THE THERMOSTAT-THE CELLAR

Five years ago one of my friends purchased a garage with a cellar and worried about how and what to do to potatoes and other vegetables, planted for winter storage in the cellar, didn’t freeze? Remember, rescued his once-popular thermostat through a balanced bridge with the two thermistors. And recently another friend of mine, being in the same life situation, asked me to help him in the manufacture of such devices.

Blind copying of someone else’s, although quite efficient, the device is not for me. And a number of considerations led to modernization of the basic thermostat.

First of all, I was not satisfied with that power supply the original variant was carried out according to the so-called transformerless scheme, where the nodes and elements under phase hazardous live voltage. After all, it is not excluded in the cellar water seepage. And the owner of the store vegetables, for example, in the roads could easily get his feet wet. What if he for a moment will affect the running of the thermostat? This helped to frame the main requirement to the thermostat: reliable isolation of the structure from the mains voltage, for example, by using separation or step-down transformer and the Executive relay.

Did not suit me, and the low-power device prototype with the radiant load in the form of 100-watt incandescent bulbs. Of course, in the upgraded design must operate heater with a capacity of not less than 1.5 kW in combination with a fan. If necessary, you can use it for quick drying cellars-greenhouses.

But then the legacy series thyristors and diodes КУ202 D245, where are the scheme-the prototype should work on my limit and overheat. So you want to install them on the radiators, to organize the forced cooling, electroisolating from each other and from the housing of the device or to use a more powerful and generally more expensive and scarce counterparts…

A circuit diagram of a controller of the prototype (top) and upgraded version (bottom)

And then I came to hand the old magnetic starter brand PME-074. This helped to solve all the problems. Besides managed when modifying an electric circuit of the thermostat to limit the use of a single temperature sensor instead of the previous two.

Those who are interested in my revision design, proven in the case, it is useful to know other details. In particular, resistors R1— RZ assembled divider 9-volt, are not associated with electrical grid, stabilized supply voltage (via Zener diode VD1 type Д814Б). In the lower shoulder it included 10-kiliany thermistor, KMT-12, easy model for the MMT-1, MMT-9, MMT-12 and similar analogues. In the upper arm of the divider — two resistors: variable R1 (resistance 1.5—2.2 kOhm, type — SPO-0.5 or SDR-4A with a linear characteristic, the adjustment knob located on the front panel with calibration adjustments) and trimpot R2 (15-47 com SPZ-16 “coarse setting”).

A pronounced dependence of the resistance of the thermistor of the temperature allows to use it as a sensor that changes the voltage at the United inputs 1 and 2 of the logic element DD1.1 microcircuits K561LA7. Adjustment knobs resistors R1 and R2 set to a threshold (temperature) operation of electronic logic. Capacitor C1 eliminates “bounce” (self-excitation) chip DD1 at the time of switching. Due to the resistors R5 and R6 output “chain” of logic elements electrically connected with transistor key UT1 (КТ972), the load which is relay K1. It, in turn, triggers the magnetic contactor K2 type PME-074, including load — domestic heater with a built-in fan with a total power of 1.5 kW and more.

However, to connect the thermostat to the household network, you need a step-down transformer. Experience suggests that any small acceptable “silovik” (for example, from a portable tape recorder, calculator). You can use a cheap network adapter with a capacity of 9-10 watts. Most importantly, apply to the diode bridge of the thermostat the required 12 V. the lower voltage can cause instability in the operation of the relay K1, and the more in danger of overheating and even burn out its windings.

The electronic part of the device, with the exception of the sensor, mounted on the PCB of one-sided foil fiberglass dimensions 70x70x2 mm and with a magnetic starter K2 accommodated in a plastic housing of a suitable size. A thermistor sensor is made of the extension for greater sensitivity attached to a small aluminum radiator.

Printed circuit Board thermostat

Temperature controller, assembled with no errors and of known good parts, starts working immediately for inclusion in the grid. Setting is the selection of the resistor 144 that ensures the proper operation of the Zener diode (checked the directory). For example, when using Д814Б as VD1, the nominal value of the resistor is determined by roughly the rate of 100 Ohms for every 1 In the difference between restabili-ized and stable supply voltages. That is, the resistance 144 to the specific conditions set schematic electrical diagram, shall be (12-9) x 100 Ohms = 300 Ohms.

It is recommended that you just mounted, connected to a source of electricity and not placed in the body of the device to “drive” within an hour or two. If it turns out that the voltage stabilization “walks” or the Zener diode is strongly heated, it is necessary to select the value of R4.

Further, by using the resistors R1 and R2 to set the temperature that must be maintained in the cellar-the vegetable store. To do this, setting the sliders to the middle position and placing the thermistor to the environment to a desired temperature, with slow rotation of the pen “correction” to find the angle of rotation of the rotor R2 at which the relay K1. Then, the cooling or heating medium, where is the sensor fix the temperature of operation of the thermostat at resistor Good handle of this “shells” on the front panel of the device to equip a pointer, and next stick the scale of paper.

Vitaly SAVELYEV, Raduzhny, Vladimir region