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I ordered a set of DC rocker switches that were advertised as "4A 12V DC SPST"

I was planning on using one to power an Arduino based project using 5V DC which will draw no more than 1 to 2 amps.

What I received were switches labeled as

10A 125V AC / 6A 250V AC

I wrote to the vendor and this is their reply:

"They will work with DC current just as well but not the other way around if they were specifically DC."

I have already returned them, as I have no intention of using a switch rated for AC on a DC circuit, but the exchange with the vendor left me with two questions

1) What is the logic (if any) behind arguing that it is safe to these switches on a DC circuit? Is it that 12V is so much lower than the 125/250V AC rating?

2) what does he mean by "not the other way around"? Do DC switches not work with AC current?

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The vendor is dangerously wrong. For a given current rating the maximum safe DC voltage will be much lower than the maximum safe AC voltage - usually by a factor of 10:1 or more.

At very low voltages (say 12VDC or less), switches designed for AC mains use will "probably" safely handle DC currents equal to their AC current rating.

Arcs form when current is interrupted OR when voltage is applied to an airgap that is below a given dimension, the dimension being affected by immediately prior history (say less than a few hundred ms) and some other factors.

With AC waveforms the voltage is zero twice in each cycle so any arc that has formed is extinguished at the zero crossing*. For an arc to continue to exist as the switch progressively opens the arc has to reform after each zero crossing, with a progressively larger gap each time. Ionisation of air from a previous arc complicates matters but arc voltage is very roughly proportional to airgap. In an opening switch an AC waveform tends to self extinguish at currents up to and somewhat above the design rating. "Somewhat" is set by the manufacturer and regulatory requirements.

  • Inductive loads complicate AC switching arc performance as the current and voltage waveforms are offset, and energy released from an inductive load can generate voltages far in excess of those being switched. Inductive considerations are too complex for easy treatment here, but very inductive loads on AC or DC may require special energy absobtion methods to control arcing during switching.

When DC is switched the voltage is applied continually and, unlike AC, there is no mechanism based on waveform to extinguish the arc on a regular basis. Once an arc has formed it continues as the gap widens and is only extinguished when the air ionisation and air currents formed thermally mean the arc can no longer be supported. For a given fully-open airgap the DC voltage at which an arc will not be sustained is very much lower than for AC.

Arc extinguishing is somewhat of a black art and can be assisted by application of magnetic fields and electrode design such that arcs ascend up a widening gap due to thermal conditions until they self extinguish. Such techniques are not usually employed in switches used in AC mains operated of "extra low voltage" DC equipment.

The switches that you were supplied were almost certainly suitable for 5V switching applications up to the rated AC current, but the supplier was still very remiss in ntheir claims.

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When the switch opens, there is a small arc that occurs. AC is self-extinguishing since the current goes to zero 120 times every second. DC does not so the switch has to survive until the arc dies down on its own. So the DC switch has to be hardier and will therefore work with AC. However, the AC switch is not as hardy and should not be used with DC.

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    \$\begingroup\$ However switches often have both AC and DC ratings. These switches might be designed for both 250V AC and 12V DC. 250V DC would be a problem, but s/he's not doing that. \$\endgroup\$ – immibis Mar 1 at 1:41
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Actually, the vendor got it backwards. All other things being equal, DC switches have better arc-extinguishing capability than AC switches, because AC switches can rely on the fact that the current goes to zero twice on every cycle anyway.

Therefore you can use DC switches on AC, but not always vice-versa. Of course, you must stay within both the voltage and current ratings of any kind of switch.

The switches he sent you were massively over-rated for your application anyway; they would have been fine to use.

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  • \$\begingroup\$ As I understand it AC switch will work in with DC in the sense that the current will flow, it just that the AC switch may not be designed to deal with the potential for arcing with DC. So it is a safety thing due to the risk of arcing/shorting out. So are the answers: 1) there is no logic, and 2) the vendor is wrong? \$\endgroup\$ – Andrew Staroscik Mar 1 at 0:39
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    \$\begingroup\$ @AndrewStaroscik You're correct. It's about interruption, not conduction. It wears the switch out and can weld the contacts. Same holds true for relays BTW. The answer is the vendor is wrong. AC only works with AC. DC works with both. \$\endgroup\$ – DKNguyen Mar 1 at 0:42

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