Single Switch Controls Sequential Operation of Multiple Power Supplies

Why It Matters

It provides designers a reliable, power‑efficient way to manage mixed‑signal power rails, reducing component count and simplifying board layout for industrial and consumer electronics.

Key Takeaways

• Analog circuit sequences 5V then 15V with single switch
• Capacitor C1 charging sets power‑up delay via RC time constant
• Discharging C1 reverses order, turning off 15V before 5V
• Design avoids always‑on watchdog, saving power
• Scalable to three or more supplies using additional MOS switches

Pulse Analysis

Power‑rail sequencing is a critical yet often overlooked aspect of mixed‑signal system design. Engineers typically rely on microcontroller firmware or dedicated sequencing ICs, which add software complexity, extra components, and continuous power draw. An analog approach that leverages a single RC network and comparators eliminates the need for firmware timing loops, delivering deterministic start‑up order while keeping the bill of materials lean.

The core of the presented circuit is a capacitor (C1) that charges through a resistor network when the main switch is pressed. As the voltage on C1 rises, it crosses two preset thresholds that trigger comparators U1B and U1A, sequentially turning on Q1/Q2 for the 5 V rail and then Q3/Q4 for the 15 V rail. On release, the capacitor discharges, causing the comparators to revert in reverse order, thus shutting off the higher voltage first. By adjusting R9‑R11 values, designers can fine‑tune the delay to match device‑specific power‑on requirements, and MOSFETs can replace the bipolar switches for higher current applications.

For product developers, this method translates into tangible benefits: lower power consumption because no watchdog runs continuously, reduced PCB real‑estate, and increased reliability through fewer active components. The architecture also scales gracefully—adding a third supply simply extends the comparator‑switch chain, as shown in the article’s Figure 2. Such flexibility makes the solution attractive for industrial control, automotive electronics, and IoT devices where deterministic power sequencing and minimal overhead are paramount.