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Author Topic: Switches: SPDT and DPDT  (Read 3089 times)

MaxamusCrasious

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Switches: SPDT and DPDT
« Reply #10 on: March 19, 2003, 04:45:05 PM »

Just don't mix ac and dc simple as that

SteveyG

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Switches: SPDT and DPDT
« Reply #11 on: March 19, 2003, 07:49:47 PM »

Quote from: Stonewall
Quote
Rogue_Weasel wrote:
Hmm, i was about to ask about this, handy. Now i just have to find out if it's ok to use switches rated for ac w/ dc...


Yes that will be fine. The only thing you need to look for is the maximum current rating and the maximum voltage rating. If you don't exceed either then you're fine using it with AC or DC.

Sory to correct you but for DC on an AC rated switch you are wrong. THe DC ratting is about 1/3 the AC ratting. Why you say? well AC goes both sides of say ground and DC doesn't so you are already at 1/2 AC voltage and I have yet to see a switch that can handle 20A AC and 20A DC without fussing the contacts (well unless it is a really big switch).


It doesn't actually have anything to do with what you just said. Its more about the the differences between the arc produced when switching an AC load or DC load. Switching a high current AC load will produce a longer but less damaging arc during the time the contacts are slowly being separated, which at the same current and voltage with DC would cause a much shorter but more powerful, hotter spark (not arc) causing more oxidation and carbonation of the contacts, so the AC rating is usually higher. The material the contacts are made from are selected to be optimised for AC or DC, but for almost all purposes, a switch rated for say 10A@250VAC will easily be fine for 10A@30VDC.

You were correct to criticise, but at least do some kind of research. Your maths is poor. Remember when they state the AC ratings, they are talking about a sinusoidal waveform, so the RMS voltage is going to be 2^0.5, not a half.
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