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Something that confuses me is why AC and DC relays are not swappable in an application.

For example, I am looking for a 12V (DC coil voltage) relay that its contacts can withstand cold-switching of a 40V/2A DC signal. I found some RT 314 SPDT 12V relays in the lab bin, and I want to use them for this DC application. But nowhere in its datasheet, it has mentioned anything about contact specification in a DC application. Does this mean this relay can not be used in any DC application?

enter image description here

I mean if they can switch 16A,250VAC....why can't they switch 40V/2A DC?

Isn't the relay contact just a conductor?

Thanks to @SteveG This is the DC-LOAD graph in the datasheet that I did not spot myself: enter image description here

If not, Do you know a pin compatible relay that can be used in a DC application 40V/2A contact rated?

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    \$\begingroup\$ You shouldn't use them for DC beyond their DC ratings. But while I can't recommend you use them, 2A is much less than 16A so they might survive. Consider the implications of their failure. Will something catch fire, unattended? Is somebody always watching, and does he have a separate OFF switch? Is this a one-off prototype, only used in the lab, or are you selling it to a litigious customer? If you're selling it, you probably shouldn't be raiding the lab bin! \$\endgroup\$ Aug 24 '17 at 12:53
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    \$\begingroup\$ Further down the data sheet is a graph which plots Max DC load breaking capacity against voltage for a purely resistive load. At 40V it can break just a little over 2A. Its up to you if you want to use it so close to its limit. \$\endgroup\$
    – Steve G
    Aug 24 '17 at 13:09
  • \$\begingroup\$ @SteveG Wow thanks for spotting that graph....now I get the idea...I am very bad at reading datasheets :( \$\endgroup\$
    – Sean87
    Aug 24 '17 at 13:12
  • \$\begingroup\$ @BrianDrummond Thanks, you almost pointed out every possible failure case...luckily this is a one-off prototype. \$\endgroup\$
    – Sean87
    Aug 24 '17 at 13:13
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    \$\begingroup\$ Then, if you use it, DOCUMENT the hack so that it can never accidentally be copied into production. \$\endgroup\$ Aug 24 '17 at 13:14
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A relay is not just a conductor, it's a switch. When opening and closing a switch under load, there will be an electric arc. When switching off a AC load, the arc will go away in the next zero-crossing. With DC loads, the arc stays there longer. Therefore, most relays have lower DC ratings if they are rated for both, AC and DC.

This relay will probably work with DC loads, but due to the missing rating, you don't know how long it will work under which conditions.

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enter image description here

As Manu310us said... it's all about arcing. Using the wrong relay with a high DC current can quickly set your relay on fire.

Nothing demonstrates this better than this video...

So don't use an AC relay for switching DC loads. Using DC relays for switching AC loads is much less problematic, but you can usually find a cheaper AC relay which does not include the mechanical arc interrupter mechanism.

NOTE: the same rules also apply for mechanical switches.

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    \$\begingroup\$ Nice demo, makes it easier to belief the facts stated in the datasheet. \$\endgroup\$
    – Manu3l0us
    Aug 24 '17 at 14:36
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    \$\begingroup\$ That arc scared the hell out of me...I'll look for a rated relay for my application...to hell with AC relays :D \$\endgroup\$
    – Sean87
    Aug 24 '17 at 17:24
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When relay contacts make or break a circuit there is an arc.

When AC current makes or breaks the arc - the arc sometimes happens near the zero crossing of the sine wave, which equals minimum arcing. When the make or break happens above or below the zero crossing the arc trades a small amount of metal between the contacts. Where the arc occurs randomly during the positive and negative part of the sine wave the trade of metal between the contacts goes back and forth.

When D.C. Current makes or breaks in a relay contact there is most always an arc ( no zero crossing). The arc and trade of metal on the contacts is always in the same direction - one relay contact loses metal and that metal builds up on the other contact.

Well designed systems that use D.C. Relays - will try to close the relay before conducting high currents. I see this practice in high horse power D.C. Drives.

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Judging by that graph, you are working right on the limit. You could put a snubber (capacitor with a bit of resistance in series to limit discharge current when closing) across the contact. You can even shunt the resistor with a diode so it is directly across when opening, turning the arc into a relaxation oscillator & extinguishing it, then discharges through the resistor to protect the contacts. I did that on a piece of equipment that was switching 110Vdc and burning the relay up, and it worked. Probably about 0.1 to 0.22uF & about 2.2R to 3.9R (or more if you use the diode).

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  • \$\begingroup\$ I've seen 250VDC circuits that use back-to-back Normally-Open contacts. This is like a logical AND gate that could be simplified. (IF B AND B then energize C.) It looks odd and pointless, but the reason for it is to have multiple places where the DC current gets opened so that contact wear is minimized and there is less arcing. That's one way of dealing with persistent arcing. \$\endgroup\$
    – Kate Moon
    Jun 9 at 6:27

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