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I'm in the process of finding a suitable relay for switching the elements in a mini oven I want to use for solder reflow.

I've found what I think will be a good candidate, however I'm a bit confused by the maximum power ratings.

Here's the relay in question: http://www.rapidonline.com/Electronic-Components/Mi-ss-106l-6v-10a-Spdt-Relay-60-4598

Das Datasheet: http://www.rapidonline.com/pdf/60-4598.pdf

The ratings state a max load of 2500VA / 300W. The 2500VA seems to be an AC rating @ 250V and the 300W is for DC, but even taking that into consideration, the disparity between 2500VA and 300W seems huge?! I know that the RMS voltage of 250VAC is considerably less (circa 160V) but @ 10 amps that should still be around 1600W?!

What's going on here, can someone explain this rating disparity in greater detail?

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  • \$\begingroup\$ The 250 VAC itself is the RMS voltage! Unless explicitly stated as Vpp it's can be presumed that the value is RMS. \$\endgroup\$
    – Kathir
    Dec 17, 2020 at 2:54

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When the relay opens it will draw an arc. With AC power the current drops to zero 100 or 120 times per second (depending on whether you have 50Hz or 60Hz power), and this will allow the arc to extinguish. With DC power you don't get this automatic interruption, and an opening arc may last longer, burning the contacts in the process. That's why relays are allowed to switch only a fraction of the AC power if DC.
It's not uncommon to see 250V AC relays only rated for 30V DC.

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  • \$\begingroup\$ Thx very much, that explains a lot! Arcing eh, I'd have never thought about it - what a crazy phenomena :) \$\endgroup\$
    – Jim
    Nov 13, 2011 at 11:47

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