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Old 2nd December 2010, 04:49 PM   #11
AndrewT is offline AndrewT  Scotland
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decibel dungeon and many posts in this Forum.
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Old 3rd December 2010, 01:10 PM   #12
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Quote:
The reversible mains plug requires that your mains switch must break both Live and Neutral. Never only one. In the UK, where we cannot reverse the plug, we have to use 2pole switching, just in case some eejit wires up the socket or the plug top wrongly.
I was thinking about that, but I don't understand. If you interrupt only one line between Live and Neutral, in an attached transformer the current should not flow. Or I'm missing something?

Thanks, Ralf

PS. I've found references about the bulb tester. Thanks.
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Old 3rd December 2010, 03:36 PM   #13
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by giralfino View Post
I was thinking about that, but I don't understand. If you interrupt only one line between Live and Neutral, in an attached transformer the current should not flow. Or I'm missing something?
Indeed, any interruption - whether it be in L or N - stops current from flowing.
AndrewT is talking about two pole switches, but despite having reversible plugs here on the mainland, the use of them is not mandatory, at least not in devices such as hifi or TV. All of my commercial hifi has single pole switching.

I think when it comes to mains electricity, the British have a few more rules and regulations. In The Netherlands, sockets are permanently powered (i.e. there's no switch), and plugs don't have a fuse.

Last edited by jitter; 3rd December 2010 at 03:38 PM.
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Old 3rd December 2010, 07:04 PM   #14
AndrewT is offline AndrewT  Scotland
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Hi,
if one uses a single pole ON/OFF switch, it can only break one Line.

If it breaks the Live Line than all the appliance downstream of the switch is connected to neutral, i.e. 0V

If the single pole switch breaks the Neutral Line (because the plug has been inverted) then the whole appliance from the live incomer to the Neutral switch pole is Live, i.e. 220Vac on the mains cabling, 220Vac on the mains fuse (both ends), 220Vac on the transformer primaries (both Live and Neutral), 220Vac on the switch (neutral pole only). That is just waiting for an accident to happen.

In the UK we have many belt and braces rules. The two pole switch is just one of them and we can't invert our BS1363 plugs.
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Last edited by AndrewT; 3rd December 2010 at 07:07 PM.
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Old 3rd December 2010, 07:50 PM   #15
jitter is offline jitter  Netherlands
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Originally Posted by AndrewT View Post
If the single pole switch breaks the Neutral Line (because the plug has been inverted) then the whole appliance from the live incomer to the Neutral switch pole is Live
Not the whole appliance, only the primary winding of the transformer or the primary side of a SMPS. But since this is inside the appliance, it doesn't really matter. I don't know if there's even a rule that says how to wire L and N on a Schuko socket, so I think it's a fair assumption that half the appliances are plugged in one way and the other half the other way.

Even floor or table lamps come with single pole switches over here and in this case the potential danger is a lot bigger because the contacts in the E14 or E27 fitting are exposed when no bulb is installed. When installing a bulb, I always take care not to come close to the threaded part with my fingers, in 50% of the cases this part carries mains voltage, even when switched off.
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Old 10th December 2010, 10:36 AM   #16
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I think I'll go with a 9V AC transformer. If I'm not wrong, this transformer with a rectifier will have an output of (9 x 1.4) - 1.4 = 11.2V DC (or better, between 9V and 13.5V considering fluctuations in the main 220V). These values are OK for my amp.

Now I'm asking advice for the bridge rectifier and smoothing capacitors:
I have this rectifier: D5SBA20 - is this OK for my amp?
What capacity do I need for smoothing? I have 2x4700uF caps but I suspect they are not big enough.
And what voltage rating do I need? Is 16V enough or is it better 25V?

Thanks, Ralf
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Old 10th December 2010, 11:33 AM   #17
AndrewT is offline AndrewT  Scotland
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Originally Posted by giralfino View Post
I think I'll go with a 9V AC transformer.
And what voltage rating do I need? Is 16V enough or is it better 25V?
16V caps should be OK, but you should really check (measure) your worst case highest voltage from the transformer before getting this close on cap voltage rating.
Safer to go with 25V caps and those will allow you to use a 15Vac transformer for your next project.
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