How to make temperature controlled Fan


Circuit Diagram:
Main Parts:


Parts
Symbol
Value
Linear Potentiometer
P1
22K
Thermistor
R1
15K   @ 20°C n.t.c.
Resistor
R2
100K   1/4W

Resistors
R3,R6
10K   1/4W
Resistors
R4,R5
22K   1/4W
Resistor
R7
100R   1/4W
Resistor

R8
470R   1/4W
Resistor

R9
33K     4W
Polyester Capacitor
C1
10nF   63V

Zener Diode
D1
BZX79C18    18V 500mW
SCR
D2
TIC106D   400V 5A
Diodes
D3-D6
1N4007  1000V 1A
PNP Transistors
Q1,Q2
BC327    45V 800mA
NPN Transistor
Q3
BC337 45 V 800mA

Female Mains socket
SK1

Male Mains plug & cable
PL1




Circuit operation:
R3-R4 and P1-R1 are wired as a Wheatstone bridge in which R3-R4 generate a fixed two-thirds-supply "reference" voltage, P1-R1 generate a temperature-sensitive "variable" voltage, and Q1 is used as a bridge balance detector.
P1 is adjusted so that the "reference" and "variable" voltages are equal at a temperature just below the required trigger value, and under this condition Q1 Base and Emitter are at equal voltages and Q1 is cut off. When the R1 temperature goes above this "balance" value the P1-R1 voltage falls below the "reference" value, so Q1 becomes forward biased, pulse-charging C1.
This occurs because the whole circuit is supplied by a 100Hz half-wave voltage obtained from mains supply by means of D3-D6 diode bridge without a smoothing capacitor and fixed to 18V by R9 and Zener diode D1. Therefore the 18V supply of the circuit is not true DC but has a rather trapezoidal shape. C1 provides a variable phase-delay pulse-train related to temperature and synchronous with the mains supply "zero voltage" point of each half cycle, thus producing minimal switching RFI from the SCR. Q2 and Q3 form a trigger device, generating a short pulse suitable to drive the SCR.

Notes:

  • The circuit is designed for 230Vac operation. If your ac mains is rated at about 115V, you can change R9 value to 15K 2W. No other changes are required.
  • Circuit operation can be reversed, i.e. the fan increases its speed as temperature decreases, by simply transposing R1 and P1 positions. This mode of operation is useful in controlling a hot air flux, e.g. using heaters.
  • Thermistor value is not critical: I tried also 10K and 22K with good results.
  • In this circuit, if R1 and Q1 are not mounted in the same environment, the precise trigger points are subject to slight variation with changes in Q1 temperature, due to the temperature dependence of its Base-Emitter junction characteristics. This circuit is thus not suitable for use in precision applications, unless Q1 and R1 operate at equal temperatures.
  • The temperature / speed-increase ratio can be varied changing C1 value. The lower the C1 value the steeper the temperature / speed-increase ratio curve and vice-versa.

Warning! The circuit is connected to 230Vac mains, then some parts in the circuit board are subjected to lethal potential! Avoid touching the circuit when plugged and enclose it in a plastic box.
0 Comments
Disqus
Fb Comments
Comments :

0 comments:

Post a Comment