Current draw of cab clearance lights?

[cojhl2]

Hmm, that's interesting. The package the switch came in (Radio Shack) said it was good for 3 amps @125VAC or 1 amp @250VAC. So I figured there was some kind of inverse relation between the voltage and the amp rating.

CTD12V

What they are saying is the switch can carry a given amount of power. P=EI.



So if the voltage doubles then the amperage must decrese by 1/2 to keep the power constant.



Then if this is true, it should carry 125/12*3 amps in a 12 system right??



That does not seem correct for sure.



Somebody get on here and splain this to me ooohhhhhh!!!!

 

[CTD12V]

What they are saying is the switch can carry a given amount of power. P=EI.



So if the voltage doubles then the amperage must decrese by 1/2 to keep the power constant.



Then if this is true, it should carry 125/12*3 amps in a 12 system right??



That does not seem correct for sure.



Somebody get on here and splain this to me ooohhhhhh!!!!

:-laf I guess this is like the blind leading the blind, huh?



Ok, it is a given that a switch can only carry a certain amount of power before it overheats, shorts, or something. I was assuming this was based on the internal resistance of the switch based on its construction, materials, etc. That's why I was looking at the Ohm side of the equation. However, if Wattage is the key to this problem . . . Aha! It now makes much more sense when I solve for it.



P = E*I So since this switch is rated for (I) = 3 amps at (E) = 125 volts, then it is rated for (3*125) = (P) = 375 watts.



So changing (E) to 12 volts and solving for (I):



375 = 12 * I



375/12 = I



I = 31. 25 amps



Also, as KAlder pointed out, I guess the voltage is closer to 14 Volts when the alternator is charging, so if I substitute that value it comes out to be about 26. 7 amps.



Anyways, this way of determining the rating makes perfect sense to me. And usually, when something makes perfect sense to me, I'm wrong. :{ So if somebody knows the real answer, PLEASE speak up!!



CTD12V

 

[cojhl2]

Well see CTD12V this is the problem,, I don't think that mini switch will carry 30 amps.



We gotta have help!!!!!!

 

[Thomas]

. . . this is like the blind leading the blind, huh? . . .

Perhaps the only true statement in the past few post!

Just use the switch—it's way overkill for your intended purpose.

The current carrying capacity of the switch has essentially nothing to do with the current in the circuit and any current in the circuit you plan to interrupt is so slight its measure would best be expressed using milliamps.

 

[cojhl2]

Perhaps the only true statement in the past few post!



Just use the switch—it's way overkill for your intended purpose.



The current carrying capacity of the switch has essentially nothing to do with the current in the circuit and any current in the circuit you plan to interrupt is so slight its measure would best be expressed using milliamps.

Well that is true, but doncha see the thread has moved to a request for knowledge beyond this one simple application.



What is the rule on this switch and how does the rating work? 120V w 3A and 240V w 1A. This cant mean 12V w 30A can it?

 

[CTD12V]

Perhaps the only true statement in the past few post!



Just use the switch—it's way overkill for your intended purpose.



The current carrying capacity of the switch has essentially nothing to do with the current in the circuit and any current in the circuit you plan to interrupt is so slight its measure would best be expressed using milliamps.

That's what my gut feeling tells me. I'm going to go measure the amps in the circuit today just to satisfy my curiosity, but like cojhl2 stated, we want to know how to actually calculate the ratings for future applications where the amp loads might be a little more critical. So can ANYBODY PLEASE tell me in actual concrete equations or something, where I'm making errors in my calculations in the last post?



BTW, I ended up using a 20 amp 12VDC rocker switch from Autozone for the cab light interrupt switch that I was originally asking about in this post. I have no worries about exceeding that rating with 5 cab lights. However, I prefer using as small of switches as are safely possible, so in future applications I would really like to know how to calculate switch ratings for my particular purpose so I have more options than the bulky, sometimes cheesy switches at the auto parts stores.



Thanks.



CTD12V

 

[Thomas]

. . . So can ANYBODY PLEASE tell me in actual concrete equations or something, where I'm making errors in my calculations in the last post? . . .

There have been no suggestions that you're making any errors in your calculations.

You're using the formulii inappropriately plus entering bogus information in the formulii.

 

[CTD12V]

You're using the formulii inappropriately plus entering bogus information in the formulii.

Ok, I should have been a little more inclusive in my request. Thanks for telling me--That's the kind of thing I want to know. I suspected I was misapplying the formula, but I just would like to know how I'm using it inappropriately, and what the correct formula to use would be. The fact that the switch has a different amp rating for two different voltages implies that there is SOME kind of relationship between the two factors, and the wattage formula appeared to me to be the best way to correlate them.



So far the breakdown of Ohm's Law on The12Volt website is the most info I've been able to find. I've searched and searched and can find no info on how a rating for a switch's capacity is derived, or how to determine the advisability of using a particular switch for an application involving different parameters than how they tested it. There HAS to be some kind of a formula that relates the various factors involved in a switch's use. I just can't find them, and so far nobody has told me any particulars.



I hate to keep bugging people about this, but surely somebody can give me a short rundown of the basics that will allow me to figure it out on my own, without them having to spend an hour explaining it.



FWIW, when I tested the "headlights on" wire a bit ago, I got a reading of 570 on my multimeter at the 2000 microamps position, so I am making what appears to be a dangerous assumption (based on my track record so far ) that my switch is carrying 570 microamps.



Thanks,



CTD12V