Electronic – AC polarity detection


How can I detect AC 'reverse polarity' (single phase) (without the use of ground/earth) and drive a led when the polarity is reversed?

It should at least detect 230 VAC but it would be cool/informative for future readers if the solution could work in US and EU (approximately 120 VAC to 250 VAC).

I did find this (non-isolated!) circuit, but I can not get it to simulate (the simulator complains that it needs ground or the LED blinks on both halves of the wave).
ac polarity detection ?
Source: 1 & 2

My questions are then:

  • Is the above schematic really functional and how does it work?
  • Is there a better or alternative way (MCU or perhaps even an IC) that does this?

Most current answers are leading me to believe one needs a reference. Now I'm thinking, why not create one? Something like a 'virtual ground' or somewhere along that line?

Update 2:
Today I read that older single-phase watt/hour-meters will run backwards if neutral and hot are reversed.. Why is this and couldn't the reason for this behavior be used in a 'equivalent circuit' that detects 'reverse polarity'?
Also, I wonder: could a hall-effect sensor be used to recognize a unloaded 'hot' wire, since a hot wire clearly emits a diversity of electromagnetic fields and elf radiation?

Best Answer

As other posters have suggested, this can't be done via conductive measurement alone. However, non-contact AC testers will properly differentiate line from neutral without a reference ground via capacitive sensing. This type of hand-held tester often has a flat plastic blade at the tip. Inserting the blade in the line slot will cause the LED to glow; inserting in neutral or ground will not glow:

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There are a number of kits for this type of circuit, as we as many sample schematics available in a few minutes of searching (Search string: non-contact ac voltage detector circuit). They're essentially just high gain amplifiers that drive an LED.

Using this type of sensor permanently mounted near the Neutral line can serve as an indicator that your plug has been inserted the wrong way, provided that you have a reliable way (i.e. not battery) of powering the circuit.