DC vs AC Rated Switches

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Kyle Boatright

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I was reading about failed switches in Sport Aviation archives. The author said four out of five sport planes use AC switches instead of proper DC switches and the AC switches will weld the contacts.

I'm sure welded switches have happened. Never come across a first person account of one, even though I've been pretty active in the EAB community for almost 30 years.
 

rv7charlie

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Might be time to mention this again; the Aeroelectric Connection really should be on all our 'must read' lists before messing with electrics in a homebuilt.

There is good reason to have a clear understanding of the AC vs DC issue in switches. But, the blanket statement that AC switches won't work on DC is not a universal truth, and for our aviation purposes, with most switches, it barely applies. The issue about DC in switch contacts is the total energy in the arc as the switch opens. If you explore switch ratings, you'll find that many (obviously not all) mfgrs will give both AC and DC ratings for their switches. If you read carefully, you'll see that the AC rating will be for either 120 or 240 VAC at 'x' amps, and the DC rating will be for the same current at (typically) 24VDC. While a DC arc is harder to break than an AC arc, the massively reduced energy at <24V means that most any switch with an AC rating based on 120 or 240V can handle the same current at <24V, even if the spec sheet doesn't show it. Switches that don't have a 'snap' action (some rockers) will be more vulnerable to DC arcs than snap action, but most switches that have an AC current rating equal to our DC needs will be fine.
 

Mad MAC

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My understanding is that there is no way to de-rate* an AC rated switch to DC as the its dependent on the details of the switch design.
I seem to recall most DC switches use a more expensive blade material.

*Except maybe destructive testing or an EE degree and a few years of on the job experience.
 

rv7charlie

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The snipped text image is from the earlier-linked EAA article. The author starts with a 120VAC rated switch, then tries to make his case by talking about DC at 120V, then ups the voltage to 240 in an attempt to reinforce his point. But the most an a/c will see is 24V; the stuff most of us fly, 12V.

Garbage in, garbage out.
 

J.L. Frusha

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So, why aren't folks using capacitors across the poles, like on a points/plug/condenser ignition, to prevent arcing across the contacts?

..., but..., wtf do I know?
 

rv7charlie

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To be clear, if your goal is to just get a plane flying, and you're not that motivated by the 'education' side of homebuilding, you should use a DC rated switch. Problem solved. (That also applies to engine choice, avionics, etc.)

But understanding the engineering & physics behind switch design can be a useful tool, similar to what gave us the first 'fiberglass' homebuilts, and is currently giving us 3D printed parts.

I'd respectfully challenge 'like you're supposed to' (in both this specific and in general), unless 'supposed' is more clearly defined.

The better way to minimize arcing on DC contacts is with a reverse-biased diode, just as you see across the coil of a relay, but the reason you see them on relay coils but not on most other applications is to protect the switch from the release of stored energy in the coil when power is removed.

Aeroelectric Connection......
 
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