i keep on popping fuses on my driving lights!

Yeah, hopefully within the next week! I already took a bunch of pics, I just need to start typing it up.

 

sweet! looking forward to it

 

No, when the voltage goes up, the current goes up. The light bulbs, when they are operating, have only a small fluctuation in resistance with filament temperature.

More volts will result in more current, which is why the output of the light goes up with higher voltage. That's also why putting too high a voltage through a bulb will blow it--the current goes up too, until the filament can't handle it and melts.

 

Did some reading, this is how I think it works.

Either I just gained a better understanding of ohms law, or I am going to embarrass myself.

As voltage goes up, so does current. At higher current, the filament glows brighter and hotter. As the filament heats up, it's resistance rises. Higher resistance will drop current back down.

Watts/volts=Amps 130W/14V=9.28A 130W/12V=10.83A

Volts=Current x Resistance if we solve for resistance we get Volts/Current=Resistance

14V/9.28A=1.5ohms

12V/10.83A=1.11ohms

That small change in resistance makes a significant difference in current draw.

I am open to a calm discussion. I am here to learn as well as help others.

 

Your logic is right, but I think your approach is slightly flawed. Your assuming that the lightbulb's wattage is fixed and therefore resistance is not fixed. Although true to some extent in real life (resistance does change slightly), for the purposes of understanding, assume that the light bulb's resistance is fixed.

The bulb's wattage rating is based on the system it is designed for. In other words, the 130W bulb rating is assuming a 12V source. Therefore, its representative resistance is 1.11 ohms at 10.83A. Now, assume that the resistance is fixed, perform ohms law again but this time using 14V, and you get 12.7A. Therefore, this bulb's wattage at 14V is no longer 130W but rather 178W and is therefore brighter.

Note: your approach is a quick and dirty way to determine maximum current draw for picking wire gauge and fuse size (I calculate the range from 8 volts to 14 volts).

 

As promised.... http://www.tacomaworld.com/forum/technical-chat/103969-electronics-basics-reference.html

 

Yep, I was thinking watts is a constant. It makes sense that it changes, since the same bulb is brighter at higher voltage.

What formula do you recommend that we use to determine max amp draw in an application like the OP is discussing?

I know from experience that a 25amp fuse and 12AWG wire works for a pair of 130W lights, so I'm not concerned about anything burning up with that setup. It bugs me not to understand how to calculate the actual amps draw. If I measure the resistance of the bulb with it off, will that be close enough to calculate amp draw using ohm's law?

 

Ok, here's how you'd do it... The two formulas are P=VI and V=IR. The power rating of the bulb (130W) is in reference to its operating voltage (12V). Therefore, from P=VI, we've determined that the current is 10.8 Amps. Now, using this new information, plug it into Ohms law (V=IR) to determine the resistance of the light bulb.... 12=10.8*R, solving for R=1.11 ohms.

Next, assume that the bulb's resistance doesn't change. Now we're interested in knowing the worst-case scenario for the current draw. This is important because as we all know, the battery voltage fluctuates as it gets old or charges/discharges. Assume that the battery could be anywhere between 8 volts and 14 volts. Therefore, using ohm's law (V=IR)... 8=I*1.11, solving for I=7.2 A. Now do the same for 14V... 14=I*1.11, solving for I=12.6A.

In conclusion, your bulb draws anywhere from 7.2A to 12.6A depending on the state of your battery voltage. Since you have two bulbs, multiply your largest number by two (12.6*2=25.2A), and this is total draw of both bulbs on the battery. Therefore, choose a wire and fuse sufficient enough to handle this load. You should use at least 14 gauge wire and at least a 25A fuse. Since your battery is rarely at 14V, your 25A fuse should be ok.

For more info, see my electrical basics thread (link in my signature).

Note: what you suggested would also work...you could measure the resistance of the bulb with an ohmeter and use that to determine the current change as a function of voltage change. But what I presented above is a mathematical way to determine current draw by simply knowing the bulb's wattage.

 

im just about to take em off the truck and smash these stupid lights with a hammer. im finally just gonna take it up to the shop and let them rewire them

 

Just had another thought op.

Have you opened the lights themselves up and looked for wiring connections shorted to the case (other than the ground wire)?

 

no never opend them up yet. what am i looking for again?

 

Follow the positive wire from the outside of the light to where it connects to the bulb. Look for any exposed wire or terminals that could be touching ground.

None of the terminals on your relay are touching your fuseblock? Looks kinda close in your pics.

This is not rocket science, there has to be a short somewhere. You should not be blowing a 30 amp fuse.

Keep on it man! It is time consuming to do a nice wiring job, and I hate to see anyone pay a shop to do it. It's so hard to know how nice of a job you are going to get. I've seen some crappy, careless wiring jobs that people paid way too much for.

 

i cant possibly think of any where else it would be imma go check right now...

 

maybe i dont have the relay hooked up right? what terminals go to what?

 

Very interested in such a thread. Would be very useful and appreciated. Thanks in advance. Gonna subscribe to this thread.

 

I posted the relay terminals and what they hook to earlier in this thread.

Something else you could do is disconnect the lights from the relay, leave the switch and wiring to the fuse block hooked up. The lights won't go on, but you could still activate the relay for testing purposes. If the fuse keeps going out, you know you have an issue between the fuse and the relay and not further down the line.

Where are you powering the switch from? If the same 30 amp fuse is supplying power to the switch, you could have damaged the wiring you ran under the dash.

 

yeah that switch isnt fused and its lighted...

 

going to wally world for fuses...results on this solution when i get back

 

alright i checked the inside of the lights i think the ground and power were touching so i electrical taped those up and then i checked the relay. i had it wired wrong so i put it the right way. i turned on the lights with the key in the "on" position with out having the engine on, turned on the lights and they worked. i turned them off then on again then back off then on again and they worked just fine. i turned on the engine drove 30 yrds then hit the lights and the fuse blows again...what the h*ll is goin on!!!