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Old 29th June 2010, 09:05 PM   #1
ssmith is offline ssmith  France
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Default Simple question on reverse polarity protection

Hi all,
Quick question -- I have a pair of stereo mini alephs that would benefit from some kind of reverse polarity protection -- because the fuse, DC blocker and soft start are on the live wire, and although I rewired my apartment properly and made the power cords, there is always the risk of someone less careful using a badly wired IEC cable and causing a problem.

I found what seems to be a good simple rundown on options for reverse polarity using a diode -- Reverse Polarity Protection. -- and though of trying option 1, a diode running from neutral to live which trips the fuse in case of reverse polarity.

I was wondering though what the experts think of this -- and also what value diode would be suitable. I know there are fancier and probably better options for the task, but I like the idea of only having to solder in one part.

thanks in advance, stefan
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Old 29th June 2010, 09:37 PM   #2
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There is no need, current will flow through the fuse, blocker and soft-start whichever way you wire the mains plug or lead live and neutral.

The diode idea is crazy, what if somebody puts the wrong fuse in with a badly wired lead or plug? Nice fire starter.
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Old 30th June 2010, 10:05 AM   #3
AndrewT is offline AndrewT  Scotland
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amps with output protection or limiters usually have a pair of diodes from output rail to supply rail to limit the back emf to ~ supply rail volateg across the output devices.
Adding a second pair of diodes across the main smoothing caps also prevents reverse voltage across the amplifier & smoothing caps.
All four of these diodes do virtually nothing when the amp is operating within specification.
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Old 30th June 2010, 11:00 AM   #4
sangram is offline sangram  India
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^^ I think he wants to know about mains reverse polarity protection, where the user can face some danger of completing a circuit if the fused 'live' is connected to neutral and the fuse blows.

Anything wrong with a 3-pin plug? The diode idea is crazy, I agree. Why would you deliberately want to short-circuit the mains supply inside and equipment?
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Old 30th June 2010, 05:33 PM   #5
asbjbo is offline asbjbo  Norway
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Yeah, it seems like the "problem" is that the fuse and DC blocker are on the live AC wire, which could turn out to be the neutral AC wire if someone plugs in the socket the other way around, or as described from thread starter:
Quote:
the risk of someone less careful using a badly wired IEC cable
Perhaps this would be a good time to review some basics of AC current, as opposed to DC current?

Of course, where I live, AC is balanced with 115 volts on either side, so there is no "live" or "neutral", just two equally live wires. I strongly prefer two-pole mains switches.

Last edited by asbjbo; 30th June 2010 at 05:36 PM.
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Old 30th June 2010, 09:18 PM   #6
ssmith is offline ssmith  France
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Quote:
Originally Posted by asbjbo View Post
Yeah, it seems like the "problem" is that the fuse and DC blocker are on the live AC wire, which could turn out to be the neutral AC wire if someone plugs in the socket the other way around
exactly. I was under the (false?) impression that that is a problem, in that the live should always travel in through the fuse and dc blocker to the transformer.

Quote:
Originally Posted by asbjbo View Post
Of course, where I live, AC is balanced with 115 volts on either side, so there is no "live" or "neutral", just two equally live wires. I strongly prefer two-pole mains switches.
Where I live the AC is not balanced There's live in and neutral out. I gather though I've got it badly wrong, so will go ahead and review the basics of AC current...
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Old 1st July 2010, 07:37 AM   #7
asbjbo is offline asbjbo  Norway
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I don't think there is a problem, unless (as sangram says) there is a series of events that cause the fuse to blow and live voltage to exist on the neutral line. The usual protection against that are plugs that can be inserted one way only. Even then, it is a good precaution to unplug equipment before you open the lid and start poking around inside to investigate the reasons for a failure.

Remember that AC current alternates between positive and negative half periods, so half the time, live voltage is higher than neutral, and half the time, it is lower. The current goes back and forth. In other words, half the time, a diode between live and neutral would just be a shorted connection. It works for DC, though.
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Old 1st July 2010, 08:07 AM   #8
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The important point is to always regard every wire from the wall socket except the safety ground (if one is present) as potentially live. From the point of view of the mains transformer it doesn't matter on which wire the switch is. If the circuit is open no current flows and the secondary of the transformer has no voltage, regardless of how the plug is oriented.

Remember: The transformer does double duty. It steps down the voltage and provides galvanic isolation between mains and the device.

Having said that, I agree with asbjbo: Two-pole mains switches are a good idea.
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Old 8th July 2010, 05:47 AM   #9
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Still if u want to detect the reversal of Live and neutral.
Connect one AC coil relay between neutral and safety earth and use the contacts appropriately.

Gajanan Phadte
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Old 8th July 2010, 07:37 AM   #10
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Quote:
Originally Posted by gmphadte View Post
Still if u want to detect the reversal of Live and neutral.
Connect one AC coil relay between neutral and safety earth and use the contacts appropriately.

Gajanan Phadte
Unless you have a Ground Fault Protector, of course. That would probably trip it.
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