Current draw question

[davidz71]

I have a question for the electrical wizzards on the forum. I found the following formulas for watts, volts, amps regarding current draw:

watts= volts X amps

amps= watts
volts

While researching led replacement bulbs for replacing my 1141 bulbs in selected fixtures and in lieu of my 3 flourescent fixtures, I have seen some flourescent fixtures with two 8 watt bulbs (16 watts) rated at 1.3 amps draw and others rated at 1.5 and 1.6 amps. According to the formula above it should be 1.33 amps (they must have rounded down) but why the 1.5 and 1.6 amp figures? Could this be a typo, the result of less efficient ballast for the higher rated amp fixtures or the bulbs themselves? Are these formulas set in stone or is there a fudge factor involved? My inquiring mind wants to know.

As an afterthought, I have also seen the 1141 bulb 18.4 watt bulb rated at 1.44 amp draw and 2.1 amp draw which is what the 1156 26.9 watt bulb is rated for. Is this just lack of attention to detail in the printing of this information?

[nangoff]

Craig, I'd buy a couple and series them up with a ammeter and see what you get. Don't know exactly but possibly the assembly could pull higher wattage till the components warm up so the vendor has to put the higher ratings on to size supporting equipment (fuses, wire, etc.) GG

[Excella CM]

Use this: W = IxIxR

[nangoff]

W=IxIxR is the same as current times volts. Also, "R" is very difficult to measure while under load so that's why most folks use IxE.

[nickcrowhurst]

Craig, the equations are accurate. The 1.33 amps is the theoretical current when 12 volts is applied to a 16 watt resistor. However, the voltage from a charged "12 volt" battery is way over 12 volts. Ohm's Law tells us that for a given resistance, current is proportional to the applied voltage. The resistance in this case is 12 volts divided by 1.33 amps, which equals 9 ohms. As a charged 12 volt battery will be at about 12.7 volts, the current will increase to 12.7 volts divided by 9 ohms, equals 1.4 amps. I suspect that some manufacturers are rating the bulbs for the real world of RVs, where 12 = more than 12! That enables RV manufacturers to rate their wiring and breakers for the higher current. Just a guess on my part. Nick.

[davidz71]

Thanks Nick. I hadn't thought about the fact that my battery when fully charged is more like 12.7 volts.

[john hd]

craig

also keep in mind most auto bulbs are really 14 volt when comparing wattage.

they are just called 12 volt to not cornfuse people.

john

[davidz71]

Originally Posted by john hd

craig

also keep in mind most auto bulbs are really 14 volt when comparing wattage.

they are just called 12 volt to not cornfuse people.

john

Hi John,
That also makes sense because the alternator guage on my t2500hd is always showing over 14 volts even though I know they aren't the most accurate things. Well, I do know that I am using less current from the leds and that is what I was after. One more fixture with dual 48 SMT leds and I'm through replacing 1141 bulbs.

[nangoff]

Craig, It would be good to find out, if you can, what the maximum ambient temperature the LED's can operate at. LED's are skiddish on elevated temperatures, they don't like it...so before you invest a lot of money on them its best to find out all of their charactoristics; trailers can get hot some times and when you turn the LED's on while hot they may fail. Making matters worst they don't like elevated voltages either (increases operating temperature), therefore some include voltage regulators to thwart this.

Gerry

[markdoane]

I think the difference in amp draw may be due to differences in the type of driver used to power the lamps. If the lamps use a high efficiency push-pull transistor inverter, the power draw would be the lower rated 1.3 amps; if a transformer style ballast is used, it would dissipate the extra 2 watts as heat, and those two watts would increase the amp draw to 1.6 amps.

So I'm backing your "less efficient ballast" theory.

[john hd]

Originally Posted by davidz71

Hi John,
That also makes sense because the alternator guage on my t2500hd is always showing over 14 volts even though I know they aren't the most accurate things. Well, I do know that I am using less current from the leds and that is what I was after. One more fixture with dual 48 SMT leds and I'm through replacing 1141 bulbs.

craig,

you would have some fun up here in the great white north! there are mornings when i leave for work with 2 batteries that are around -10 degrees i get voltage readings upwards of 15 volts!

when they warm up a bit under the hood they drop down about a volt or so. don't under estimate the voltmeters on the newer trucks, i have checked mine against my simpson and a el cheapo DMM and the one in the truck is prety close.

any work with batteries as far as voltage and current must be compensated for tempature.

this will give you a chuckle if you are into old school electric theory. also from a wisconsinite!

Automotive Ignition Systems - Google Book Search

john

[davidz71]

John,
I scrolled through the first 20 pages of the link to the book and thought about my first year of engineering at Vanderbilt. Now I remember why I got out of engineering.

[davidz71]

Originally Posted by nangoff

Craig, It would be good to find out, if you can, what the maximum ambient temperature the LED's can operate at. LED's are skiddish on elevated temperatures, they don't like it...so before you invest a lot of money on them its best to find out all of their charactoristics; trailers can get hot some times and when you turn the LED's on while hot they may fail. Making matters worst they don't like elevated voltages either (increases operating temperature), therefore some include voltage regulators to thwart this.

Gerry

Gerry,
I have 5 led bulbs of various type now. I'm going to watch them carefully as to the amount of time I run them and general temperature. The one over the stove will stay on more than any of the others so it will get the real workout.