It is also transformed to a 7.5 V supply voltage by D2 and C2. The
sensor needs an alternating drive voltage at a level not higher than 1.5
V. This potential is obtained from the pulsating direct voltage by
network R2-R3-C3-C4, which removes the direct voltage component and
lowers the level to 1.4 V. At the same time, the network functions as a
50 Hz bandpass filter. To ensure that the drive voltage for the sensor
does not fall outside the common-mode range of op amp IC2, an offset
potential of 3.9 V is applied to the sensor as well as to the voltage
reference source of the op amp.
The charged capacitor thereupon provides a steady current to the triac.
When T1 is cut off for some time, capacitor C6 is discharged via
resistor R7. Capacitors C1 and C7 are discharged via R9, so that after
the mains has been switched off, no dangerous potential remains at the
pins of the mains connector (K1). The humidifier is best built on the
PCB shown in Figure 2, which is available ready made (see Readers
services pages towards the end of this issue). Bear in mind that parts
of the board will carry mains voltage, which makes careful working and
the enclosing of the board in a plastic case imperative. The humidifier
may be converted into a dehumidifier by interchanging connections 1 and 3
to sensor IC1.
Parts list
Resistors:
R1 = 470 Ω, 1 W
R2, R3 = 10 kΩ
R4 = 1 kΩ
R5 = 56 kΩ
R6 = 6.8 kΩ
R7 = 4.7 kΩ
R8 = 470 Ω
R9 = 2.2 MΩ
R10 = 39 Ω, 1 W
P1 = 1 kΩ preset
Capacitors:
C1 = 0.47 µF, 250 V a.c.
C2 = 470 µF, 16 V, radial
C3, C4 = 0.33 µF, metallized polyester, 5%
C5 = 0.1 µF, high stability
C6 = 47 µF, 16 V, radial
C7 = 0.047 µF, 250 V a.c.
Semiconductors:
D1 = zener diode, 8.2 V, 1.3 W
D2 = 1N4001
D3 = zener diode, 3.9 V, 500 mW
D4 = zener diode, 2.4 V, 500 mW
T1 = BC557B
Integrated circuits:
IC1 = NH-3 (Figaro)
IC2 = TLC271CP Tri
1 = TLC336T (SGS)
Miscellaneous:
K1, K2 = 2-way terminal block for board mounting, pitch 7.5 mm
F1 = fuse-holder with 630 mA slow fuse.
Resistors:
R1 = 470 Ω, 1 W
R2, R3 = 10 kΩ
R4 = 1 kΩ
R5 = 56 kΩ
R6 = 6.8 kΩ
R7 = 4.7 kΩ
R8 = 470 Ω
R9 = 2.2 MΩ
R10 = 39 Ω, 1 W
P1 = 1 kΩ preset
Capacitors:
C1 = 0.47 µF, 250 V a.c.
C2 = 470 µF, 16 V, radial
C3, C4 = 0.33 µF, metallized polyester, 5%
C5 = 0.1 µF, high stability
C6 = 47 µF, 16 V, radial
C7 = 0.047 µF, 250 V a.c.
Semiconductors:
D1 = zener diode, 8.2 V, 1.3 W
D2 = 1N4001
D3 = zener diode, 3.9 V, 500 mW
D4 = zener diode, 2.4 V, 500 mW
T1 = BC557B
Integrated circuits:
IC1 = NH-3 (Figaro)
IC2 = TLC271CP Tri
1 = TLC336T (SGS)
Miscellaneous:
K1, K2 = 2-way terminal block for board mounting, pitch 7.5 mm
F1 = fuse-holder with 630 mA slow fuse.
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