Can't decide on the PCB color? #1 Red

In case anyone is still wondering what color to choose for a PCB, below are photos of my PCB (made by a hobbyist).

Just curious—does the chosen color have anything to do with your project's intended use? 🤓

Since the PCB I designed is related to temperature, I chose the color red.

#PCB# #color#

Overview

This circuit is a linear, temperature-controlled fan driver. Unlike simple on/off thermostats or PWM (Pulse Width Modulation) controllers, this circuit varies the DC voltage supplied to the fan smoothly. This "linear" approach eliminates the high-frequency switching noise often associated with PWM and prevents the "coil whine" that occurs when fans are run at very low voltages where they struggle to spin.

Key Functional Blocks

1. Temperature Sensing (The Input)

• Components: TH1 (10k NTC Thermistor) and RV1 (10k Potentiometer).

• Function: These two components form a voltage divider connected to the base of transistor Q1.

  • TH1 (NTC): An NTC (Negative Temperature Coefficient) thermistor decreases its resistance as temperature rises.
  • RV1 (Pot): This allows the user to manually set the temperature threshold (calibration).
  • Logic: As the device gets hotter, the resistance of TH1 drops, changing the voltage at the base of Q1. This signals the circuit that cooling is needed.

2. Control & Error Amplification (The Brain)

• Component: Q1 (2N5401 PNP Transistor).

• Function: Q1 acts as the control element.

  • As the temperature rises and the base voltage drops (due to the NTC), the PNP transistor turns ON more heavily.
  • It amplifies this small change in temperature into a control signal that drives the power MOSFET.

3. Power Driving (The Muscle)

• Component: Q2 (IRF540N N-Channel MOSFET).

• Function: This is a high-power transistor that handles the actual current for the fan.

  • Instead of acting as a simple switch (fully ON or fully OFF), it operates in its linear region (acting like a variable resistor).
  • As Q1 conducts more current, it increases the voltage at the Gate of Q2. This lowers the resistance between the Drain and Source, allowing more current to flow through the fan.
  • This results in the "smooth ramping" mentioned in your text. The fan speed increases gradually rather than in steps.

4. Hysteresis (Stability)

• Components: R1 (100k) and R5 (220k) (and potentially the interaction with R3).

• Function: The text mentions "shutdown hysteresis prevents frequent cycling."

  • Without hysteresis, if the temperature hovered exactly at the threshold (e.g., 40°C), the fan would rapidly turn on and off (chatter), which is annoying and wears out the motor.
  • The feedback resistors (likely R1 and R5) create a "lag." Once the fan turns on, the threshold temperature for turning it off is lowered. For example, it might turn on at 45°C but won't turn off until the temperature drops to 35°C. This creates a stable "dead band."

5. Coil Whine Prevention (The Zener Diode)

• Component: D1 (5.1V Zener Diode).

• Function: The text states the circuit "avoids applying voltage just at the threshold... preventing... coil whine."

  • Fans often make a buzzing or whining noise at low voltages (e.g., 1V–4V) because the magnetic field isn't strong enough to spin the rotor smoothly; it just vibrates.
  • The 5.1V Zener diode likely sets a minimum voltage threshold. The circuit is designed so that the fan stays completely OFF until the temperature is high enough to drive the voltage past this 5.1V point. Once it turns on, it starts at a healthy speed (silent operation) rather than a slow, whining crawl.

6. Filtering

• Component: C1 (100uF Capacitor).
• Function: This capacitor is placed across the fan terminals (parallel to the fan). It smooths out any voltage ripple and provides a burst of current when the fan motor demands it (motors are inductive loads), ensuring stable operation and reducing electrical noise.

Summary of Operation

1. Cold State: The NTC resistance is high. Q1 is OFF. Q2 is OFF. The fan receives 0V (or is disconnected) and is silent.
2. Warming Up: Temperature rises. NTC resistance drops. Q1 begins to conduct.
3. Activation: Once the threshold is passed (and the Zener voltage is overcome to avoid whine), Q2 begins to conduct. The fan voltage ramps up smoothly from ~5V to 12V.
4. Hot State: Q2 is fully ON (low resistance). The fan receives the full 12V for maximum cooling.
5. Cooling Down: As temp drops, the fan slows down smoothly. Due to hysteresis, it stays on slightly longer than it would in a simple on/off circuit, preventing rapid cycling.

The circuit description was created using A.I.

Source: *https://www.youtube.com/watch?v=YGpXFbxD_Wk&ab_channel=REDShade*

I did not create this circuit, I just designed it for my own use.