FREE VB COURSE - Index of

HANDS-ON DIY REFLOW OVEN
the middle of the oven. Ensure that the
thermocouple remains electrically isolated
from the rest of the oven, in order
to avoid creating a hazardous situation.
The two leads of the thermocouple are
connected to IC1, a MAX6675. This IC
computes the temperature at the tip of
the thermocouple based on the voltage
generated by the thermocouple and
the ambient temperature. The microcontroller
can query the temperature
via a serial interface.
IC4 and IC5 are connected to the
microcontroller via resistors. These two
ICs are optotriacs with integrated zerocrossing
detection and snubber networks.
That makes them very easy to
drive from the controller. Power is
applied to the two heating elements as
necessary via these two ICs.
The controller contains several settings
that must be stored in an EEP-
ROM. IC6 is included for that purpose.
32
S1 S2
D3
D2
S5
S3
R5
COMPONENTS LIST
Resistors:
R1,R2 = 150Ω
R3 = 10Ω
R4 = SIL array 8 x 10kΩ
R5 = SIL array 4 x 1kΩ
R6 = 10kΩ
P1 = 10kΩ
Capacitors:
C1,C4,C5,C12,C13,C15 = 100nF
C2,C3 = 27pF
C6-C9,C14 = 10µF 16V radial
C10,C11 = 470µF 16V radial
D1
K6
S6
S4
R3
LCD1
70%
Semiconductors:
B1 = B80C1500 bridge rectifier, 80V
piv, 1.5A
D1,D2,D3 = LED, red, low-current
IC1 = MAX6675
IC2 = AT89C52/24JI, programmed,
order code 050319-41
IC3 = MAX232
IC4,IC5 = S202S12
IC6 = 24LC64
IC7 = 7805
Miscellaneous:
K1 = connector for thermocouple
Thermocouple, K-type
K2 = 9-way sub-D socket (female),
Alert readers may quickly come to the
conclusion that 64 KB is rather generous
for the number of settings to be
stored. That’s certainly true, but the
circuit does in fact need that much
memory because it is also used to
store the measured temperature once
per second during the soldering
process. That allows the operation of
the oven and the corresponding temperature
profile during soldering to be
examined afterwards.
This information must be sent to a PC
in some way or another. Here we use
an old faithful: the RS232 serial port.
As usual, the port is implemented
using a MAX232 IC and associated
components.
Pushbutton switches S1–S6 provide
the operating controls for the oven
controller. LEDs D1–D3 and LCD1
keep the operator informed while the
oven is in use.
P1
R4
C2
C8
X1
C3
C7
IC3
K2
T
K1
C14
R6
IC1
C5
C6
C1
IC2
C9
+
IC6
C10
C4
C15
C12
B1
C11
IC7
C13
R2
R1
TR1
(C) ELEKTOR
050319-1
IC5
IC4
050319-1
HEAT 2
HEAT 1
MAINS
PCB mount
K3,K4,K5 = 2-way PCB terminal block,
lead pitch 7.5mm
K6 = connection for LCD backlight LCD1
= LCD module, 2x16 characters, e.g.
order code 050319-72 or PLED
version 050319-73
S1-S6 = pushbutton, ITT type D6-R
Tr1 = mains transformer, primary 230V,
secondary. 6V (e.g. Monacor/Monarch
VTR-3106)
X1 = 12 MHz quartz crystal
PCB, ref. 030519-1 from The PCBShop
Disk, source and hex code files, order
code 030519-11
17 wire links
Not much needs be said about the
power supply. It is very basic and perfectly
ordinary. Note that no fuse is
included on the circuit board. An
external fuse must be used for the
input voltage. Besides a normal fuse,
which is usually located at the rear of
the equipment, a thermal cutout is
also necessary. It must be fitted such
that it switches everything off if the
temperature of the oven becomes too
high. That prevents the oven from
overheating if something goes wrong,
which in turn prevents everything
from catching fire.
The (fused) 230-V supply voltage is
connected to K3. The two heating elements
are connected to K4 and K5.
Installation
Installation of the controller in the oven
will be different for each type of oven.
K3
K4
K5
elektor electronics - 1/2006