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I'd like to build a fun audio amp. Its a blue tooth module, so there are no controls. I just want a simple on/off switch. This amp is to be used in a workshop, and because I live in Las Vegas (where it gets quite hot in the summer) I don't want to leave the power supply engaged all the time. I'm not all that worried about speaker pop on startup. I'd like to shut off power upstream of the switching power supply. I was considering using this switch, Knife Blade Switch just because it looks cool. I intend to use it in this circuitSimple Audio Amplifier with A/C Power Cord

What I'm not clear about is safety using this switch. I'm guessing the power supply output is approx 2 or 3 amps at 14v. I don't know the input power ratings. I'm thinking worst case is that somebody might bridge the open knife blade to blade seat with a sweaty hand?

The parts aren't all on hand yet, so I don't know the actual operating current in a single leg of that A/C circuit.

The other option is to use a double pole, single throw switch of the same design, to shut off both legs of the 120v wiring, but that seems much more dangerous to me. Somebody could bridge the 120v terminals on the hot side of the switch. (In fact, why do they even make a double pole single throw switch like that? That just seems unsafe.)

Note: the intent here is that this amp assembly hangs on the wall, about ten feet from the door of the shop. I am not worried about tossing a tool on the workbench and having that short out the switch. I am not worried about hitting the switch with my hands when I enter the dark shop (while trying to find the light switch)

Your thoughts here? Is this crazy unsafe?

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  • \$\begingroup\$ Yes its unsafe. Don't use that on the AC side of your PSU ! \$\endgroup\$ – Icy Nov 6 '15 at 11:32
  • \$\begingroup\$ Using that switch in the +ve output from PSU to amp should be OK, as long as the workshop isn't the flying metal swarf sort of shop. Definitely not on the input to the PSU... \$\endgroup\$ – Brian Drummond Nov 6 '15 at 13:01
  • \$\begingroup\$ It is INSANE to the point where I wonder if this is a serious question. Of course NO exposed circuit connected to mains power is safe by ANY definition of the word. \$\endgroup\$ – Richard Crowley Dec 16 '16 at 19:20
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Exposed 110V is a horrible idea and very unsafe. Never do this.

14V is going to be generally safe in the dry conditions: for example, UL 1310 "Safety Requirements for Class 2 Power Units" requires "A maximum potential of 42.4 Vac peak / 60 Vdc for exposed wires / terminals." I personally would not go above 24V DC.

A separate issue is temperature: an incompletely closed switch may heat up so much as to cause a burn. This depends on the design of the switch and on maximum current consumption -- you mention "2-3A" which is not much; moreover, this is likely a peak current, not continuous. Also, a lot of bluetooth receivers reset if the power is too low, and this causes their current consumption to be reduced.

Yet another thing is a damage to amplifier. Someone walking on a dry carpet can have few thousands volts of static electricity and all of this will go to your amplifier's power input if they touch the metal part. You will likely be fine if you use automotive grade parts -- they are usually designed to withstand this.

The biggest practical danger with that switch is the accidental short circuit. You really want to make sure that the switch is the only exposed metal part of this circuit. Otherwise, if someone shorts the switch directly to the other transformer leg, the transformer will be overloaded and may get damaged; also the object that shorted the circuit may get very hot thus presenting the burn hazard.

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Hmmm. Sounds like this is safe.

Simple Light switch

But that doesn't mean you can't have fun with it...

Steam Punk Mechanism 1

Steam Punk Switch 2

Reference, lots more here.

Hmmm. I'm not ready to give up on the knife switch yet, Maybe I'll put it inside the box behind glass with a wooden actuator, or figure out a way to use a cool mechanism with an old skool traditional light switch. I'm not exactly a rookie when it comes to mechanism design.

update: I also ran into this implementation of a knife switch. 3d printed knife switch

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To safely use that switch to connect/disconnect AC to your amplifier I would consider having it control a relay. The relay handles the high voltage AC supply thus isolating you from danger.

However, you'll still need a permanently connected low voltage supply to be able to activate the relay's coil when you wish to use the amplifier.

So scout around for some relays that have the required AC voltage and current ratings for the contact and try and find one that doesn't need much power to energize the coil. Then look for maybe a wall-wart to supply the low power and safe voltage to activate the relay coil.

Here's a CEC (California Energy Commision) compliant power supply that says it uses less than 0.3 watts in standy: -

enter image description here

It's got a European plug arrangement but I'm sure you can find one that is compatible with the US.

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  • \$\begingroup\$ So you might as well just switch the low voltage output from the amplifier PSU. \$\endgroup\$ – Icy Nov 6 '15 at 11:36
  • \$\begingroup\$ Which defeats the purpose of powering off the power supply. Always on power supplies in an ultra hot workshop is just a bad practice. \$\endgroup\$ – zipzit Nov 6 '15 at 11:43
  • \$\begingroup\$ @zipzit the standby power taken by the amp's power supply will likely be several watts but this is dependant on ultimately how big the power amplifier is in terms of wattage. A bigger power amp means more standby power in the power supply. However the power needed by a relay coil (maybe 1 watt max) would imply that the standby power of the wall-wart need only be a fraction of a watt and, by comparison, hardly produce any self heat. \$\endgroup\$ – Andy aka Nov 6 '15 at 11:48
  • \$\begingroup\$ CEC compliant wall warts have to be less than 0.5 watts in standby. The alternative is to use a battery that can feed enough power to the relay and then, when power is on, trickle some DC from the main power supply to keep the battery charged. See this article: edn.com/electronics-blogs/powersource/4311906/… \$\endgroup\$ – Andy aka Nov 6 '15 at 11:54
  • \$\begingroup\$ @Icy No, that would be pointless and not save the power. \$\endgroup\$ – Andy aka Nov 6 '15 at 12:03

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