power switch question - fundamental

Discussion in 'Shock Brother's DIY Amps' started by Meteorman, Oct 13, 2021.

  1. Meteorman

    Meteorman Tele-Afflicted

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    I notice some schematics (e.g. Fender Pro Jr, Vox AC15) show a main power switch that hosts/breaks both the mains lines.
    And some of the older Fender schematics (Princeton) show a power switch that breaks only the hot line, with the common going straight to the transformer.
    Why the difference ? Does it matter ?
    Is this a safety thing I need to worry about ?
     
  2. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    I'm just guessing here, maybe it is done that way to prevent shock hazard, if the plug somehow gets reversed, and the single pole switch switches off the neutral and leaves the hot line voltaginated.
     
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  3. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    It's technically required (CE, but not UL?) to switch both hot and neutral since the chassis is metal, and the user comes in contact with it. If the power cord is detachable (IEC), only hot needs to be switched.

    Building in the US, you don't need to worry about it, and switching only hot with hot fused before the switch is sufficient. If you're building in a repro chassis, you're not likely going to get a DPST or DPDT Carling to fit well anyway.


    Edit: removed the half my post that made zero sense. :)
     
    Last edited: Oct 13, 2021
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  4. Nickfl

    Nickfl Friend of Leo's

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    Old way vs new. Breaking one side of the connection was once considered safe enough, now it's more standard to switch both sides of the primary, probably required for UL listing these days. The modern way is objectively safer, but the old single pole power switches are hardly a death trap...
     
  5. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    Opening both power supply conductors is required in many parts of the world as a safety measure. For the most part, in the US you're only required to open the hot conductor.

    From a strictly electrical point of view, all you have to do is break one conductor. If you're going to break only one conductor it should always be the 'hot', leaving the neutral intact. If the supply wiring is working properly, that persistent neutral provides some measure of protection should something horrible happen with whatever it is turned off.

    Mostly, it's a theoretical exercise. It's probably best to just follow whatever convention is observed wherever you are.
     
  6. sds1

    sds1 Tele-Afflicted

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  7. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    Note that the above references UL/EN standards; He probably meant UL/EU, and kettle leads. Very probably English in origin. A lot of electronics adopt aspects of the European standards, it just makes it easier when selling globally.
     
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  8. Timbresmith1

    Timbresmith1 Tele-Afflicted

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    Brits are at 220v iirc...
     
  9. aging_rocker

    aging_rocker Friend of Leo's

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    Close...240v
     
  10. trancedental

    trancedental Tele-Meister

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    246-254 VAC in North London! :eek:
     
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  11. Bitsleftover

    Bitsleftover Tele-Holic

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    Switch both. But mainly, I absolutely love your avatar!
     
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  12. NoTeleBob

    NoTeleBob Friend of Leo's

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    Safe in the USA *if* you have a polarized plug and *if* the outlet is properly wired. The first you can see, the second hides sometimes.
     
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  13. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    Build your amp to make future owners happy, break both. That said, I use a lot of recycled "stuff", and if a scrounged switch only has one pole, I break the hot.
     
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  14. rdjones

    rdjones Tele-Meister

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    On ~240V systems both sides are 'hot', it's a balanced feed.
    So in areas where mains power is balanced both sides must be switched for safety reasons to meet codes.

    On some amps it's simply a matter of using a common part for use in multiple mains regions.
     
  15. rdjones

    rdjones Tele-Meister

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    IEC or not, if the mains feed is balanced (ie, 240V) both sides are hot and MUST be switched.
    In the US (unbalanced) neutral should never be connected directly to chassis.
    From the mains, only an earth ground should be connected to chassis, and as close as practical to the power entrance point.
     
  16. 2L man

    2L man Tele-Holic

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    European mains is technically not different. We have neutral and hot wire but the mains plugs do not have polarity so hot can come to either input.

    European mains network is three phase 120 decree system instead of 180 decree two phase.

    New buildings have required to have also Safety Earth wired connectors at least 30 years and at least 10 years "30mA leak current safety relays" to kitchen and bathrooms because 230VAC is much more dangerous than 120VAC.
     
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  17. dougsta

    dougsta Tele-Meister

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    But only the best mains plugs in Europe are found in the UK, Ireland and Malta!
    Even if you remove the earth bar (solid brass not like the flimsy hollow tube I've seen when I lived in France used) you still can't insert it as it's required to uncover the latching covers over the supply. Also comes with a built-in fuse holder.

    You only step on them once, you tend not to do it again :)

    IMG_9905.jpg IMG_9906.jpg
     
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  18. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good discussion. Modern commercial amps (eg, Fender) in the US switch both. It's a good idea, since in the US also, hot/neutral reversal in the outlet is pretty common. Plus, tho neutral isn't ground, there are cases where switching both prevents a ground problem in the wall (also um, shockingly common) from zapping you.

    Plus: There aren't many disadvantages. The switch tends to be a bit bigger. But OTOH, it also gives you a nice place to tie the neutral to the PT primary -- no splicing/reliability/appearance issues.

    Finally, a much more common problem is the modern builds/kits/instructions (looking at you, Mojo and StewMac) where they switch neutral and not hot. OK, it's not gonna kill you most days. But why do it the wrong way?
     
    Last edited: Oct 18, 2021
  19. Paul-T

    Paul-T Tele-Holic

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    I don't think I agree with some of the statements here about neutral being much the same as hot/live.

    Neutral would be pretty close to earth potential; it will be connected to earth back, iirc, at the step down tranformer or local distribution point. It SHOULD be at a potential of only a few volts. But of course the issue is that neutral is often flipped.

    I switch both on all my builds, always.

    And, yes, British plugs are great - but they need to be.
     
  20. LightningPhil

    LightningPhil Tele-Meister

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    It would be even better if after breaking the 2 power connections, the amp sides are then connected to ground and a robust capacitor drain resistor is connected across the filter caps. That way the amp would become safe quickly. But then we're requiring substantial extras in the power stage and would have to certify that they all operate safely. It's roughly how a lot of HV and grid kit works. Perhaps overkill for an amp.
     
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