I have a push button make/break switch rated at 2 A, 48 VDC. I want to use it on my aircraft in an 8 A, 12 VDC circuit. Can I use this switch in this circuit or will the switch burn up?
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1\$\begingroup\$ Question regularly answered, for example in electronics.stackexchange.com/questions/624438/…. \$\endgroup\$– Mister MystèreSep 10, 2022 at 23:36
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2\$\begingroup\$ Note that the current rating applies when the switch is closed, while the voltage rating applies when it is open. You don't multiply these, since they never happen at the same time. \$\endgroup\$– DarenWSep 13, 2022 at 1:08
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\$\begingroup\$ @MisterMystère I've closed that one as a duplicate of this one since the other one has a negatively scored accepted answer. \$\endgroup\$– Null ♦Sep 13, 2022 at 16:52
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5 Answers
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tl; dr: nope. The switch will fail.
I think you’re misinterpreting how switch ratings are derived. While the two proposals would appear to handle similar amounts of power (96W), switches are specified like wires: they’re rated for their insulation strength and current-carrying capacity.
Switch voltage rating means its insulation strength, both in the switch body and between its open contacts. That is, the max voltage that won’t result in arcing.
Switch current rating is based on acceptable heating in the switch when carrying current. That is, the max current at max ambient temperature that won’t result in the switch burning up.
In your case, since you’re switching only 12V, it’s the current rating that really matters. 8A is much more than your proposed 2A switch can handle: it will heat up and fail in short order. Further, the low-rated switch will have higher IR drop in it than a higher-rated one, robbing your load of adequate voltage.
To be safe, choose a switch that can handle at least 2x the worst case load, that is, 16A. Then you’re assured that if there is a short, it will be the fuse that goes and not the switch.
If your concern is panel space, you could use a relay, much like automobiles do for headlamps. Then you can use your 2A switch to control the relay, which would switch the 8A load.
If you’re unsure about any of this, and even if you aren’t unsure, consult a qualified aircraft mechanic or avionics technician.
answered Sep 8, 2022 at 0:17
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No, you can't use that switch in that circuit; both current and voltage ratings need to be satisfied separately.
The voltage rating has to do with isolation and contact arcing, the current rating has to do with heating up, so you can't just multiply the two, arrive at a wattage, and take it from there.
If the current in the circuit is higher than 2 A, the switch will get hot and (eventually) burn up. At 8 A, it will be gone fast.
BTW: In an airplane, you really shouldn't use that switch in that circuit.
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6\$\begingroup\$ I assumed it was a model airplane, but now that you mention it, it could be a real airplane... \$\endgroup\$ Sep 8, 2022 at 15:32
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If the circuit is really using 8 Amperes, then a 2 A switch will definitely burn, likely within seconds.
If the circuit is instead only using 8 Amperes for a split second, then 3 A for the remainder of the time, then the switch will slowly overheat and then burn. It could take minutes or weeks, but it'll burn. It will burn before any other failure mechanism; see following.
If the circuit is instead only using 8 Amperes for a split second, then 1 A thereafter, then the switch will work, but it will eventually fail, probably with its contacts welded closed.
Switch specifications (all of them) must be individually met, or else the rated number of actuations cannot be achieved. Using a 4x higher-voltage switch does not imply that it can switch 4x the current at lower voltages - these are all separate specifications.
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Switch ratings must be satisfied simultaneously. I.e. rated current must be equal to or greater than actual current* and rated voltage must be equal to or greater than actual voltage. That switch is not suitable for your application.
*Minimum contact current is a thing but is not applicable in this case
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Switches that are rated like this, in fact, do exist.
It is normal for a switch to be rated e.g. 12V / 5A and at the same time, e.g. 24V / 3A or something like this. The same switch can as well be also rated 125V AC / 10A.
(It is very much normal for a switch to be rated higher for AC and lower for DC.)
On the other hand, one cannot safely assume any "reasonable" derating of a switch other than what the datasheet says. You do have read the datasheet, didn't you?
Absent this, the only possible derating of a switch is "at most the voltage written on it, at most the current written on it and be careful for currents constantly less than 5% of the rated current".
If the switch has two ratings for different voltages, the rule is applied to any V/A combination separately.
If the switch is rated at AC only, do not assume anything about DC (the reverse usually is OK).
p.s. to save the next question, this whole thing applies to the relay contacts as well.
p.p.s I profoundly hope that the aircraft in question is a model one that carries no pilot or passengers.
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1\$\begingroup\$ Those varying specs reflect the difficulty of interrupting DC at higher voltages due to arcs. This has no bearing on thermal based ampacity. In a real plane, use FAA approved gear if you want to keep your license. They would find out post-accident, which means you are defending a mistake that did cause an accident. \$\endgroup\$ Sep 8, 2022 at 23:37