PCB Hotplate Slightly Bigger Edition

Greetings everyone,

So here's the "PCB Hotplate slightly bigger edition" which is Version 2 of my previously made PCB Hotplate project.

The most significant change in this project was the size of the hotplate, which is approximately 99mm x 98mm in total and the heating area, which is approximately 65mm x 98mm. (The previous hotplate was tiny, measuring only 60mm x 90mm in total, with a heating area of only 60mm x 55mm.)

The new model is powered by an XIAO M0 DEV Board and features an OLED display that shows the current temperature of the heating coil.

XIAO was chosen due to its small size and the ATSAMD21G18 microcontroller, which is an ARM Cortex-M0+ processor. Seeed Studio created it, and it is part of the Seeeduino XIAO family.

The board is small, measuring only 20.3mm x 17.8mm, but it is powerful, with a 32-bit, 48MHz ARM Cortex-M0+ processor, 256KB flash memory, 32KB SRAM, and a variety of communication interfaces such as I2C, SPI, UART, and USB.

Like the previous project, this version has a coil setup that is driven by a Mosfet IC that cuts off the power when a certain temperature is reached, then reconnects the power when the temperature drops, which is currently set around 210°C as declared by the code below.

const int threshold = 210;//cuttoff temp

This hotplate is powered by a 12V 4A source; if we use a 12V 10A SMPS, the temperature will rise faster and reach 210°C+; the current maximum temperature observed is 207°C.

I hate to say it, but the temperature reading of this hotplate still needs to be fine-tuned; the NTC was placed at one edge of the coil, and the temperature around that area and the temperature around the center of the coil are different, as evidenced by the two different readings in the attached image. An external temperature meter was temporarily added to determine the temperature difference between the center and edge of the PCB.

These were the materials used in this built-

• Custom PCB
• MPS2314 buck converter IC
• XIAO M0
• SSD1306 OLED Display
• DC Barrel jack
• ON OFF DIP Switch
• MPS2314
• 10K Resistor
• 10R Resistor
• 7.5K Resistor
• 75K Resistor
• 40K Resistor
• 100uf Capacitor
• 10uf Capacitor
• 1H inductor
• LED
• M7 Diode
• PCB Standoffs
• Female header pins
• 12V SMPS
• Solder paste
• Test PCB for reflow
• Patience

Yes, this project was a success; we added a test board that took less than a minute to properly reflow.

The solder paste used is regular Sn-Pb 63-37, which reflows at temperatures between 180°C and 200°C. Low melting solder paste will be more effective and faster with this hotplate, but regular solder paste will also work.

PCBs are used here due to the fact that we can create coils directly on the PCB layer and use them as heating elements. Let me explain how.

As we all know, when electricity is passed through any material that has some resistance, heat is generated.

We make coils for the PCB design; these coils are essentially long copper lines, each 1mm wide. This coil has a resistance of 2.2 ohms, so if we pass electricity through it, the coil heats up, and we can use this setup as a small heating element.

Also, making a PCB design and getting it fabricated without doing any physical editing is an easy and hassle-free thing.

One of the few disadvantages of using PCB as a heating element is that the PCB used here has a Tg rating of 140°C, which means that heating the FR4 of the PCB above this temperature will deteriorate the material and burn the board.

After 10-15 uses, the coil region of this PCB will turn yellowish, then dark brown, and finally black.

Copper traces will also burn out over time, as I found with my previous mini hotplate project, which is currently in a brown state and has been used more than 20 times; it is still working, but I have a feeling it will soon burn out; when it does, we can simply change the board and transfer all the non-burned components to the new board.

I made a breadboard version first to see if the sketch works, The led imitates the mosfet gate, which drives the coil, and the temperature sensor is heated by a lighter, which raises its temperature.

This 10K NTC is encased in a round spade terminal with epoxy and salvaged from an old inverter circuit, but an alternative can be easily found on the internet.

We connect NTC in series with a 10K resistor, and add the middle pin between NTC and resistor with pin A0 of XIAO.

• VCC of OLED with 5V Pin of XIAO
• GND to GND
• A2 with LED's positive pin
• SDA to A4 and SCL to A5

Here's the code that was used.

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

#define SCREEN_WIDTH 128 // OLED display width, in pixels
#define SCREEN_HEIGHT 64 // OLED display height, in pixels


Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, -1);

int ThermistorPin = 0;
const int Mosfet = 2; 
const int threshold = 210;//cuttoff temp

int Vo;
float R1 = 12000;
float logR2, R2, T;  // Tc;
float c1 = 1.009249522e-03, c2 = 2.378405444e-04, c3 = 2.019202697e-07;


void setup(){
  display.begin(SSD1306_SWITCHCAPVCC,0x3C);
  pinMode(Mosfet, OUTPUT);
  digitalWrite(Mosfet, HIGH);
  display.clearDisplay();
  Serial.begin(9600);
}

void loop(){
  Vo = analogRead(ThermistorPin);
  R2 = R1 * (1023.0 / (float)Vo - 1.0);
  logR2 = log(R2);
  T = (1.0 / (c1 + c2*logR2 + c3*logR2*logR2*logR2)) -273.5;
 // Tc = T - 273.15;
  Serial.println(T);
  display.clearDisplay();
  display.setTextSize(2);
  display.setTextColor(WHITE);
  display.setCursor(70, 0);
  display.println("C");
  display.display();
  display.setTextSize(2);
  display.setTextColor(WHITE);
  display.setCursor(3, 0);
  display.println(T);
  display.display();
  delay(1);

  if (T > threshold) {
    digitalWrite(Mosfet, LOW);
  } else {
    digitalWrite(Mosfet, HIGH);
  }

}

The LED stays on throughout, but when the temperature reaches the threshold (210+), the LED turns off. After a few seconds, when the temperature drops below 210, the LED turns back on, and this cycle continues indefinitely.

The schematic of this project consists of three parts or sections, which are the following:

• Buck converter circuit
• XIAO MCU and OLED setup
• Coil with mosfet as switch setup

The buck converter circuit is built around an MPS2314 integrated circuit.

Monolithic Power Systems' MPS2314 is a buck converter IC (integrated circuit) (MPS). It is a synchronous, rectified step-down converter with an input voltage range of 4.5V to 23V and an output current of up to 3A.

The MPS2314 incorporates a variety of features including a high efficiency, low RDS(on) internal switch, soft start, over-current protection, thermal shutdown, and programmable output voltage. It also supports a wide range of switching frequencies and can be operated in either pulse-width modulation (PWM) or pulse-frequency modulation (PFM) mode.

This IC is commonly used in a variety of applications such as point-of-load regulators, set-top boxes, routers, and other consumer electronic devices.

Buck converter circuit steps down the 12V source into 5V for the XIAO and OLED to work.

The second section is the XIAO Breakout board, which connects XIAO I/O pins to OLED Screen pins as well as a CON3 Port, which will be used to connect the thermistor and the Mosfet's gate pin with this setup.

A potentiometer has also been added to the board for manually controlling the temperature of the coil; this feature will not be used and will remain idle, but it was added for future modifications.

The coil section is straightforward; the coil is connected to a mosfet as a switch setup that is linked to the input side of the circuit, and two diodes are added in series with the coil's VCC to slightly reduce the voltage going to the coil.

In the PCB Editor, we create an outline of 98mm x 99mm and place all of the components on one side, then separate the coil zone and component zone with a few rectangular slots to keep the PCB components from being heated by coil heat transfer through the FR4 medium.

This was an issue in a previous project.

We add coils on the top and bottom sides of the board with a thickness of 1 mm for a total length of 4329.8 mm, which is extremely long.

I used the allaboutcircuits trace resistance calculator to determine that the total resistance should be around 2.14 ohms, and the actual measured resistance was 2.2 ohms, which was very close.

As for the PCBs in this project, they were sent to SEEED Studio for samples.

An order was placed for a Blue solder mask with white silkscreen, as this was same color i got for previous project.

PCBs arrived in less than a week, which was super fast.

The quality was very good considering the price, which was also pretty low.

The PCB quality of both PCBs was just awesome!

Seeed Fusion PCB Service offers one-stop prototyping for PCB manufacture and PCB assembly, and as a result, they produce superior quality PCBs and Fast Turnkey PCBA within 7 working days.

Seeed Studio Fusion PCB Assembly Service takes care of the entire fabrication process, from PCB manufacturing and parts sourcing to assembly and testing services, so you can be sure they are getting a quality product.

After gauging market interest and verifying a working prototype, Seeed Propagate Service can help you bring the product to market with professional guidance and a strong network of connections.

Next, we prepare for the PCB assembly process.

This project's PCB assembly process consists of two major steps: first, we add SMD components, and then we place THT components.

• We begin by applying solder paste to each component pad individually using a solder paste dispensing seringe.
• Next, we pick and place all the components in their place using a ESD tweezers.
• Following the pick and place process, we place this board on a mini reflow hotplate, which heats the PCB from the bottom up to the solder paste melting temperature.
• Fun fact, the previously made Mini Hotplate is being used here to reflow the new Hotplate PCB.
• Following the reflow, we use a standard soldering iron to add two M7 diodes on the back side of the board.
• Then we put all of the THT components in their proper places, such as the header pins in the XIAO and OLED pads, the switch and DC jack, and the capacitors and inductor.
• Board is now assembled.

Following board assembly, we connect the 12V SMPS and measure the voltage across the MPS2314 Boost module output, which should be around 5.3V.

This means the circuit is operational, and we can proceed to the next step, which is to connect a thermistor to its CON3 pad.

The SMPS used is a 12V 4A whose input voltage ranges from 100V to 240V AC and was used to power a 12V DC aquarium pump. This is an isolated supply that is relatively safe to handle and keep on a desk without risk of electrocution.

Next, we add the Thermistor in its place using a soldering iron.

The thermistor is connected to the XIAO's VCC and A0 pins, and a 10K resistor is connected between A0 and GND to form a voltage divider.

Thermistor is essentially a resistor whose resistance changes when heat is applied to it; we connect it in series with a 10K resistor and apply 5V to it. The voltage divider is used to measure thermistor resistance, which is then used to calculate temperature.

• We add OLED Display and XIAO in its place first.
• Then, on each edge, we add four PCB standoffs, which lift this PCB from the base and serve as its stand.
• This board is now complete.

Here's the finished product: a hotplate that works properly and can reach temperatures of more than 200°C+, making it ideal for reflowing larger PCBs.

Because the onboard temperature sensing was not working properly, we added an external temp meter's NTC to the bottom side of the PCB using kepton-tape, which will keep the NTC in place because it is heat resistant.

The maximum temperature recorded is 207°C, which is high; if we replace the power supply with a much higher current one, we can get more temperature readings in less time, but the deterioration rate will also increase.

Overall, this project was a success.

A few things need to be tweaked, including the location of the temperature sensor. Currently, the temperature sensor is located on one edge of the coil, and as we tested this hotplate, we discovered that the temperature on the outside is different than the temperature on the inside. This is due to the fact that inside traces are shorter in length, and thin traces have higher resistance than traces near the edges, which are longer.

Because middle traces have less resistance, the center of the coil heats up first, and maximum readings are taken only at the center of the coil, so the best place to put a thermistor is in the middle, but we can't just drill a hole and add a thermistor with a nut and bolt because that will obstruct PCBs and cause gaps.

As a result, the best option is to use thermal epoxy and glue the thermistor to the bottom layer of the PCB in the middle region.

Then we must fine-tune the temperature readings by taking reference readings from the precise external temperature meter and changing the value of R1 in the sketch so that the onboard thermistor reading and the external meter reading match.

Overall, I'm pretty happy with the result. This hotplate is large enough to reflow large PCBs, not to mention it is inexpensive and can be rebuilt if the board is damaged.

All the documents related to the project are attached, if you need any help regarding this project, DM me or comment.

Special thanks to Seeed Studio for providing PCBs for this project, do check them out if you need great PCB service for less cost.

Peace out, and I'll be back with a new project soon!