Currently I am working on a capacitive sensing and actuation system powered both by battery and AC/DC (Wall Wart) power adapter. The system works well when powered by battery and bench top power supply. The system sometimes behaves unpredictably when connected to AC/DC power adapter.

The AD/DC power adapter is 6V and is rated at 1000mA. The capacitive sensing/scanning part of the system draws less than 1mA. The sleep/standby mode draws in the uA range. The actuator needs about 500mA for a very short period and activated sparsely. About 50 times in a 24 hour period. The actuators active time duration is about 30 milliseconds.

I have measured the voltage and current usage with a multimeter, I don’t see any issues.

Any thoughts on what might be causing the instability of the system when used with an AC/DC adapter. Thought on next steps to identify the issue(s) and develop a solution.

Thanks in advance for your help!

  • 1
    $\begingroup$ Source a better quality AC supply. Now you know what cheap wallwarts are like. $\endgroup$
    – Solar Mike
    Oct 6 '20 at 5:39
  • 1
    $\begingroup$ @SolarMike Only if the source team understands the concept of purchasing quality parts $\endgroup$
    – user7083
    Oct 6 '20 at 17:23
  • $\begingroup$ "Wall Wart" isn't a brand, is it? If not it shouldn't be capitalised. $\endgroup$
    – Transistor
    Oct 6 '20 at 23:01

The actuator is rated for 500mA, but note that if it is electromagnetic, the transient inrush current at turn-on can be several times as big as that for a period of tens to hundreds of milliseconds. During that time, your wallwart voltage may sag down as it struggles to deliver the large current transient and if that happens, then the rest of the circuit may go offline or reset itself.

(Note also that upon shutdown, the actuator will generate a kickback voltage spike greater than the rated voltage of the wallwart which may also disturb the circuit.)

You should attempt to electrically isolate or "bypass" the actuator from the control circuitry so that any transients that it generates will not upset the control circuitry. The electrical engineering stack exchange would be a good place to get advice on how to accomplish this.


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