Dabney Puts Up Resistance
The usual morning stroll taken by Dr. Crankshaft was interrupted by the ringing of his cell phone (yes, the Dr. too had finally entered the 21st Century and obtained one of the gadgets, much to his delight.) It was Dabney calling, and he asked the Doctor to drop by the garage for some consultation.
When the Doctor showed up at the garage, Dabney was laying under the dash of his 1954 Ford Club Sedan. The smell of electrical smoke wafted in the air.
"What do you know about electricity, Doc?" asked Dabney as he got out from under the dash.
"Well, it depends on what you're referring to, Dabney," replied the Doctor, "I don't think I'm qualified to wire an industrial site, but automotive and residential electricity are pretty much one of my many levels of expertise," he said with a certain air of confidence.
"I thought I knew a fair amount myself," said Dabney, "but I can't make this 6-volt electrical system function with a 12-volt battery without burning out my dropping resistors."
"Hmmm," mumbled the Doctor, "why don't you take me through your reasoning?"
"Okay," said Dabney, "I know my electrical formula is: V=IR, or voltage equals current times resistance. Therefore, if I have 12 volts and I want to reduce it to 6 volts, the formula says: 12 volts = current times 2 ohms. So, if the current is one amp (like the radio, for instance) then I need a 2 ohm resistor. I added up the current draw from the lights and gauges and got a rough total of 15 amps, so I figured a good-sized 2 ohm resistor could take it. Every time I wire one in it burns up. I could put a resistor in line with each device and bulb, but that's a lot of resistors."
"It's no wonder," said the Doctor. "You forgot about the watts."
"Watt?" said Dabney
"Did you say Watt or what," replied the Doctor.
"No, I said watt," said Dabney. "Maybe you'd just better explain it."
"Okay," said the Doctor. "You are correct in using a 2 ohm resistor to drop the voltage from 12 to 6 volts, but the resistor has to be able to dissipate enough watts."
"But my formula doesn't include watts," said Dabney.
"Too bad, my boy, but you need an additional formula," said the Doctor. "The formula is Amps times Volts equals Watts. So this is really quite easy. All you need to do is plug in how many total amps will be drawn in the whole circuit you want to run, then multiply by the 6 volts you need. In your case it's 6 volts times 15 amps, so that equals 90 watts."
"So what?" replied a frustrated Dabney.
"Stop with the whats," said the Doctor. "What all this means is that you need a 2 ohm resistor that is capable of dissipating the heat generated from 90 watts of power. Those little puny 2 ohm resistors you are using won't do the trick. You need a wire-wound resistor with a rating of 90 watts or over. It's going to be about the size of a cigarette pack."
"Where can I find something like that?" asked Dabney.
"That's a good question," said the Doctor. "You can start calling electronic/electrical supply houses, but first I would suggest you check the J.C. Whitney catalog. They probably sell dropping resistors, since they've been selling stuff like that for nearly 50 years."
Dabney got out his catalog and, sure enough, there they were. "I can't resist" he said and got on the phone and ordered one.