DIY Solder
ReFLow Oven
Part 2
by
Phil Prosser
with PID Control
When we left off last month, we’d finished assembling the PCBs and the
hardware. Now it’s time to put it all together – and get cooking!
J
ust in case you missed this project’s first
instalment last month, let’s briefly recap. We’re taking
a bog-standard ‘toaster oven’ and making a controller
for it, which allows it to be turned into a reflow oven for soldering PCBs with lots of (or even a few) SMD components.
We do this without any modifications to the toaster oven
at all – in fact, there is only minimal mains wiring to be
done within the controller. What’s more, we’ve made it
very safe to use.
If you want more details than that, we suggest you look
up the first part in the April 2021 issue. Now, let’s get on
with the show!
Putting it together
Everything mounts inside a commercial plastic case,
with the components mounted on a baseplate made from
1.5mm-thick aluminium. Cut it to 200 × 115mm and drill
all the required holes, as shown in Fig.8. Deburr all the
holes and clean it up.
32
We haven’t shown a cutting/drilling diagram for this
simply because of its size but we have prepared one; it can
be downloaded from the May 2021 page of the PE website
and printed out at 1:1 size.
Covering the panel with masking tape before cutting out
the display window has two benefits: (a) you can much
more easily mark the position on the tape (along with other
hole locations) and (b) it tends to make the waste stay in
place, resulting in less mess!
Practical Electronics | May | 2021
We previewed the completed controller last
month. Here it is again showing where
everything goes. Again, this shot was taken before the
Presspahn shield was fitted to cover exposed mains.
Similarly, drilling diagrams for the front and rear
panels, along with a cutting and drilling diagram for the
Presspahn safety shield can also be downloaded in a single
PDF document. Now would be a good time to get those
diagrams and cut/drill the components.
We applied masking tape to the front and rear panels
and marked cuts and holes on this. For the LCD and the
IEC connector, we used a Dremel with a cut-off wheel to
cut just inside the marked cut lines, then used a file to tidy
the holes. This gave a neat result.
Use the aluminium plate as a template in the bottom of
the case to mark out and drill the holes which will be used
to attach the plate to the base.
Be careful to leave a minimum of 40mm of room to the
front panel for the LCD connector.
Now you can start to fit the components to the baseplate.
Apply a small dollop of heatsink paste under the solid-state
relay before mounting it.
Mount the PIC32MZ PCB using 15mm nylon standoffs.
These ensure that the board is well insulated, with sufficient
creepage distance from the base plate. Do not substitute
metal standoffs. You can then attach the metal plate to the
bottom of the instrument case and move on to the front
and rear panels.
For the rear panel, attach the dual IEC connector, binding posts and DC socket securely. We can solder wires to
these in situ later.
It is now time to mount the LCD screen and front panel
PCB. We used glue (actually, silicone sealant) to avoid messy
screws through the front panel, and to make it bombproof. You
just need to be a bit careful in application. Start with the LCD.
If your cut-out leaves a gap around the LCD screen,
you may be able to see the white of the backlight assem-
To avoid screw heads protruding from the front panel,
we decided to glue the LCD to the rear of the panel using
silicone sealant. The masking tape showed us where the
glue was to go.
Once you’ve drilled out the baseplate (download the PDF
from the May 2021 page of the PE website) it can be used
as a template for drilling the four required holes in the case
(these align with four of the pillars moulded into the case).
Practical Electronics | May | 2021
33
DUAL IEC MAINS
INPUT PLUG & OUTPUT SOCKET
EARTH PINS
ARE LINKED
9V DC
INPUT
THERMOCOUPLE
AMPLIFIER
MODULE
CJMCU
GND OUT GND VCC
1
10
8
6
4–
INPUT
–
+
CON10
9
10
SOLI STATE
SOL
SOLID
RELAY
(230V/40A)
1
2
9
10
10-WAY IDC
RIBBON
CON11
2
LO
O
LOAD
CON5
1
7
(DOTTED LINES
SHOW
PRESSPAHN
BOX FOLDED
OVER SSR
AND MAINS
WIRING,
HELD DOWN
BY THREE
MOUNTING
SCREWS)
3+
1
2
7
CON8
6
1
2
20-WAY IDC RIBBON CABLE
19
20
USER INTERFACE
128 x 64 LCD MODULE
Fig.11: this wiring diagram shows you which wires need to go
where to complete the controller. Besides making sure that the
ribbon cables have the red wires going to pin 1 of the plug and socket at both
ends, and that the IDC connectors are correctly crimped, the main thing to note is the way the 10-wire ribbon cable from
CON10 is split up and routed to two places. Only five wires in this cable are used; the other five should be cut short. When
finished, use cable ties to tie all the bundles of wires together, so nothing can move around, and don’t forget to add the
Presspahn barrier.
bly through the gap. Use a black marker to colour in the
white backlight around the edges of the LCD panel before
mounting it.
Put masking tape across the front panel cut-out and temporarily mount the LCD, making sure that the connectors
are on the bottom. The tape should hold the LCD pretty
well flush with the front panel.
Also, apply masking tape around the LCD edges to facilitate tidying up the silicone after you have applied it.
Refer to the accompanying photo.
Next, attach the front panel control board. Put one nut
(or several washers) over the rotary encoder shaft to set a
minimum depth, then mount it to the front panel using the
supplied nut. Check the pushbuttons operate properly and
do not get stuck on the front panel. If they do, carefully file
the holes a bit larger with a round file.
Once it is all good, tighten up the nut on the rotary encoder and check that everything is sitting neatly. Adjust
if necessary.
Then, using a matchstick or small timber offcut, build
up a dollop of silicone at either corner of the LCD. Do the
same with the control board, at the end far from the rotary
encoder. Watch out for the pushbutton; do not get silicone
onto this, or it will stop it working. You do not need to use
a lot of silicone – a ‘dollop’ at either corner is more than
enough. (We used far more than necessary!)
34
Once the silicone has set, attach the on/off toggle switch
in the usual manner, and push a knob onto the rotary encoder. You are now ready to start the wiring.
Wiring it up
Fig.11 shows the wiring that’s needed to finish the controller. As you do the wiring, keep in mind that twisting
pairs or bundles of wires together and/or covering them in
heatshrink tubing will keep the whole thing neat.
It’s important to note that this also contributes to safety,
as it stops wires that might break off from moving around
and contacting other parts of the circuit. See our photos for
an idea of what it should look like when you’ve finished.
Start by running light-duty red hookup wire from the
middle pin of the barrel connector to the front panel on/off
switch, then from the other terminal of the on/off switch to
the + power input of the PIC32MZ controller board. Run lightduty black wire from the DC socket ground (outer connector,
as shown in the photo) to the PIC32MZ’s GND power input.
Twist these together and use heatshrink to make the connections tidy. Then plug in the two ribbon cables you made
earlier, one from the CON11 on the CPU board to CON2 on
the front panel, and the other from CON8 on the CPU board
to the DIL header on the back of the LCD adaptor board. In
each case, make sure the red stripe side of the cable goes
towards the pin 1 side on the connector.
Practical Electronics | May | 2021
1.2k
100nF
CON
GND
CON10
The code for programing is
available in the download
package from the May 2021
page of the PE website.
CON9
BACK
OF
PICKIT 4
8
7
6
(PGEC) 5
(PGED) 4
(GND) 3
(VDD) 2
(MCLR) 1
SPI2/I2S
1
BACK
OF
PICKIT 4
8
7
6
(PGEC) 5
(PGED) 4
(GND) 3
(VDD) 2
(MCLR) 1
JP5
1k
100nF
1
100nF
1
CON23 ICSP
Fig.12: When programming
a blank PIC32 or there is a
firmware update, this is how to
connect a PICkit 3/4 or similar
to the board to reflash the chip.
10k
D15
REG3
PORTB
1
390
1.2k
100nF
Thermocouple input wiring
The two binding posts are mounted 20mm apart, allowing
the Jaycar QM1284 thermocouple to be plugged straight in.
This provides a professional-looking solution. However,
CON
as mentioned earlier, if you run the thermocouple wire through a grommet in the
rear panel and connect them directly to the
screw connectors on the thermocouple amplifier board, the
temperature readings will be more accurate.
The downside is that you now have a captive thermocouple wire, so changing the thermocouple is a tedious job.
The thermocouple and also the solid-state relay signals
BACK
connect to CON10 (PORTB)
on the PIC32MZ board. We
OF
8 connector onto one end of
suggest that you crimp
an
PICKIT
4 IDC
7
a length of 10-way ribbon cable.
This can then be plugged
6
(PGEC)
into CON10 and the wires
at 5the other end separated and
(PGED) 4
stripped to make the required
connections. Make sure
(GND) 3
(VDD) 2to the IDC terminal marked
that the red striped wire (MCLR)
goes
1
as pin 1.
With this cable, some fiddling is required. We couldn’t
think of an easier way for this short of adding a PCB, which
seemed over the top. Pull the wires apart to separate out wires
1 (red), 6 and 7 (together), 8 and 10. Snip the other wires
off short as they are not needed. Mark wires 7 and 8 as ‘–’
with some heatshrink or colour it with a permanent marker.
Connect wire 6 to the solid-state relay input + terminal,
and wire 7 to the SSR – input. These can be wedged under
the screw terminals; do them up tight.
Connect wire 1 to the ‘Out’ connection of the thermocouple amplifier, wire 8 to its ground and wire 10 to the
positive power input on the thermocouple amplifier.
We arranged the cable lengths so that it is possible to
encapsulate the thermocouple amplifier in heatshrink
tubing and zip tie it to the binding posts. This places the
thermocouple amplifier in reasonable contact with the
thermocouple plugs. Remember that this amplifier has correction circuitry that accounts for the temperature of the
thermocouple plug, so the closer it is to this plug, the better.
If you purchased the recommended thermocouple amplifier with a purple PCB, there will be a mounting hole. You can
use this to mount it to the rear panel with a nylon machine
screw and nut, close to the binding posts/banana sockets.
GND
Hopefully, when you soldered the LCD adaptor to the
LCD screen earlier, you connected pin 1 on that board to
pin 1 on the LCD. If not, rotate the IDC connector plugging
into the LCD adaptor by 180° to compensate.
The specified dual male and female IEC connector allows
a conventional IEC mains power cord to supply power to
the unit, and also makes it easy to connect up to the oven.
Strip out a length of 10A mains flex or an unused 10Arated mains power cord to get the brown, light blue and
green/yellow striped wire that you will need to wire this
up to the SSR.
For the following mains wiring, keep all the wires as
short as possible to maximise safety (the earth wire is less
critical, but it’s still better to keep it short if possible.)
Use a short length of the light blue wire to join the two
neutral connectors on the socket together. These are both
marked with an ‘N’. Then crimp an eyelet lug onto one end
of a short length of green/yellow striped wire, solder the
free end to the earth connector on the mains socket and
attach the eyelet to the baseplate using a machine screw, a
shakeproof washer (under the eyelet) and two nuts.
Cut two lengths of brown wire and crimp eyelets to one
end of each, then solder the free ends to the incoming
and outgoing Live terminals on the mains connectors. It
doesn’t matter which wire goes to which load terminal on
the SSR - this is AC after all, so current must be able to
flow in both directions.
Note that you could connect to the SSR using bare wires
rather than eyelet lugs, but we feel that using a crimped connector makes it a bit tidier. Just make sure they are securely
crimped. Apply insulation to all of these connections, and
double-check them, then cable tie them all together, so that
if one comes loose, it can’t go anywhere.
CON10
The thermocouple amplifier we used has a purple PCB.
If you search ebay or AliExpress for ‘AD8495’, then you
should be able to find one which looks like ours.
A view of the rear panel connections – again, this is before
the Presspahn insulation barrier is installed. Don’t forget it!
Practical Electronics | May | 2021
35
A male-IEC-plug-to-female
mains socket (such as this
one from CPC Farnell, part
number PL15014) means no
modifications are required for
the toaster oven.
The
Presspahn
barrier is
essential for your
safety – there are
exposed mains voltages
inside the case which must
be covered.
Tidying it up
Once you’ve finished all the wiring, use cable ties to tie
each bundle of wires together. This is especially important for the mains wiring, which must all be tied together
securely, and also the red and black wires from the DC
socket to the front panel on/off switch and to CON4 on
the control board.
Make sure that these wires are tied so that they can’t
move around inside the case (eg, by tying them to the
nearby ribbon cables) and that if one breaks off at either
end, it can’t go anywhere.
Now is also a good time to attach the Presspahn insulation barrier to the bottom plate using machine screws,
shakeproof washers and nuts. Refer to the photos to see
where it goes. Once the lid is on the case, it should isolate
the mains section from the rest of the controller.
Initial testing
For the following tests, do not connect the mains lead.
Use only the 9V plugpack. Make sure that jumper JP5 on
the CPU board is inserted. There must also be a jumper
on LK2 in the position shown in Fig.4. You don’t need
a jumper on LK1; if there is one there, it doesn’t matter
which position it is on.
Now switch the device on and check the LCD. Adjust
the LCD bias voltage using trimpot VR1. This may require
some experimentation; the LCD will initially show no image or a washed-out image. Adjust the bias from one end
toward the other until you get a good image.
Next, check that the user controls work by pressing the
right-hand button (EXIT); a screen with four boxes should
appear. Rotate the encoder knob; you should see each of
the four quadrants be highlighted in turn.
Now we set the initial PID coefficients. Press the lefthand button/rotary encoder knob (SEL) when the ‘adjust
PID settings’ screen is highlighted. You will be presented
with a screen asking if you are sure. Rotate the dial to ‘Yes’
and click SEL. Enter 100 for P, 0.5 for I and 670 for D.
This configuration is super critical – if you do not do
this, the thing will most likely show 0°C, and definitely
not work.
Next, set the reflow settings by pressing SEL when the
‘Setpoints’ screen is highlighted. You will be presented
with a screen asking if you are sure. Rotate the dial to
‘Yes’ and click SEL. Enter 150C for Preheat Temp and
225C for Reflow Temp.
Then set the Sensor TEMPCO settings by pressing the
left button (SEL) when the ‘Tempco and Offset’ screen is
highlighted. You will be presented with a screen asking if
you are sure. Rotate the dial to ‘Yes’ and click SEL. Enter
0 for OFFSET (this is in °C), and 0.161 for TEMPCO.
Check that the home screen now shows approximately
the right ambient temperature. Boil a jug of water, insert
the thermocouple and check that the home screen shows
something close to 100°C. Remember that thermocouples are not super-precise devices, and an error of a few
degrees is OK.
To check that the SSR drive is working, use the following steps:
1. Reset the system by cycling the power
2. Set the target temperature a bit above the ambient
temperature
3. Watch the LED on the solid-state relay (it is next to
the input). This should light up every few seconds, in
time with the lightning bolt on the screen going from
an outline to a full lightning bolt
4. Turn the set temperature down to around 20°C, and hold
the tip of the thermocouple between your fingers, so
the measured temperature is above the set temperature
5. Check that after a few seconds, the lightning bolt and
red LED on the solid-state relay stop lighting.
Four holes must be drilled in the front panel (follow the drilling diagrams in the downlod package from the May 2021
page of the PE website) but there is also a cutout required for the display. We used a Dremel to cut out the rough hole then
finished it off with a fine file. The same approach can used for the IEC mains socket cutout on the rear panel.
36
Practical Electronics | May | 2021
Pin
1
6
7
8
10
Role
Analog input
Heater control
GND
GND
+3.3V
Connect to
Thermocouple amplifier output
SSR input 3+
SSR input 4Thermocouple amplifier GND
Thermocouple amplifier power supply
Table 1 – CON10/PORTB connections (other pins not used)
Note that with a PID controller, there can be a lag in its
response to changes in temperature and settings.
Live testing
You can now switch off the power and connect the oven
to the IEC mains output socket on the controller via the
IEC/mains socket adaptor.
Before connecting the mains input IEC lead, doublecheck your wiring, and ideally have a friend triple check
it. Check that:
n No stray wires are poking out of crimps, terminals or
the insulation
n There are no wires stripped too far, leaving lots of exposed copper that could contact with something
n The IEC ‘N’ connector goes to the other IEC ‘N’, diagonally across the connector
n The earth connector is solidly connected to the baseplate
n One wire from each of the IEC live pins goes to one
SSR ‘LOAD’ terminal.
Close the case and securely screw it together; make sure
there are no exposed mains wires. Turn the oven to its
maximum temperature setting, and then switch on all elements. Dial the timer for 20 minutes or so, plug the oven
into the controller, power up the controller and set the
temperature to 20°C. Plug the controller into the mains
and switch it on.
The oven should not be on yet, unless your house is
particularly cold. Turn the dial on the controller for a
setting of 40°C. You should soon see the lightning bolt
on the controller coloured in, indicating the heater is on.
If your oven is like ours, you should see a light on the
oven indicate it is switched on. After a few seconds, you
should see the measured temperature start to creep up.
The thermal inertia of the oven will cause a delay of 20
seconds or so; the temperature will likely overshoot the
Above: the board used during reflow test, showing solder
paste applied to pads. The amount shown here is more than
enough! Below: some sample boards during reflow test.
target. As explained above, our controller is optimised for
high temperatures, and you will see overshoot in the order
of 15°C or so at low temperatures. Just watch to see that
heating switches off before it reaches the target temperature.
Try setting the controller to 60°C, and watch the controller switching on and off. Once heated, the oven takes quite
a while to cool down. Remember that when reflowing, you
must open the door at the end of the cycle.
Reflowing
We reflow soldered a couple of boards with SMD components to demonstrate the operation of the oven. As shown
in the pictures below, if you are applying solder paste by
hand, use a syringe and put less than you think it will
take! The biggest mistake most people make when reflow
soldering parts is to add too much solder paste.
We stuck the thermocouple to the edge of the oven using tape, and fiddled it until the thermocouple was close
The rear panel sports the 9V DC input socket (left) with the polarised thermocouple terminals alongside. At the right end
is the twin IEC mains output/input socket which is the raison d’etre for this project: mains comes in to the right-hand
(male) socket; controlled mains to the toaster oven comes out of the left-hand (female) socket.
Practical Electronics | May | 2021
37
Help, it’s not working!
Nothing on the LCD screen
• Check that the LCD bias pot is set correctly. Turn it fully
anticlockwise, then slowly turn it clockwise until you see
something on the display
• Check that the microcontroller is running
• Check your parts and soldering, especially look for bridges
across the microcontroller pins
• Check the output of the 5V and 3.3V regulators.
Reflow display showing target and actual temperature profiles.
to the test PCBs. You need the sensor to be as close as
possible to the boards (maybe even touching), to ensure
the temperature profile achieved is right in the vicinity
of your board.
The temperature profile that the oven ran is shown
above. You can see that the temperature fell after we
opened the door a crack. We left it like that for about 20
seconds, then opened it fully to allow the board to cool.
Don’t move the board until it cools, as the solder will still
be liquid for a while!
At about 180°C, the flux activates and the solder starts
to reflow. By the time the oven hit 225°C, and we opened
the door, the board had fully reflowed and settled down.
Ideally, you should give your oven a trial run on a noncritical PCB before soldering anything really expensive.
But if you have a hot air rework station, you can probably fix anything that goes awry on the first couple of tries,
until you get it fully dialled in.
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My oven is going crazy
• Have you used an oven with a smart controller? This project
won’t work with it!
The temperature readings are very wrong
• Is the thermocouple connected backwards
• Are the tempco and offset in the software right for your
amplifier
• Use a DVM to check the voltage on the thermocouple amplifier output. It should be about 1.25V. If not, read the panel
on thermocouple amplifiers
• Put the thermocouple tip in a cup of hot water. Watch to see
if the voltage increases.
The temperature readings are slightly wrong
• Is your thermocouple in the oven next to your workpiece
• Check the thermocouple tempco and offset is correct for
your thermocouple
• Try putting the sensor tip in iced water and boiling water. If
the readings are off by more than a few degrees, check for
construction errors.
The oven won’t heat
• Ummm – you did check that the oven worked normally before
making any modifications, didn’t you?!
• Check that your oven’s temperature is set to maximum and
that it is switched on and both heating elements are selected.
• Is the thermocouple reading the right temperature?
• Set the temperature on the controller for say 100°C and watch
the SSR. It has a red LED that indicates when it is on
• Watch your oven power light. Is it switching on in unison
with the SSR light?
The oven is running too hot when set for fixed
temperatures
• At low temperatures, even with the optimisations we made,
the thermal mass of the oven means that there is still a lot of
overshoot. Also, the thermal mass of the elements and oven
means it takes a long time to cool down
• Try starting it up in advance and give it time to settle before
using it.
Settings are lost at power-off
• Use the save option after making changes.
• Check the PIC microcontroller pins for shorts on the side
close to the EEPROM
• Check the orientation and soldering of the EEPROM chip.
38
epts oft.indd 1
Reproduced by arrangement with
SILICON CHIP magazine 2021.
www.siliconchip.com.au
Practical Electronics | May | 2021
04/02/2021 12:00
