Q Error Correction
I'm writing in reference to the Troubleshooting article from the April '08 issue. You talked about wiring a capacitor up to the lights, but this in turn would cause a huge spark each time you turn the lights on. The simple solution is to increase the gauge of wire sending power to the headlights. This is like when someone upgrades their fuel pump but still leaves the stock 16-gauge wire to power it, and when you turn on the signal light you hear the fuel pump pulsate to the blinks. I've solved this issue on my car by doing such an "operation" and just thought people should know so no one ends up getting shocked over such a simple solution.Thanks,David Swenson
A David, I went back to review my answer to see if I worded something that would lead readers to the wrong conclusion. Normally, I put a new column aside for a couple days so I can read it later and see if it still makes sense. If I'm running late for the publication deadline, I might let it go, knowing my technical editor will catch any big errors. Explaining technical concepts is difficult without being able to get immediate feedback, but via print I can't see when eyes glaze over. In contrast, when I am teaching a class of student installation technicians, I can see by their expressions when I need to go deeper into an explanation.
In this case, I found a couple things that I want to clarify. I ran across a couple of Internet posts and a how-to video that probably gave you the mistaken idea about how this works.
First, a capacitor will not cause a spark each time you turn the lights on. A spark can only be caused by electrical energy jumping across an air gap or an insufficient dielectric, so if the capacitor is connected securely in circuit, no spark will ever occur. During installation, a trained technician will either have the battery disconnected, or at least the breaker will be in the off position so no current flows, and therefore no spark can occur. This is mostly to prevent the ring terminal from welding itself to the capacitor's terminal, since the spark isn't likely to cause serious injury.
My suggestion of using a relay is the one part that might cause a small spark that will be enclosed within the relay housing. When the contacts of the relay make and break (close and open), a spark could jump across the air gap as the distance between the contacts grows larger. But by recommending the use of the resistor on the primary side of the relay, the capacitor will pre-charge, so that there's a much lower voltage difference between the capacitor level and the battery level. Leave out the resistor and the repeated arc will cause the relay contacts to pit or carbonize over a short period of time. With the resistor, the relay will last for ages.
Changing the wire to the headlights to a larger gauge would also not help. The lights dim due to an upstream force, in this case the battery/alternator combination. In our homes, it's called a "brownout." The only time that the headlights would dim when nothing else is changing is if the headlight resistance value were to change-it doesn't. Ohm's Law tells us that if the supply voltage is stable and the headlight resistance is 2.7 ohms, the current will always be 4.7 amperes. The light intensity will remain constant, since 4.7 amperes through a 15-foot length of 16-gauge wire will result in no serious voltage drop and if the voltage supplied remains constant, the light output will also remain constant.
But the voltage doesn't remain constant. As accessories turn on and off, and the engine rpm varies, so does the voltage produced by the alternator and the load output voltage of the battery. Now we have a change that will affect the light output, and the source isn't the light or its wiring.
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