The invention relates to a temperature measuring circuit comprising a plurality of temperature sensors, each of which is formed by a temperature-dependent measuring resistance having two current terminals and two voltage tapping terminals each arranged between an associated current terminal and the temperature-dependent measuring resistance, and an evaluating circuit which via the current terminals sends a constant current through each temperature sensor and measures the voltage drop caused by the constant current between two voltage conductors connected to the voltage tapping terminals.
In temperature measuring circuits of this type the temperature measurement is based on the measurement of the resistance value of the temperature-dependent measuring resistance, the resistance measurement taking place by current-voltage measurement in a four-wire circuit. The use of the four-wire technique gives a higher measuring accuracy because voltage drops caused by the current flowing through the measuring resistance at lead and contact resistances do not enter the measurement result. A disadvantage is that each temperature sensor is connected by four conductors to the evaluating circuit and consequently with a relatively large number of temperature sensors, as are often required for detecting different temperatures at different points in technical plants or processes, the number of connection conductors required rises in proportion with the number of temperature sensors and often becomes undesirably large.
The problem underlying the invention is to provide a temperature measurement circuit of the type set forth at the beginning which whilst completely retaining the advantages of the four-wire technique requires a substantially smaller number of connecting conductors between the temperature sensors and the evaluating circuit.
According to the invention this problem is solved in that the temperature sensors are connected via the current terminals in series, that the two voltage tapping terminals of two temperature sensors which follow each other in the series circuit and are associated with two current terminals connected together are connected to a common voltage conductor, and that a resistance is inserted into the connection between at least one of the two voltage tapping terminals and the common voltage conductor.
In the temperature measuring circuit according to the invention due to the series connection of the temperature sensors only two current conductors are required for the connection to the current source, irrespective of the number of said sensors, and the number of the voltage conductors required is reduced in that in each case two voltage tapping terminals belonging to two consecutive temperature sensors in the series circuit are connected to a common voltage conductor instead of to two separate voltage conductors. The total number of connecting conductors required is therefore only three greater than the number of the temperature sensors compared with four times the number of temperature sensors in conventional temperature measuring circuits. Accordingly, the saving of connecting conductors is the greater the greater the number of temperature sensors present. Due to the fact that each common voltage conductor is connected to the two associated voltage tapping terminals not directly but via resistances, it is ensured that the voltage measurement takes place practically with the same accuracy as with the conventional four-wire technique with separate voltage conductors.
Advantageous further developments and embodiments of the invention are characterized in the subsidiary claims.