Current draw of cab clearance lights?

[CTD12V]

Ok, this a stupid question, but how do I find out the current draw of the cab clearance lights? I want to install an inline switch to be able to turn off the clearance lights while the headlights are on if I need to, but I don't know how much current the switch needs to carry (or margin of safety in the rating, for that matter). The clearance lights use 194 bulbs, but I don't recall seeing an amperage spec on the package or in the factory service manual.



Thanks!



CTD12V

 

[WDaniels]

Why turn them off? Their draw is almost nothing. If you want to reduce what draw there is, convert to LED's. I did and they are a snap to change.

If you still want to determine their amp draw, use ohms law to caculate it.

WD

 

[DrDiesel]

It`s negligible. Run a 20 amp switch and you be fine.

 

[Philip]

A chart I found says the 194's pull 1. 25 amps at 12 volt.



http://www.optronicsinc.com/liteco9.htm



Scroll down to the bottom of the page.

 

[CTD12V]

Good deal. Thanks for the link Philip.



WDaniels, my electrical knowledge (as far as calculating current is concerned) is somewhat lacking, so that is why I want a way to determine how many amps I was dealing with so I wouldn't be skimping on the components. I erred on the side of overkill when I installed the clearance lights and used 14 gauge stranded wire. I will see if I can find out more about Ohms Law, since I want to be able to calculate the draw on some other circuits as well.



I'm not worried about the current draw of the lights in and of themselves, I just like to have the option of turning them off for troubleshooting sake and to be able to flash them in greeting/farewell if I want. I want to use as small of a rocker switch as possible on my dash, but didn't want to install a switch too small to handle the current draw.





Thanks for the replies.



CTD12V

 

[Philip]

Just use a small switch and use it to control a relay. Most auto style relays have a 30 to 40 amp rating. Then your not concerned with the switch rating. It take maybe 50 milamps to fire a relay.

 

[CTD12V]

Yeah, I thought about going that route too, just figured a single inline switch was easier. I guess now I just have to look for switches.



BTW, can you get an automotive relay that is normally closed, rather than normally open as seems to be the case with the Bosch relays I have pulled out and looked at in the PDC?

 

[cojhl2]

My understanding of DOT rules say if you have a light it must work and be on when it is proper to be on.



Why would you want to turn them off?



Edit: Oops I went back and reread,, re turning them off!!

 

[Thomas]

. . . BTW, can you get an automotive relay that is normally closed, rather than normally open as seems to be the case with the Bosch relays I have pulled out and looked at in the PDC?

The Bosch micro relays I've seen in the Dodge PDC's DO have a normally closed terminal.

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•• Click to Enlarge ••

 

[Philip]

The 5 pin bosch relays have a terminal that is normally normally open and one that is normally closed. You just pick the terminal you need to use.

 

[CTD12V]

The 5 pin bosch relays have a terminal that is normally normally open and one that is normally closed. You just pick the terminal you need to use.

Yeah, now that I think about it, I guess I remember seeing both. For instance, I think I recall that the trailer lighting relay functions as normally open, and the relay I modified in the high beam/fog lamp interlock used to function as normally closed (it's nice to have the fogs stay on with the high beams now ).

 

[WDaniels]

CTD12V,

Here is a real good site that explaines ohms law and on how to find everything out about current, resistance, voltage, wattage etc but were afraid to ask. http://www.the12volt.com/ohm/ohmslaw.asp

WD

 

[KAlder]

amps x ohms = volts. measure resistance at clearance lamp common power to ground (w/ circuit dead). take the ohms from your reading and divide by 12(12v system), there is your amperage. but when the alternator is charging you have more like 14 volts, but that will cause amperage in the circuit to go down. that is why low voltage kills starters, takes more amps for the same job at lower voltage.

 

[cojhl2]

amps x ohms = volts. measure resistance at clearance lamp common power to ground (w/ circuit dead). take the ohms from your reading and divide by 12(12v system), there is your amperage. but when the alternator is charging you have more like 14 volts, but that will cause amperage in the circuit to go down. that is why low voltage kills starters, takes more amps for the same job at lower voltage.

Didn't you mean go up KAlder. The higher the voltage the higher the amps for a given resistance.

 

[Thomas]

amps x ohms = volts. measure resistance at clearance lamp common power to ground (w/ circuit dead). take the ohms from your reading and divide by 12(12v system), there is your amperage. but when the alternator is charging you have more like 14 volts, but that will cause amperage in the circuit to go down. that is why low voltage kills starters, takes more amps for the same job at lower voltage.

Huh? Using your method, as the resistance (ohms) increases with a constant voltage the current (amperage) INCREASES! I don't think so! Also, the resistance of the filament will increase drastically as the filament goes from the ambient temperature to white hot making the cool filament temperature useless with a proper application of Ohm's Law.

A fairly accurate measure of the current flow through the filament would be to divide the wattage of the bulb by the applied voltage—somewhere near 14 volts. The General Electric catalog shows all of their 194 bulbs as 4 watt bulbs. This yields something less than a third of an amp per bulb. This is in agreement with Sylvania's ". 27 amp" catalog listing for their 194 bulbs.

 

[KAlder]

sorry i got my algebra backwards, yes the amperage will go up. should have divided voltage by ohms, not ohms by voltage. oops! maybe what i learned in the electrical course may have worked better for me if i hadn't dropped out of math!!

 

[CTD12V]

sorry i got my algebra backwards, yes the amperage will go up. should have divided voltage by ohms, not ohms by voltage. oops! maybe what i learned in the electrical course may have worked better for me if i hadn't dropped out of math!!

:-laf :-laf :-laf That's the kind of thing I do!



Thanks for all the replies everyone. This is good info (for me, at least).



Now for another stupid question. Can I use Ohm's Law to find out the 12VDC rating for a mini switch rated for 3 amps at 125VAC? Or is there some other factor since it is changing from AC to DC. (BTW, this switch would be for a separate application, not taking the full load of the clearance lights. )



Thanks! Keep it coming.



CTD12V

 

[CTD12V]

I guess to be a little more specific on that last question, I assume that a switch's current rating is based on its resistance? So if I can find the resistance of the switch, I should be able figure out the amperage rating at a given voltage?



However, the formula Amps=Volts/Ohms doesn't work since it gives the current in circuit, not the current rating of the switch, so maybe I'm totally off in left field here.

 

[cojhl2]

If they rate at 3 amps it's 3 amps AC or DC regardless of the voltage.



Hope this helps. .



If in question go big!!

 

[CTD12V]

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.



I want to use this switch for interrupting the "headlights on" signal to the overhead console display which causes it to dim too much to be able to read it easily if my headlights are on when it's not totally dark outside. I doubt that particular wire carries 3 amps anyway, and I have tested the switch breifly in the circuit and it doesn't seem to be getting hot, but I don't know for sure how much current it is carrying. Can I put a multi meter inline in the circuit and test the amperage when the headlights are on? My DMM is a cheapie from Checker Auto Parts, so the instructions on it are pretty pitiful.



I appreciate you guys help!



CTD12V