switch ratings theory

[meatshieldChris]

Trying to wrap my head around switches. I can't find a switch that fits my specific application, but I can find a switch that has a higher voltage rating, but a lower amperage rating.

So assuming it's rated for 125v, at 0.5A, does this mean that I can run it at under 62.5v at 1A? or 31.25v@2A? or 20v@3A (perfect)? Like, is it watts that you need to keep from exceeding as long as you stay under the max voltage?

 

[Mr Happy]

Two aspects of switch design are: (1) resistance and conductor size -- how much current the switch can carry without overheating when closed; (2) arcing and contact damage when opening or closing the switch.

AC/DC is also a factor in this, so for instance there is no such thing as a "125 V 0.5 A" switch; you must also observe whether this is an AC or DC rating.

The simplest rule of thumb is that the switch be rated for at least as much current as you want to pass through it, with at least the same voltage, and for AC or DC as specified.

You can relax these rules a bit under some circumstances, but it is not as simple as halve the voltage, double the current.

 

[lctorana]

Yes. Unless there are two sets of ratings (often you get 120V and 240V ratings or 240VAC and 12VDC ratings both for a switch), you really have to treat the two figures as absolutes, and not multiply them to "see how much wattage they can handle".

So for the 125V, 0.5A switch, the switch might arc if subject to more than 125VAC, and the contacts might burn out if subject to over 0.5A.

Excessive voltage and current are two separate problems.

For high-current, flashlight use, we need automotive switches, as they are the ones with high current ratings, and low contact resistance.

 

[greg_in_canada]

It also depends on the type of load. Some relays are rated at 40A for resistive loads, 30A for (incandescent) lamps, and 20A for motors.

Greg

 

[lctorana]

It also depends on the type of load. Some relays are rated at 40A for resistive loads, 30A for (incandescent) lamps, and 20A for motors.

Greg

Oh yes indeed. Derate for inductive loads. Good point.

 

[Wiggle]

So assuming it's rated for 125v, at 0.5A, does this mean that I can run it at under 62.5v at 1A? or 31.25v@2A? or 20v@3A (perfect)? Like, is it watts that you need to keep from exceeding as long as you stay under the max voltage?

Nope, they are really two separate specs. The amp rating is independant of voltage as the effect of .5A is the same heat-wise regardless of voltage rating. The voltage spec is more to do with the breakdown of insulating materials in the switch. You should try to keep the switch within both ratings, but for short periods it's usually safe to exceed current within reason.

 

[meatshieldChris]

ah, I understand. guess I need to figure out a MOSFET+low current switch then. Anyone have a link to setting that up for 8.4v/3A? Need some recommendations on the MOSFET and if any resistors are needed on the gate.

 

[sylathnie]

Simple FET switch -> http://www.candlepowerforums.com/vb/showthread.php?t=182482

I'm using this design in multiple lights with varying MOSFETS and resistors. It has held up very well.