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Big toggle switches: estimating its DC rating from its AC rating.

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astronomerroyal

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I wish to put a large toggle switch on a power carrying line (from a bicycle generator + charge controller setup). The line typically carries 120W (12V and 10Amps) so I need a toggle switch that can toggle that sort of *DC* power flow. Yet they all seem to have only AC ratings.

The loads I'll be driving aren't necessarily purely resistive, and may involve short inrush currents of up to 20 Amps, perhaps. Also it won't be toggled very often.

I have a switch rated at 6A 125VAC, do you think this is adequate in the DC realm?

I found this curiously enthusiastic document about switch ratings,

www.aeroelectric.com/articles/Switch_Ratings.pdf

but thought I'd get your opinions also. Thanks in advance.
 
The current restriction is when you switch it OFF, not ON - as the contacts open, an arc forms, with AC this arc vanishes as the AC drops to zero - with DC it continues until the gap is wide enough for the arc to extinguish naturally. So a switch can turn OFF higher AC currents than DC, the actual curent carrying capacity is the same.
 
Thank you both.

Mr/Dr Goodwin, that's an interesting point. Presumably AC assures that the arc's lifetime is less than 1/120 s, for 60Hz... Still, I'd have thought that the switching ON phase would suffer the same effects.

Ken, the mouth-watering switch with the LED is worth a trip to RS (which I see was recently robbed by armed assailants - let's hope no people were hurt or switches stolen.)
 
Thank you both.

Mr/Dr Goodwin, that's an interesting point. Presumably AC assures that the arc's lifetime is less than 1/120 s, for 60Hz... Still, I'd have thought that the switching ON phase would suffer the same effects.

Ken, the mouth-watering switch with the LED is worth a trip to RS (which I see was recently robbed by armed assailants - let's hope no people were hurt or switches stolen.)

I'm gonna take a kick at the can and Nigel can smack me down if I goof up. :) As I understand it, the Off arcing is more important since as the contacts move apart, the distance between them starts out small and increases, so when the gap is small, the arc is easier to start. Since it's easier to maintain an arc than to start one, this lets the arc continue to sustain longer than bringing the contacts together does. When coming together, the contacts have to get quite close to start the arc--much closer than the arc length which can be sustained when pulling an arcing pair of contacts apart.

The effect is easy to see with a stick welder. You have to get the electrode right close to the workpiece to strike the arc, but once it's going, you can pull the electrode back quite a way and still maintain the arc. For instance, once I've got an arc established, I can pull back about 2 cm and keep the arc going, while there is no way I can strike an arc from 2 cm away--I have to get down to within millimeters to strike it.

Make any sense?


Torben
 
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That arc welder from Torben is actually a very good example even while it is an ac load but it certainly visualises the principle of an lot longer arc when trying to brake the electrical circuit.

We have similar issues with manual switches on the HV power network. You can close switches direct, but under a fault you can not immediate break the electrical circuit. The handle needs to be turned over before an opening of the switch is allowed, by then the feeder would have tripped anyway.

DC is continuous, and a lot bigger gap is required to extinguish the arc, otherwise the contacts can weld together under certain circumstances.
 
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That arc welder from Torben is actually a very good example even while it is an ac load but it certainly visualises the principle of an lot longer arc when trying to brake the electrical circuit.

How did you know my welder is AC? :) Most good stick welders have the option of AC or DC, but yeah, mine is AC only.


Torben
 
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why not switching this load with a mosfet
no problems with arcs no problems with inrush curents (the types that handle 10 a 15 Amp will not be in the danger zone if the inrush current is 2X bigger)

Robert-Jan
 
Torben

Just a lucky guess I think.

It didn't even cross my mind that some welders have a dc option as well.

Regards, Ray
 
I see, the welder analogy was good indeed. I was blinded by an obsession with 'microscopic time-reversal symmetry.' Is the arc 'sustained' in the same way that secondary strokes of lightning follow the lowest-resistance path of the initial stroke?

I considered a power mosfet - although I've never used one, and don't own any. The big toggle was amongst an assortment I bought on ebay. Off topic: I do want to eventually make a 12v heated bird-bath, with thermostat and night-day sensor, using a microcontroller. An ideal application for a MOSFET switch I imagine. To heat the water at 12v, I'd have to run at a large current (>10A) since commercial examples seem to be ~150W. I'd need a 1 Ohm heating element. Any recommendations for such a thing, or is that impracticably low?

This turned out to be an interesting discussion. Thanks.
 
What is the mechanical construction that you have in mind

if you have a bird wash bath with some metal incorparated i would say don't look for the heater element on 12v (very hard to find and if you find one it's going to be expensive)

try some hi wattage resistors that heat up the metal part

ceramic can also be heated but takes longer to heat up

Robert-Jan
 
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