DIFFERENTIAL THERMOSTAT The present invention relates to the applications of integrated circuits and particularly, to a novel application of the I.C. comparators of the type CA 3094 produced by RCA» or their equivalents* It is the object of the present invention to provide a differential thermostat) sensitive and precise* whose performances are unaffected by changes in ambient temperature and which* last* but not least* have an improved linearity»i Conventional bridge circuit configurations for differential measurements do not offer a good linearity.
It is a further object of the Invention to provide electrical means in order to allow for an I.e.' of the above mentioned type to actuate a conventional electromagnetic relay*' According to the present invention* there is provided a differential thermostat comprising: two series connected thermistors which represent the electrical means responsive to temperature* the said thermistors being current supplied from a stabilised power source and balanced by using compensating resistors i series with each thermistor* the mid-point of the described assembly being connected via a protecting resistor to one of the input terminals of the I.C. (integrated circult)the two said thermistors and their corresponding reelstors constituting two legs of a bridge measurement circuit* whose two other legs are represented by two resistors whose midpoint is connected to the second input terminal of the ∑*C; furthermore* in order to obtain an improvement of linearity» parallely to each group thermistor - resistor Is connected a compensating resistor*! the I.C.» proper in a comparator configuration with a relay coil across its output; on said relay coil is mounted*, in parallel, a capacitor for protecting against surges·> In order to achieve the optimal values for the resistors and to obtain the high characteristics mentioned above» the design has ^ been performed by using the linear programming on computer* a linear programme which minimises the standard deviation from a predetermined temperature Further features and details will become ^apparent from the description hereinafter of a preferred embodiment of the invention which is accompanied* for sake of clarit only* by a single figure of drawing which represents a wiring diagram of the said differential thermostat.
In the drawing* the thermistor t^ and its corresponding balancing resistor are connected n series to the second thermistor t^ and its resistor E^ .i The mid-point of this group is connected* via protecting resistor RQ» to the nput terminal 2 of the CA 3094· The extremities are connected - one to the positive pole of the power supply* and the second one is grounded »i o the same input terminal is also connected the mid-point of a second group* parallel to the first one and composed by resistors Rj and R^, acting together with ^ and ^r* Th other input terminal 3 of the I.C.i type CA 3094 is: connected, via a protecting resistor R^* to the other mid-point of the bridge measurement circuit* said mid-point being the common point of the resistors R,. and R^*' The output circuit of the CA 3094 comprises the relay coll as a load connected by the other terminal to the positive pole of the stabilised power supply* In accordance with the description hereinbefore* the operation of the circuit may easily be explained as follows: ' When one of the thermistors is subjected to a temperature variation (the other one remaining in its initial state) and the difference of temperature attains a predetermined value* the variation in thermistor resistance causes a modification in the voltage level at the input of the I.C*» activating the comparator* whose output will operate the relay* Thus* the device accomplishes a function as a differential thermostat, and its linearity Is much improved* I The capacitor mounted in parallel on the relay has a protecting function *i By way of example» using a thermistor with a rated resistance of 50 at 250%* and characterised in that its temperature coefficient in the range 0-50°C is 11*6» when the resistances were determined using the linear programming, the linearity of the circuit was eatremely improved** The values of the resistances thus calculated are: Rj^ , ■ 220 r ¾ β 100 r » 120k R. β 115k 4 R5 « 8.6 k R^ ■ 10 k 100 k *7 Rg ■ ■ 100 k Rg » 1 m R1rt °» 120 k When changing the 50k thermistor by another one, the ratio between the new and old thermistor 'resistances will be reflected also9 in the same extent» by the new calculated compensating resistors··