Amp switch ratings

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by loverocker, Mar 16, 2005.

  1. loverocker

    loverocker Member

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    Are some switches rated higher than typical mains power supplies?

    Most of the toggles and other switches I see used for Standby switching are general purpose and rated at a few amps at 250V/125V AC, and not the 300-500V DC that they see in use. I assume this is OK, because amp makers have been doing it for years, but are properly-rated switches available?

    Got any pointers? URLs?
     
  2. Wakarusa

    Wakarusa Member

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    Strictly speaking the switches you see used are underrated for the operating voltages, but not so badly as to make them unsafe. Higher voltage rated switches are much more expensive, larger, and difficult to come by.

    The voltage rating is a statement on the switch's ability to break a circuit and is a combination of the distance between open contacts (the air gap) and the speed with which the contacts open. Too little gap operated too slowly at too high a voltage will draw an arc instead of breaking the connection. Not really an issue in a MI amp using standard switches.

    More important is the current rating (the ability of the switch contacts to dissipate heat).
     
  3. loverocker

    loverocker Member

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    Hmmm... I agree that the current carrying capacity is fine (maybe 50-300mA or so through the Standby switch). I was surprised that the Carling switch info I found didn't mention a DC voltage rating. Then I realised I'd *never* seen a spec that mentioned ratings for high DC voltages...
     
  4. Wakarusa

    Wakarusa Member

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    In 24 years of electrical work I don't recall ever seeing a DC voltage rating on a switch. Could just be that I wasn't paying attention tho'
     
  5. saros141

    saros141 Guest

    Some toggle switches I've seen will say, for instance, both 6A 120VAC, and 3A 240VAC on them.

    Extrapolating that, one would get 1.5A at 480VAC... but since we're changing the V:A ratio from the first rating by a factor of 4, let's be safe and drop the A value by a factor of 4. So 375mA at 480VAC would seem reasonable. More current than most HT supplies draw anyway.

    Now considering the AC rating on motor run oil caps is generally considered safe to reinterpret as a DC rating 1.4 times higher, 480VAC becomes 672VDC.

    So I'd venture a switch rated for 6A at 120VAC could be used in the HT rail for standby purposes in circuits under 375mA and 672VDC.

    None of this comes from any textbook or training, it's just my own (hopefully) common-sense interpretation... if there are glaring flaws in this thinking, I'd like it pointed out to me for my own safety and others!
     
  6. loverocker

    loverocker Member

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    Well, I lack the smarts to argue with your maths! :) But it's the insulation breakdown level - between the internal contacts and the toggle and body of the switch - that makes me believe that we can't just multiply the volts and amps to estimate equivalence.

    Plus there'd be that internal arcing at some point.
     
  7. John Phillips

    John Phillips Member

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    Most switch bodies are made from Bakelite or other phenolic-type resins, which have extremely high insulation ratings - I have a little electrician's screwdriver marked for 1500VDC, for example (and the handle isn't even all that thick). My old AVO mulitmeters with solid Bakelite housings are rated to 3KV.

    It's not that which limits the voltage rating, it's the contact spacing and switch speed, as Todd said; this is far below what the switch body will stand in any switch - it has to be, or the current would leak around through the body rather than being broken at the contact.

    I don't know of any amp with a B+ of over 700V (the Hiwatt 400, and at least one MusicMan, are the highest I know of at almost exactly 700), and they use standard switches.

    If you think about it, the arc risk is not too high really - it mostly occurs when the contact is broken... and in fact when a standby switch is turned off, because both sides of the switch (should) have filter caps on them, there is no voltage across the switch at the moment of switching! - it only rises as the cap on the far side discharges.

    I have seen quite a few failed standby switches from internal arcing it's true, but not that many really - and I'm not sure I haven't seen as many power switches go the same way. They may even be at more risk due to switching a highly inductive load (the PT) which actually causes a voltage spike as the power comes off.

    I've never seen a switch fail - ever - from insulation breakdown to the chassis.
     
  8. TieDyedDevil

    TieDyedDevil Member

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    The DC rating of toggle switches, if you ever do see it marked, is much lower than the AC rating. As John said, the issue is arcing. WIth AC, the voltage goes through zero twice each cycle (every 8.3 milliseconds, or 10 milliseconds in Europe) and gives any arc a chance to quench. If you break DC then any arc will sustain much longer and cause carbonization and pitting of the switch contacts.

    The susceptibility to arcing is dependent on the circuit, but some designers really didn't give that much thought and did in fact put the standby switch in an inductive circuit. The first two schemos I checked at ampage.org/ffg, a Vibro-King and a Blackface Twin Reverb, have the standby switch breaking an inductive circuit: the VK in the PT secondary windings and the TR between the first filter cap and the choke.
     
  9. saros141

    saros141 Guest

    Very interesting, thanks for posting that. I guess using motor run oil caps as a reference is not very applicable here.

    In regards to switch spacing, I can concur with John, I researched Grayhill sealed rotary switches a bit and the ones with the highest ratings had a 45 degree index, where those with a 36 or 30 degree index had lower ratings. As it turned out, for my application (impedance selector), the only rating that really mattered was its "carry" rating... it's "make" rating was irrelevant, as I wouldn't dare switch impedance on the fly.
     
  10. loverocker

    loverocker Member

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    Thanks JP and TDD - great explanations. I'll go look at those schematics to see where the inductive element is in relation to the Standby.

    I wonder if the voltage spike you guys mention is one of the causes of the 'pop' you get when switching off some low-cost amps? Is there a way of suppressing such spikes - something to place across the transformer to absorb the energy realeased?
     

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