During the initial testing phase, the system exhibited unstable behavior, particularly when the fan and control electronics were operating simultaneously.

The 12V rail, derived using a linear regulator (7812), showed noticeable voltage drop under load. This resulted in erratic system operation, including unreliable fan startup and inconsistent control behavior.

At first, the issue was suspected to be related to firmware timing or PWM interaction. However, further measurements revealed that the root cause was insufficient bulk capacitance on the rectified DC supply.

The original design used a 1000 µF reservoir capacitor, which proved inadequate for handling transient load demands from the fan and control circuitry. Under load, the input voltage to the 7812 regulator dropped below its required dropout margin, causing the regulated 12V rail to sag to approximately 10.6V.

To resolve this, the reservoir capacitance was increased from 1000 µF to 5000 µF. This significantly improved voltage stability by reducing ripple and maintaining sufficient headroom for the regulator during peak load conditions.

After this modification:

• The fan started reliably across the full PWM range

• The control system became stable

• Temperature regulation operated consistently

This debugging process highlighted the importance of proper power supply design, especially in systems combining high-power loads with sensitive control electronics.

Ensuring adequate bulk capacitance and understanding regulator limitations are critical for stable operation.