Technical Field

The invention relates to a method and a device for connect¬ ing, e.g. a current source, to a load, in dependence of tempe¬ rature, connecting and disconnecting the current source being 5 carried out by means of a temperature sensing circuit-breaker, the process being repeated as long as the ambient temperature of the temperature sensing circuit-breaker falls below a prede¬ termined value. Prior Art 10 It is previously known to use electromechanical thermoswit- ches in connection with the control of heating appliances, for example, heating pads for vehicle seats. These thermoswitches exhibit a substantial difference between switch-on temperature and switch-off temperature,-which means that if there is a switch

15 on temperature of 15 C and a switch-off temperature of 30 C, the current to the heating appliance will be cut off when the temperature has raised to 30 C, not to be supplied to the heat¬ ing appliance again as long as a person is sitting in the chair, as the heat of the person's body will maintain the temperature

20 of the thermoswitch higher than the switch-on temperature of

15 C. The temperature of the heating appliance can thus decrease to 15 C without the current being switched on again, which is unsatisfactory.

Other, more complicated, electronic solutions are also avail-

25 able on the market, but these have the disadvantage of being expensive. The Problem of the Invention

The object of the invention is to eliminate in a simple and inexpensive manner the above disadvantages in switching on

* 30 and switching off in dependence of temperature an electric cir- f cuit, for example, connecting and disconnecting a current source

♦ to and from a load, where reconnecting the current source is to occur when the temperature falls below a selected breaking temperature.

Summary of the Invention

The invention relates to a method and a device for repeated¬ ly connecting and disconnecting, in dependence of temperature, an electric circuit. The device includes a temperature sensing circuit-breaker in series with a current conducting element.

The invention is characterized in that a thermal mass is provided between the current conducting element and the temperature sensing circuit-breaker, the mass absorbing loss energy from the current conducting element and giving off heat to the temperature sensing curcuit-breaker. When the circuit-breaker reaches a predetermined temperature the current source is disconnected and the mass cools off. The process is repeated as long as the ambient temperature of the circuit-breaker is below the predetermined level.

Features characterizing the invention are defined in the accompanying claims.

List of Drawing Figures

An embodiment of the invention will be described below with reference to the annexed drawings, wherein

Fig. 1 shows a circuit diagram of an exemplified embodiment of the invention.

Figs. 2a and b illustrate the process of switching on and switching off as a function of time at two different ambient temperatures.

Disclosure of an Embodiment The device shown in Fig. 1 is intended for controlling an electrically heated vehicle seat, where 1 designates the tapping point for the heating element 2 which is to be controlled with respect to temperature. The device comprises a heat sensing cir¬ cuit-breaker consisting of a bimetallic spring 3 having on its end a contact button for cooperation with a contact plate 4. The spring 3 is arranged such that it makes contact with the plate 4 if the ambient temperature and that of the spring is lower than T , T _Q being the breaking temperature of the breaker 3, 4. If the temperature is higher than T the bimetallic spring flexes so that contact ceases. A battery 6 is connected to the input terminal 5 of the spring 3. The base electrode 8 of a transistor 9 is connected to the output terminal 7 of the con¬ tact plate. In the examplified embodiment the transistor 9 is

a current controlling NPN-transistor. The collector of the transistor 9 is connected to the terminal 5 of the bimetallic sprin , and the emitter of the transistor is connected to the tapping point 1 of the device. Between base and emitter there is a resistor 10. In association with the transistor 9 a thermal mass 11 of a predetermined size is provided. When the transistor 9 is conducting it gives off loss energy, which is absorbed by the thermal mass and converted into heat. One end of the bime¬ tallic spring 3 is in contact with the thermal mass 11. The device operates in the following way. It is assumed that the breaking temperature T of the circuit-breaker is 20 C and that the ambient temperature is -10 C (see Fig. 2a). The bimetallic spring 3 is thus in contact with the plate 4 and the battery 6 emits current to the base 8 of the transistor 9, which then becomes conducting. The current increases and the heater element 2 is heated. When the transistor 9 is conducting it gives off a certain amount of loss energy to the thermal mass 11, which converts the energy into heat which is conducted, at a certain delay, to the bimetallic spring 3, The spring is thus heated, and when it reaches breaking temperature T (20 C) the currect connection to the transistor 9 is cut off. The energy loss of the transistor then ceases, which brings with it that the tem¬ perature of the thermal mass 11 decreases. When the temperature becomes lower than T the bimetallic spring reassumes it ^' s contact with the plate 4 and the transistor 9 becomes conducting again, after which the temperature of the thermal mass 11 increases. The process is repeated as long as the ambient temperature of the circuit-breaker is lower than T (Fig. 2a). By the thermal delay in the mass 11 a procedure of switching off and on will thus be enacted in dependence of the initial temperature of the thermal mass 11 and the ambient temperature, until the steady state temperature of the thermal mass 11 exceeds the breaking temperature T of the circuit-breaker 3.

In Fig. 2b the corresponding process is illustrated when the ambient temperature is 5 C, the time of the first switching on then being shorter. In addition, the time between switching off and switching on again will become longer, and the time bet¬ ween switching on and switching off will be shorter than in the

process according to Fig. 2a.

It is obvious that the device can be used for other purposes than those stated above. Thus the heating element can be a mirror heater or some other heating means. By selecting thermal mass and switching on temperature the invention can be used as a thermostate in various heating/cooling devices or, generally, for switching on and switching off an electrical circuit in dependence of temperature and time, that is, as a thermal timer. The device can also be connected to alternating current, and the transistor can be replaced by some other current con¬ ducting element which produces loss energy which can be utilized in a thermal mass.

It is also obvious that other thermal breakers than bime- tallic breakers can be used, e.g. temperature operable semi-con¬ ductors, so called thermistors or other temperature dependent breakers.