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I have 3 buttons (R13-81 PUSH SWITCH) for a school project. In the specifications, it is written that they take Voltage Rating (AC) : 125V (Source)

Problem is I only have 3.3V or 5V as power supplies. Will it still work ? I don't understand if it takes between 0 and 125V or if it has to take 125V every time. Also, the output needs to be 3.3V maximum... What should I do ?

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    \$\begingroup\$ 125V is what they are rated for. So they will work perfectly fine at whatever lower voltage. \$\endgroup\$ – Bart Jun 10 '17 at 18:03
  • \$\begingroup\$ If you read the specs of everything even the wire you are using has a 120V+ rating. It's like a truck rated for 5T can run safely with 1kg of load, electric components functions within the rated range, whose lower bond is assumed 0 unless otherwise explicitly specified. \$\endgroup\$ – user3528438 Jun 10 '17 at 18:05
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    \$\begingroup\$ I know what you mean. Once I got HR-rated tires. I wasn't able to actually drive 112 mph, but I was pleased to discover the tires still worked even at slower speeds. Those tire designers think of everything! \$\endgroup\$ – Harper - Reinstate Monica Jun 11 '17 at 1:34
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Your question is wisely asked. In general, an AC-rated switch should be derated for DC use. AC ratings are usually higher than the DC rating, if available, for the exact same switch. There's a reason for that.

AC has zeroes, twice per AC cycle. A manually opened (or closed) switch happens "randomly," relative to the AC cycle. Any "arcing" drawn out as the metal contacts open up will soon be extinguished because the AC cycle will hit one of those zeros.

DC doesn't have any zeros. So there is no inherent self-extinguishing process and therefore they may have to cope with more prolonged arcing when operating at the same voltage.

In your case, the DC voltage is so low and is also so much lower than the AC switch rating, that there is almost certainly no problem using it. I wouldn't worry at all. But it was still wise to ask. (There is also the problem of current compliance, but you've not mentioned it. I'm assuming from the context of your question that we aren't talking about large currents.)

P.S. A manual switch is just a bit of mechanical components arranged to make it easy for a human to operate, and includes metal conductive contacts that are placed into direct contact with each other when the switch is engaged. The metal contacts are just physical bits of metal and do not have to have a specific voltage in order to work. They will make contact, regardless, because the mechanical design for operation is independent of the voltage it is switching.

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    \$\begingroup\$ They will make mechanical contact, regardless, but if you are switching very low voltages and/or currents, they may gradually accumulate an insulating layer of dirt, oxidized metal, etc, that has enough resistance to generate thermal noise that is comparable with the signal you are switching, or restricts the current flow. But that is unlikely to be a problem switching 3.3V or 5V and milliamps of current, compared with switching a few microvolts and nanoamps. \$\endgroup\$ – alephzero Jun 11 '17 at 1:27
  • \$\begingroup\$ If you've ever been in a really old house and the light switches have a very firm "SNAP" action especially when you turn them off -- those are DC rated switches, a throwback to the Edison age. There is a spring mechanism to hurl the DC contacts apart quite definitively. \$\endgroup\$ – Harper - Reinstate Monica Jun 11 '17 at 1:39
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    \$\begingroup\$ anyway most places I see use a 3:1 through 10:1 derate for DC on an AC switch. I once saw a 600VDC light-rail-vehicle main contactor. No air on the car, so they couldn't use locomotive contactors. So they used a 1200VAC 3-pole contactor made for 3-phase, with all three contacts wired in series! Crazy! \$\endgroup\$ – Harper - Reinstate Monica Jun 11 '17 at 1:48
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The voltage rating on switches is the maximum voltage that the switch can safely handle. Likewise, the current rating of a switch is the maximum current the switch can safely handle.

Your switches are fine for use with 3 or 5 volts.

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In general, the voltage rating of "passive" components like switches and connectors is their max rating, and lower voltages may be used with no difficulty. (In this case the rating is basically for the insulation within the switch, and the distances between bare metal components at different voltages. Staying within the rating assures that the insulation will not "break down" due to voltage stress and that arcing between parts of the switch will not occur.)

There are two separate concerns, however, unrelated to the voltage rating. First off, one must keep the current through the switch below its max current rating. Your switch has a half-amp rating, so you need to keep current below that level. Most uses for signal and control applications don't exceed a few milliamps, but if you are, say, powering a motor you could see higher currents.

Secondly, arcing. When the switch "breaks" (turns off) and the contacts in it pull apart, it's possible for there to be an arc formed between the contacts, causing them to briefly heat up and be damaged, and, in rare cases, causing the current to continue even when the switch is supposedly off. This is mainly a problem with "inductive" loads -- relays, solenoids, motors, etc. If you are switching such loads you need to study up a bit on how to deal with this situation.

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  • \$\begingroup\$ Thanks for warning me about those issues. However, I'm using a simple generator with very low voltages so I guess it shouldn't be a problem :) \$\endgroup\$ – Paul Lemarchand Jun 10 '17 at 20:25
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SPST off-(on) .5A @ 125VAC Plating undefined ( possibly silver micro-plated over nickel over beryllium copper)

Usually these are derated for DC to about 1/4 of the AC rating due to inductive load arc needs to be extinguished rather than zero crossing current of AC.

All relays are however gold plated upto 2A to permit low current circuits to conduct without oxidation risk (high) However when switch supplies, there is often a low ESR filter cap and thus a surge current. THis can be a bad thing or a good thing. If the cap is too big, and very low ESR, it can pit the silver oxide and raise the resistance from 50 mOhms to much higher value.

But a little "wetting " current usually 10% of rating is all you need to burn thru oxidation insulation on silver plating.

So depending on your surge current on contact, 0.5A should be OK but contact will wearout some material with a low ESR cap of 1 Ohm giving V/1Ohm= 5A. Ceramic and plastic caps have even lower ESR and gives higher peak currents. So beware Low ESR caps are good for noise reduction but bad for switch load surge currents and contact wear. In well designed system, it could last for 50k cycles. If really well designed, even more and much less if poorly design surge current is applied on contact.

So designer beware.

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