Friday, September 26, 2014
Temperature Sensor Circuit for Remote Distance Applications
The circuit shows a temperature sensing device which can be used to indicate at a remote point when the temperature passes through a certain value or to give an alarm when this occurs.
The sensing unit itself contains a 2N930 transistor. The base-emitter voltage of this device appears across R1 and (as the base current is far less than the collector current) the voltage at the upper end of R2 will be the emitter-base voltage multiplied by (R2 + R1)/Rl.
The base emitter volt· age changes with T a temperature coefficient of —2.2mV/°C and this change is multiplied by the same factor before being applied to the LM339 circuit. The potential at point A is set by the resistors R3 and R4. As _the temperature of the sensor transistor rises, the voltage at point B falls. At the time this voltage falls below that at point A, the output of the LM339 voltage comparator will go high. lf, however, the input connections to the LM339Aare reversed, the output will go low when the temperature of the sensor falls below the preset point.
The LM339 contains four separate voltage comparators in one package; only one of these comparators is used in the circuit shown. The other three comparators could be used with another three temperature sensing transistors so that an indication is given when the temperature passes through three other preset values. The`value of R5 should be chosen so that the current passing through the remote sensor unit is about 10pA. lf the temperature range over which operation is required is narrow, the ratio R2/R1 may be large so that the system is- very sensitive to small temperature variations. A potentiometer may be substituted for R3 and R4 so that the temperature at which the comparator switches is variable.
The voltage at point B is,highly linear over a very wide temperature range (about -65°C to +150°C) and there- fore the potentiometer which replaces R3 and R4 can be given a linear calibration. A feedback resistor may be connected from the output to the non- inverting input to provide a small amount of hysteresis (so that the temperature at which the output changes when the temperature is rising is different from that when it is falling); one then has the basis of a thermostat. ‘ The output current has a maximum value of about 15mA.
The sensing unit itself contains a 2N930 transistor. The base-emitter voltage of this device appears across R1 and (as the base current is far less than the collector current) the voltage at the upper end of R2 will be the emitter-base voltage multiplied by (R2 + R1)/Rl.
The base emitter volt· age changes with T a temperature coefficient of —2.2mV/°C and this change is multiplied by the same factor before being applied to the LM339 circuit. The potential at point A is set by the resistors R3 and R4. As _the temperature of the sensor transistor rises, the voltage at point B falls. At the time this voltage falls below that at point A, the output of the LM339 voltage comparator will go high. lf, however, the input connections to the LM339Aare reversed, the output will go low when the temperature of the sensor falls below the preset point.
The LM339 contains four separate voltage comparators in one package; only one of these comparators is used in the circuit shown. The other three comparators could be used with another three temperature sensing transistors so that an indication is given when the temperature passes through three other preset values. The`value of R5 should be chosen so that the current passing through the remote sensor unit is about 10pA. lf the temperature range over which operation is required is narrow, the ratio R2/R1 may be large so that the system is- very sensitive to small temperature variations. A potentiometer may be substituted for R3 and R4 so that the temperature at which the comparator switches is variable.
The voltage at point B is,highly linear over a very wide temperature range (about -65°C to +150°C) and there- fore the potentiometer which replaces R3 and R4 can be given a linear calibration. A feedback resistor may be connected from the output to the non- inverting input to provide a small amount of hysteresis (so that the temperature at which the output changes when the temperature is rising is different from that when it is falling); one then has the basis of a thermostat. ‘ The output current has a maximum value of about 15mA.
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