# The Purpose of my sensing circuit is to regulate the temperature in a Steam Sauna which operates between the temperatures 30C - 60C

by paulbrume (student)

Introduction

A Sauna is a small room designed to be heated to very high temperatures, with well-controlled . Sauna’s are used for both recreational and therapeutic purposes. There are different types of Sauna, all with different heat sources and temperature ranges.

The Purpose of my sensing circuit is to regulate the temperature in a Steam Sauna which operates between the temperatures 30°C - 60°C

Sensor

My sensor is a thermistor (a temperature sensitive resistor).

There are two types of thermistors.

1. PTC (Positive Temperature Co-efficient) thermistors for which an increase in temperature increases its resistance
2.  NTC (Negative Temperature Co-efficient) thermistors for which an increase in temperature decreases its resistance

Through my coursework I will be making use of a Negative temperature coefficient thermistor because I will be attempting to put a limit on the maximum temperature in the Sauna at which the steam generator can operate (also limiting the maximum temperature of the Sauna) by varying the potential difference across the steam generator as temperature changes.

Principle of operation of a Negative Coefficient Thermistor

Negative Temperature Coefficient thermistors are usually made from semiconductors (beads of metal oxides). They decrease in resistance as temperature rises because, a temperature rise ionises atoms in their structure, increasing the number of mobile charge carriers (electrons) and this outweighs the effect of the increased vibration of atoms. A rise in temperature thus increases the overall conductance of a Negative Temperature Coefficient thermistor.

A simple model is shown below

At 30°C the NTC thermistor has few mobile charge carriers

At 60°C some atoms ionise and increase mobile charge carrier density of the NTC thermistor

KEY

Potential Divider

A potential divider can be made by connecting at least two resistors in series across a source electromotive force in a circuit.

In a potential divider circuit the ratio of resistances determines the potential difference across a resistor. The larger the resistance, the greater the potential difference across the resistor and the smaller the resistance the smaller the potential difference across it.

In the simple potential divider circuit above

• The current, I= Vin ÷ (R1 + R2)

• The Potential Difference across R1

VR1= I × R1

= Vin × R1

R1 + R2

=  R1____  × Vin

R1 + R2

• The Potential Difference across R2

VR2 = I × R2

= Vin × R2

R1 + R2

=   R2____  × Vin

R1 + R2

The calculations above prove that the electromotive force (Vin), in a potential divider circuit is divided across each resistor according to the ratio of resistances.

The resistors used in a potential divider circuit do not necessarily have to be fixed in value, thermistors, rheostats, light dependent resistors and light emitting diodes may be used.

My Sensing Circuit

My ...