motor gate current triac when required. Continual triggering will occur if the correct temperature gradient is not established i' as occurs when there is liquid flow dissipating energy from the heater element.

Mode 1 voltage control at +V is ensured by the action of the zener diode, D2: and the output level of the control comparator IC1/A (OV in Mode In Mode 1, the SCR cathode rises as Cl is charged until zener D2 conducts and diverts Ql's trigger current from the SCR. (R2 provides the SCR trigger current, including at 'turn In this mode, the electronics supply voltage at Cl) is set by the value of the zener diode, D2, and the trigger voltage of the SCR, Ql. As charge is lost from Cl (maintaining a supply current) and tho voltage on Cl falls, the SCR will re-trigger, re-charging Cl. If the output of ICl/A goes high SCR Ql is continually triggered and excess charge is dissipated, and the output voltage set, by the zener diode D3.

The temperature gradient is established by the configuration of the bridge formed by thermistors R12 and R13, and resistors R1 and R3, in conjunction with comparator IC1/A. (R1, the combined heater and mains dropping resistor, is mounted adjacent to the thermistor R12 with thermistor R13 separated the required distance. Thermistor R13 and resistor R3 form the reference arm for both the temperature gradient establishment and the flow sensing fri 9 arms). The bridge is trimmed at manufacture to ensure balance only occurs with R12 about 20 0 C. warmer than R13.

In this manner, ICl/A, in conjunction with the other components, can control both the electronics supply voltage and attempt to maintain a 20 0 C. temperature differential between the thermistors R12 and R13 on the substrate. In the embodiment shown, the temperature gradient function is enabled if the pump motor is energised or low pressure is indicated in the pressure reservoir. The temperature gradient function is disabled only when the pressure is adequate and the pump motor is not running. (In an alternative flow sensing embodiment only, the temperature gradient controller would be on continuously).

Flow sensing is accomplished by the bridge formed with thermistors R13 and R14 and resistors R3 and R4 in conjunction with the comparator ICl/B. (Resistors R15 and R16 are relatively small resistors which may be used for adjusting the flow rate sensitivity). The flow sense bridge arm components, R14 and R4, are designed to sense the change in temperature (specifically, the change in temperature gradient) as liquid flows across the sensor. The thermistor R14 is then sensibly placed between tLe thermistors R13 and R12. If there is no flow, R14 will be at some intermediate temperature between R12 and R13 and the comparator ICl/B output will be high. This high output will generally force the pump motor off via the latch formed by IC2/A and IC2/B and the pump motor triac, Q3, off (via its gate trigger S'I switch triac, Q2). Q2 is a sensitive gate triac minimising the current source requirements of the CMOS latch IC2/B and the power supply.

As the comparator ICl/B requires a pull up current to enable its output, the pump motior must run for some minimum time when activated as set by the time constant of the components R8 and C4 (typically 10 20 seconds). Provided this minimum time has elapsed, the pump motor can then be turned off immediately flow cessation has been sensed. it 10 In operation, flow will be sensed by the pressure in the liquid reservoir falling and actuating the pressure switch. This must set the latch and turn on the pump motor.

This is initially independent of the flow sensor output (ICl/B) as there can be no pull up current provided to its open collector output until the minimum run time has elapsed (via R8, C4 and buffer IC2/D).

If the pressure switch remained actuated, i.e.

continual low pressure, and no flow was sensed, this would be indicative of a liquid "No Head" condition at the pump and the pump must be turned off. This is ensured as both latch outputs will be forced low by the action of the 0 pressure switch and flow sensor inputs. This condition is stable and can maintain the motor off and the temperature a 4, S* 15 controller (flow sensor) on.

The pressure switch input is filtered from external noise sources and protected by R5, R6 and C2.

The latch output, IC2/B, drives the motor switch.

R9 sets the trigger current for a sensitive gate triac, Q2, 20 which in turn, via R10, sets the larger trigger current 0f I 0. drive required by the motor on/off triac, Q3.

S° Mains Voltage Considerations 0 Mains voltages appear across Rll, R2, Ql, Ql, Q3 Sand momentarily across 10. Peak voltages within the circuit are limited by the specified breakdown ratings of r, QI, Q2 and Q3. Correct design and specification can ensure these breakdowns can be safely accommodated.

In summary, the following commercial benefits are readily achieved The thermal elements and bridge components are inexpensively mass produceable.

The thick film (or thin film) hybrid assembly method ensures safety isolation consistent with suitable electrical, physical and especially thermal diffusivity properties.

The heater resistor provides the 'mains dropping' component too.

i

Claims (2)

1. A liquid/fluid flow sensing apparatus including a temperature controller for connection between a mairua pil and a DC load, the DC load including a heating impedance in series with a switch means controlling current flow in said impedance, said switch means being ope' M a under the influence of a temperature sensing means such that in a heating mode, current flows through said heating impedance and in a non-heating quiescent mode, a reduced current flows to maintain a substantially constant output voltage across the impedance irrespective of the switch mode, wherein the switch means is a thyristor under the influence of a comparator such that in the non-heating quiescent mode the output of the comparator is zero and in the heating mode the thyristor is permanently triggered by said comparator under the influence of said temperature sensing means until a predetermined temperature is reached, whereupon the comparator output reverts to zero and the circuit to non-heating quiescent mode, the heating impedance is exposed to liquid/fluid flow and connected to a bridge circuit, which depending upon balance of temperatures in a no-flow/flow situation switches the heater to said heating or non-heating mode. t f

2. A liquid/fluid flow sensing apparatus substantially as hereinbefore described and f• S. illustrated in the aocompanying drawings. 9, IDATED THIS 20TH DAY OF MAY, 1992 .4 :DAVEY PRODUCTS PTY. LTD. 9, WATERMARK PATENT TRADEMARK ATTORNEYS THE ATRIUM J 290 BURWOOD ROAD SHAWTHORN VICTORIA 3122 AUSTRALIA LJD:JZ AU4556889.WPC iI i 1