I don't know much about electrical hardware, so forgive any naivety. Anyways, I've been wondering: since 2 prong plugs (the reversible ones whose prongs are the same size) can be plugged into a socket two different ways, don't they have to include more complex circuitry than their non-reversible 2 prong brethren to allow for the electricity flowing two opposite directions?

If so, then why do manufacturers do this? Is it just cheaper, or is it motivated by consumer convenience?

Based off of the relatively small sample of plugs currently available for my investigation, it seems to me like most reversible plugs are found on charging devices (e.g.: phone or laptop charger, which all have the colloquial "brick" included), while plugs built into electronics and other hardware (e.g.: a power cable for a TV or a lamp, none of which have an external "brick") do tend to have 2 prongs, but these are differently sized, rendering the plug non-reversible.

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    \$\begingroup\$ AC is always flowing in both directions alternately. It is only when there is a chance of galvanic connection to something outside the insulated housing that hot vs. neutral (line vs return) has meaning. Voltage is always relative. \$\endgroup\$ – Chris Stratton Mar 23 '16 at 3:34
  • \$\begingroup\$ True! That rather obvious fact somehow utterly escaped me. But what about devices that charge things? I may be wrong here too, but they need to use DC, correct? If that's correct, then the use of a diode (I believe) would suffice to allow only the current in one direction. So, for charging devices, they don't need to worry about orientation, because the voltage, if it can be represented by a sine wave (or even a square wave), is effectively the same if flipped upside-down. So that's why these devices don't care about orientation? \$\endgroup\$ – Ben Sandeen Mar 23 '16 at 3:42
  • \$\begingroup\$ Pretty much. Most circuits that use DC (chargers, small electronic things, pretty much anything that isn't a motor or an incandescent lamp, these days) have a bridge rectifier, which takes an AC signal and makes it all "positive". Bridge rectifiers basically give you the absolute of the input voltage, mathematically, and therefore they couldn't care less about the input voltage "orienatation". With the help of some extra circuitry, a bridge rectifier can take AC and produce DC. \$\endgroup\$ – uint128_t Mar 23 '16 at 3:48
  • \$\begingroup\$ If they are built correctly the mains and DC output are isolated without galvanic connection. If not, watch out! Note that a bridge rectifier does not provide isolation, rather that is the job of a transformer, running at mains frequency in a linear supply or at higher frequency in an isolated switcher such as a legitimate modern phone charger. \$\endgroup\$ – Chris Stratton Mar 23 '16 at 3:49
  • \$\begingroup\$ Many (most?) devices in North America without a ground often come with polarized plugs even if they don't strictly need them to be compliant to a spec. The only things in my house I can think of that do not have polarized plugs are so-called double insulated devices. These are designed not to allow single voltage exposing faults. Often Class II AC/DC power supplies like chargers fall under this category, so a polarized plug is not required. \$\endgroup\$ – user65586 Mar 23 '16 at 3:52

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