Attempts at Supplying Efficient Logic-Level Power to Sensitive Electronics

A power supply, when suddenly turned off, bleeds voltage slowly. Attached electronics experience a gradual voltage decline from 5V to 3.3V and eventually to zero. The problem is that microcontrollers and microprocessors don’t know how to behave with under-voltage. Unless they have a brown-out detect circuit enabled, which is not the default, their behavior and flash memory integrity is not defined. Flash memory can even be erased.

Flash memory erased during under-voltage
Flash memory erased during under-voltage
Goal: Design an efficient power circuit to supply either 5V or 0V, known as a logic-level power supply.

Attempt 1: Direct connection to the power supply

My first attempt at powering a cluster computer with a direct connection to the PSU ended badly.

Power supply showing a slow voltage decay
Power supply slow voltage decay

I noticed that some of my microcontrollers suffered memory erasure due to this, but I was thankfully able to enable the brownout-detect circuit (BOD) in each of them and reflash them. However, I’d like to solve the problem which is what I attempt next.


Attempt 2: Use an AC-sensing relay to cut off power

This next attempt had all the hallmarks of an elegant and efficient design: connect an AC-sensing relay to the mains and the switching part to the 5V output of the PSU. When the mains are disconnected, the relay opens and the power is completely cut off to the microprocessors. Simple and effective.

Voltage decay when AC-sensing relay cutoff
Voltage decay behind an AC-sensing relay when open

The voltage decay waveform shows a sharper voltage drop under the internal resistance of the electronics when the PSU is mechanically disconnected via the relay. Problem solved?

Unfortunately, the relay gets both untouchably hot (> 50°C) and emits high electromagnetic energy (> 200µT). Darn.
AC relay runs both hot and EMI noisy
AC relay runs both hot and EMI noisy
Is 200µT high? The highest I found the 200W PSU to get is around 5µT. That makes the AC-relay coil over forty times more electromagnetically noisy. Who knows how adversely this will affect the microprocessors?

Power supply EMI is only around 5µT

Solution: Use a high-power DPDT switch and bleed resistor

I discovered a six-pin, 2400W DPDT toggle switch that solves this problem. The plan is to establish three circuits toggled by the switch: 1) AC mains to the PSU, 2) PSU to the electronics, and 3) electronics to a bleed resistor.

High-power DPDT power switch with bleed resistor
High-power DPDT power switch with bleed resistor

The DPDT circuits are established as follows:

DPDT toggle switch with three circuits
DPDT toggle switch with three circuits

When the switch is off, the electronics are disconnected from the PSU and are connected to a 1.5Ω bleed resistor for rapid discharge. The stored charge in the smoothing capacitors in the electronics bleeds quickly leaving a square-wave-like logic-level voltage decay curve. The momentary delay while the switch toggles is taken into consideration and will not cause a short circuit anywhere.

Rapid voltage drop using a DPDT switch and bleed resistor
Rapid voltage drop using a DPDT switch and bleed resistor

This simple solution causes no excess heat nor extra EMI.

Success: Using a DPDT toggle switch and a bleed resistor it is possible to supply logic-level power to sensitive electronics without added heat dissipation nor EMI.