A temperature regulator is used for the cellar in order to ensure the most comfortable conditions for storing vegetables and fruits. There are many types of structures - some designed for use in the basement, others - for installation on balconies. The design of the balcony drawers is very simple - a box with high-quality thermal insulation, heating elements are installed inside, with which the optimum temperature is maintained. Such a mini-cellar will be very useful if the balcony is unheated, and in the winter there will be severe frosts.
The general principle of the thermostat
The simplest home-made thermostat for a cellar allows you to maintain a preset temperature value using heating elements. Depending on what the temperature difference is, you can use these elements for heating:
- Powerful wire resistors.
- Nichrome spiral.
- Incandescent lamps.
- TENY.
If you need to equalize the temperature in a small range, it is enough to install several incandescent lamps - this is exactly what is done in makeshift incubators for growing birds. But if you need to equalize the temperature in a wide range, you will need to use more effective means - heating elements or wire resistors.
The easiest way to maintain the set temperature is to install a bimetallic temperature sensor. It will turn off the heating elements when the set temperature is reached. But you can use a temperature sensor and a simple electronic device on a single chip. The advantage of this design is that it is more reliable - it does not have mechanical breakers. All switching work is performed by the microcircuit.
Sensor Calibration
For beginner hams, the most difficult thing will be to configure the device, namely the calibration of the sensor and microcircuit. For this, several actions are performed - the reader is immersed first in water, the temperature of which is 0 degrees, then in boiling water. To calibrate the regulator, you need to measure the intermediate values and put the appropriate labels.
It is possible that before connecting the thermostat, you will need to carry out the setup procedure several times. But in order not to bother with self-manufacturing and setting up the device, you can buy a finished device and put it in the cellar without any problems. Most of the sensors are designed to work with microcontrollers, at the output they have a digital signal that is transmitted via a 1-WIRE bidirectional single-wire interface. This allows you to design fairly complex devices, for example, multi-point thermometers - these are devices that allow temperature measurement in several rooms at once.
LM335 temperature controller
Among all the thermostats, the cheapest and easiest - LM335. He has several modifications - with the designations 235, 135. In the marking, the very first digit indicates the scope of application:
- The number "1" means that the device is designed to work in devices for the military-industrial complex.
- The number "2" - the element is used in industry.
- "3" - for installation in household appliances.
Appearance of a temperature regulator is the TO-92 case. The internal circuit contains 16 semiconductor transistors. Sometimes sensors can be found in the SO-8 package, however, differences are observed only in appearance - the internal circuit remains unchanged.
The principle of operation is somewhat similar to the zener diode. The stabilization voltage directly depends on temperature. With a temperature increase of 10 Kelvin, the stabilization voltage increases by 10 mV. At the same time, the operating current of the device is 0.45-5.0 mA. If the maximum current value is exceeded, the sensor will overheat and it will measure the temperature of its case.
What does the sensor show?
But how will this device behave in the temperature regulator circuit for a cellar? You need to figure this out. Suppose an absolute zero in a room is 273 degrees below zero Celsius. This is 0 K, therefore, you will have to do a small conversion of the value. It is in conditions when the temperature is 0 K, the sensor will not generate a signal.
As soon as the temperature rises by 10 K, the voltage rises by 0.01 volts. And this will happen with every increase in temperature. But you need to take into account that there are no such temperatures, and 0 degrees Celsius is 273 K. Normal conditions, according to all textbooks, are 25 degrees Celsius, or 298 K. After a few simple steps, you can determine that at a temperature of 25 degrees centigrade on the signal output of the sensor will be a voltage of 2.9815 volts.
The temperature range in which the device operates lies in the range of -40 .. + 100 degrees Celsius. Moreover, its main characteristic is linear in this range - this facilitates the calculation of stresses and temperatures. And do not forget that the absolute zero is 273.15 K. In exact calculations, even hundredths of values should not be neglected.
Temperature controller circuit
The device has a kind of instruction. The temperature controller can be made according to the schemes given in the datasheet. This is a detailed guide that describes all the possible schemes for switching on the device, its main characteristics, and features of work. And nothing extra needs to be invented - all designs have been tested for years and work stably.
All industrial samples of finished devices of temperature regulators are made exactly according to the schemes indicated in the datasheet. One thing to consider - you cannot directly control the heating element, since the signal is very weak. You will need to use an amplifier on a field effect transistor or assembly. The amplified signal can be applied to a magnetic starter or relay.
Comparator Operation
A comparator is a device with which a temperature is compared with a set value. Without knowing the basics of how it works, you won’t be able to make a thermostat for the cellar on the balcony. The circuit is based on the comparator from LM311 - it has two inputs and the same number of outputs. Inputs
- Direct - indicated by the “+” sign.
- Inverse - has the designation "-".
The operation algorithm is very simple:
- If the direct input receives more voltage than the inverse, then the output is set to a high level. The transistor opens and the heating is turned on.
- If the voltage at the inverse input is higher, then a low level is set.
- When the response temperature is reached (it is set by a variable resistor), a transition to a low level occurs - the transistor closes, and the heater is de-energized.
How to connect the device to the heater
A heating element is the load of the entire device. It is desirable that the switching elements have a margin of safety - install a relay with a degree of protection corresponding to wet rooms, or magnetic starters. The signal from the microcontroller must be supplied to the field effect transistor and amplified. Only after that it can be used to control the coils of the relay or starter, which are included in the open circuit of the power supply. Since the thermostat for the cellar will be in a humid environment, take care of safety - put the circuit breakers and RCD.