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# Introduction heating system thermostat circuit

2017-08-06 09:39
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This article is the summary of heating system thermostat circuit diagram. The purpose of this circuit is to control the heating system or central heating, keep indoor temperature despite extensive changes of external environment. Two sensors need: a place in the outdoor, in order to perceive external temperature; Another on returning pipe heating system circuit, its input before short boiler. The output of the relay contact start-stop control input must be connected to the boiler. Then the circuit when the Q1 base ground voltage is less than half voltage supply (set R7 & R9), a voltage generating and drivers in the R8 transistor Q2 and Q3 through to relay. When more than half of the Q1 base ground voltage supply voltage, caused by n t c. Thermistor lowered its value due to the increase of temperature, no voltage appears in the R8 and relay is closed. C3 allows clean switch relay. P1 ACTS as the main temperature control. As shown in figure: R3 is the outdoor sensor, R4 sensors. If you cannot find a 3 k3 thermistor for R3 can use 4 k7 value instead. Different values can be easily through the fine-tuning R5 compensation. R5 allowed to set the heating system to outdoor temperature range is 10 ° C down. R5 resistance of the higher, the heat of the heating system, and vice versa. The existing boiler thermostat should be set to its maximum value, rather than bypass: it is necessary to be on the safe side. This circuit can abandon its differential characteristics and converted into a simple precision thermostat omit R3. The  circuit is very simple, hope to help you.

Parts:

```P1______________1K   Linear Potentiometer

R1_____________10R   1/4W Resistor
R2______________1K   1/4W Resistor
R3______________3K3  @ 20°C n.t.c. Thermistor (see Notes)
R4______________2K2  @ 20°C n.t.c. Thermistor (see Notes)
R5_____________10K   1/2W Trimmer Cermet
R6______________3K3  1/4W Resistor
R7,R9___________4K7  1/4W Resistors
R8____________470K   1/4W Resistor
R10____________10K   1/4W Resistor

C1,C2_________470μF   25V Electrolytic Capacitors
C3______________1μF   63V Electrolytic Capacitor

D1,D2,D4_____1N4002  100V 1A Diodes
D3______________LED  Red 3 or 5mm.

Q1____________BC55745V 100mA PNP Transistor
Q2____________BC547   45V 100mA NPN Transistor
Q3____________BC337   45V 800mA NPN Transistor

RL1____________Relay with SPDT 2A @ 220V switch
Coil Voltage 12V. Coil resistance 200-300 Ohm

J1_____________Two ways output socket

SW1____________SPST Mains Switch

T1_____________220V Primary, 12   12V Secondary 3VA Mains transformer

PL1____________Male Mains plug & cable```