GB2207261A

GB2207261A - Temperature control circuit

Info

Publication number: GB2207261A
Application number: GB08714061A
Authority: GB
Inventors: James Fitzpatrick
Original assignee: SOUTH OF SCOTLAND ELECTRICITY
Current assignee: SOUTH OF SCOTLAND ELECTRICITY
Priority date: 1987-06-16
Filing date: 1987-06-16
Publication date: 1989-01-25
Anticipated expiration: 2007-06-16
Also published as: GB8714061D0; GB2207261B

Abstract

A control circuit for an electrical resistance storage system includes a solid state switch such as a triac (32) which controls the power supply to a heating element (30). The triac (32) is controlled by a driver circuit (34) in response to the output of a pulse generator (36) and a zero-crossing detector (38). The circuit (34) is controlled by an inhibit circuit (54) and an I.C. (46) including a comparator (50) adapted to compare the output signal from a thermocouple (44) with a preset reference and to control the driver circuit (34) and thus the triac (32) on the basis of the comparison. The thermocouple (44) senses the temperature of a refractory mass heated by the element (3). The power supply (40) for the circuits (34 and 46) is preferably provided by the mains supply powering the heating element (30). <IMAGE>

Description

Temperature Control Circuit This invention relates to circuit arrangements for controlling the temperature of an electrically heated appliance, and is particularly but not exclusively applicable to the control of a high temperature storage system for use with circulating water space heating.

It has previously been proposed to make use of electrical storage heating to supply heat to a circulating water space heating system such as a domestic heating system. For reasons of economy, it is necessary for electrical power to be supplied during off-peak periods, and therefore heat may have to be stored for time periods of up to 16 hours. It is also desirable to make the storage medium as small and compact as possible, particularly for domestic use.To meet these requirements it has been proposed to make use of a refractory storage medium heated by electric resistance heating elements to a temperature in the region of 750 0 C, and to transfer heat from this medium to the circulating water system by means of vaporizing water in the heat storage region and then condensing the steam generated on condenser tubing through which the space heating water is circulated.

In such an arrangement it is necessary to control the temperature of the heat storage medium accurately since relatively small excesses of temperature can cause a serious degradation of the materials used in the high temperature area. At the same time it is desirable that the temperature control circuit be simple and cheap to produce, and will operate without causing acoustic or electrical noise or radio frequency interference.

Accordingly it is an object of the present invention to provide a control arrangement which meets the above objectives.

In broad terms, the invention provides a control circuit for an electrical resistance storage heating system, comprising solid-state switch means for connection in series with resistance heating elements across a mains supply; a driver circuit arranged for triggering said solid state switch means into conduction, a zero cross-over detector for detecting the zero point of the mains supply to enable said driver at the zero point, a thermocouple for detecting the temperature of the storage medium, a thermocouple circuit arranged to compare the thermocouple output with the predetermined reference; and inhibit means responsive to thermocouple circuit to inhibit the driver when the output of the thermocouple is above said reference.

In a preferred embodiment of the invention, DC power to the circuitry is provided by a power supply directly connected to the mains voltage line through a voltage dropping resistor, and the mains voltage neutral is used as the zero voltage line of the circuitry power supply.

The solid state switching device is preferably a triac directly connected to the driver circuit.

In a preferred application of the invention, the triac is positioned remotely from the remainder of the circuitry on a substantial structural member of a heat storage apparatus adjacent a cable entry.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which; Fig 1 is a schematic cross-sectional side view of a storage heating apparatus embodying the invention; and Fig 2 is a block diagram of the control circuit.

Referring to Fig 1, a storage heating system comprises a cabinet 10 within which is positioned a mass 12 of refractory material heated by resistive heating 0 elements (not shown) to a temperature of 750 C for use in a vaporizing and condensing system as discussed above.

For thermal and mechanical purposes, a thick steel base plate 14 is provided at a position raised above the floor. The triac 16 of the circuit to be described is mounted on the underside of the base plate 14 within an enclosure 18 which also serves as an entry for a supply cable 20. The remainder of the circuit is positioned within an enclosure 22 in the upper part of the cabinet to allow user controls such as indicated at 24 to be located at a convenient height for the user.

Turning to Fig 2, the resistance heating elements indicated at 30 are connected in series with a triac 32 across a 240 volt AC mains supply. The triac 32 is controlled by a triac driver circuit 34 which in turn is controlled by pulses produced by pulse generator 36 in response to a zero cross-over detector circuit 38 connected to detect the zero voltage point of the mains supply.

Power is supplied to these items by a power supply circuit 40 connected to the main supply line through resistor 42. The main supply neutral serves as the zero voltage reference for the circuit. Thus the controlling circuitry is interfaced with the main supply circuit without the use of transformers or other wound components, so affording simplicity and low cost.

A thermocouple 44 of known type has its hot junction located within the storage medium and its cold junction in an ambient location. The thermocouple output is applied to an integrated circuit 46 which is suitably a known IC type AD 595 AH and comprises an amplifier 48 and comparator 50 which compares a voltage related to the thermocouple output with a reference voltage set by a potentiometer 52. If the thermocouple output is greater than the reference, the comparator output goes high and actuates an inhibit circuit 54 preventing operation of the triac driver 34.

The IC 46 includes a circuit 56 which adjusts the gain of the amplifier 48 to compensate for the effect of changes in ambient temperature on the cold junction of the thermocouple 44. An external resistor 58 may be used to set the hysteresis of the system.

A feature of the present circuit is that the output of the power supply 40 is also used as the power supply for the integrated circuit 46. To enable this to be done it may be necessary to reduce the gain the thermocouple amplifier by means of an external resistor (not shown).

It will be noted that again the main supply neutral is used as the zero reference point for the integrated circuit 46.

In the preferred application the triac 32 may be carrying a current of 35 amps for a period of hours.

Therefore the dissipation in the triac together with the high ambient temperature produced by proximity to the heat storage medium could cause overheating problems with the triac. For this reason the layout shown in Figure 1 has been adopted.

The circuits 34-40 and 54 may suitably be comprised by a known integrated circuit type CA 3059.

Claims

1. A control circuit for an electrical resistance storage heating system, comprising solid state switch means for connection in series with resistance heating elements across a mains supply, a driver circuit arranged for triggering said solid state switch means into conduction, a zero crossover detector for detecting the zero point of the mains supply to enable said driver at the zero point, a thermocouple circuit arranged to compare an output signal from a thermocouple with a predetermined reference, and inhibit means responsive to said thermocouple circuit to inhibit the driver when said output signal is greater than said reference.

2. A control circuit as claimed in claim 1 wherein a DC power supply for said circuit is provided by power supply means directly connected to the mains voltage line via a voltage dropping resistor, and the mains voltage neutral is used as the zero voltage line of the circuit power supply.

3. A control circuit as claimed in claim 1 or claim 2 wherein said solid state switch means is a triac directly connected to the driver circuit.

4. A heat storage apparatus incorporating a control circuit as claimed in any of claims 1 to 3 wherein said triac is positioned remotely from the remainder of said circuit on a substantial structural member of said apparatus adjacent a cable entry.

5. A control circuit for an electrical resistance storage heating system, substantially as hereinbefore described with reference to the accompanying drawings.

6. A heat storage apparatus substantially as hereinbefore described with reference to the accompanying drawings.