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When I run 24VAC into a full wave bridge rectifier followed by a 220uF electrolytic capacitor to turn it into ~32VDC, the source has two wires. Does it matter in what order I connect the AC wires to the input to the bridge rectifier? If so how do I determine which wire goes where? I suspect that it's totally symmetric on the input side, but I'm am full of doubt when it comes to AC. Sorry if this is just a really dumb question.

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    \$\begingroup\$ In some power systems neutral is grounded so yes AC power sometimes can have "polarity" though probably doesn't apply in your case. \$\endgroup\$ – Sridhar Nov 4 '14 at 3:58
  • \$\begingroup\$ @sridhara ... you are correct! but where neutral is grounded, a bridge rectifier probably won't do much good, because the resulting DC supply isn't isolated so you can't connect -V to ground. \$\endgroup\$ – Brian Drummond Nov 4 '14 at 11:39
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When you're looking at an AC source in isolation such as in your question, indeed there's no polarity and you can connect the wires either way round.

When combining two or more AC sources in series or parallel, the relative phasing is very important.

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    \$\begingroup\$ While not relevant in this case, I guess it is worth mentioning that transformers have polarity between the primary and secondary. In schematic symbols, this is denoted by a dot. When the primary terminal with a dot on it goes high, the secondary with a dot on it also goes high. Sometimes this matters. \$\endgroup\$ – mkeith Nov 6 '14 at 6:03
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As long as you are dealing with a closed system (like a transformer secondary winding + bridge rectifier) then no, AC is not polarized.

However, when dealing with outside power (like what comes out of the wall) we do consider AC-carrying cables to be "polarized". There's the hot wire carrying the juice and the neutral wire carrying the return. The hot wire should go directly to your device's switch / fuse, and any semi-exposed contacts must have the hot wire as protected as possible. If you don't do this on production items you will fail UL certification and be wide open for lawsuits. The common light bulb would likely fail many of today's standards, but it's been around too long to recall.

Devices that don't have a polarized wall plug will use a double-pole power switch to prevent the circuit being live up to the switch.

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  • \$\begingroup\$ Among others, the Schuko wall plug (used in most of Europe) is not polarized, so any device intended for those markets will have to assume that both wires are "hot". More: electronics.stackexchange.com/questions/96033/… \$\endgroup\$ – ntoskrnl Nov 4 '14 at 22:13
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    \$\begingroup\$ @ntoskrnl Actually all the Schuko plugs I have ever come across were polarized because they were designed to fit in both the Schuko outlets as well as French outlets. But that doesn't help when it is used in a Schuko outlet which is not polarized. \$\endgroup\$ – kasperd Nov 5 '14 at 0:10
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    \$\begingroup\$ @kasperd The polarization of the French type "polarized" plug is not standardized; for example the polarization is reverse in the Czech Republic compared to France. \$\endgroup\$ – ntoskrnl Nov 5 '14 at 15:12
  • \$\begingroup\$ @ntoskrnl As the saying goes. Standards are great let's have some more of them. \$\endgroup\$ – kasperd Nov 5 '14 at 18:24
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    \$\begingroup\$ @kasperd Obligatory xkcd: xkcd.com/927 \$\endgroup\$ – ntoskrnl Nov 5 '14 at 18:41
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AC voltage has no polarity. Therefore it does not matter how you connect the wires to the bridge rectifier. Also, there are no dumb questions. Feel free to ask about whatever is bothering you. You will learn, as will others, and you will be safer for it.

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  • \$\begingroup\$ The last three sentence are a comment, not an answer. \$\endgroup\$ – user253751 Nov 5 '14 at 0:40
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    \$\begingroup\$ I think it is potentially dangerous to tell people that "AC voltage has no polarity" without further qualifying it. Some of the other answers covered areas where polarity is important to consider. \$\endgroup\$ – mkeith Nov 5 '14 at 6:39
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At the atomic level, with the AC coming out of your transformer there is a herd of electrons which all run one way, then stop, turn round and run the other way half a cycle later. Since the electrons are running in both directions the transformer output has no polarity. The job of the rectifier is to round them up and make them all run in the same direction. Now you know which way they're going they now have polarity. That's why the input of the rectifier doesn't specify which way round to connect the wires, but the output does.

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