Rusty's Blog

Not an exercise posting.

I’ve mentioned before that I’m a ham. Might be considered an understatement there, but anyway. I was assembling a power adapter for one of my radio’s today, and went to test it. I tested it ‘wrong’ this time.

The correct method of testing a power cable is to take a multimeter, put it in ohm or short test, and make three or six tests, depending on whether it is a 2 conductor, or 3 conductor power cord. The first test is obvious, make sure you have connectivity from one end of the cord to the other, on the expected lead. In the process, you may also test to make sure that you are not providing a short between any two conductors.

What did I do? Plugged it in. Twice. The first time I wasn’t really expecting much, simply because the battery in question was dead. I kind of thought it was, but that’s a different matter.

The second time I was ticked. Then burned. Ticked first. I had plugged a radio into the cable, then plugged the cable into a battery. I was expecting the radio to power on. So when it didn’t, I was ticked. Then I realized that the wire was getting a bit warm. Well, ok, hot. No, HOT!

Can’t hold onto it long enough to separate it, but the wire attached directly to the battery is cool enough, so I ripped it off there. After the wire cooled off enough to handle, I separated the power poles, and have discarded the remains. the insulation melted, and smoked. As I say I had attempted to grab the wire to separate it ther, and ended up burning a thumb and finger.

Yes, smoke. Wire got hot enough that it melted. When that happens, unless you have some very special insulating material, it’s going to go up in smoke. Or at least melt significantly, and part of it will go up in smoke. But the important part here is that wire that’s hot enough to melt, will burn you, and most of the things you have in your house right now.

Getting back to a multi-meter, they do pass some voltage down the line. However the current involved is very small. If you have a short in the line, the meter pegs to zero ohms, or if the meter has a continuity tester, it will beep or emit a tone. There is a current limiting resistor in the path, so you won’t have the experience I ran into when plugging into a 7.5 amp hour battery.

“7.5 amp hour battery?” you ask. Ok, batteries are rated in ‘amp hour’ or in some cases ‘cold crank amps’ I won’t go into what cold crank amps is, it’s pretty specific to car starting batteries, and if you want to know more, I’d suggest looking to your auto mechanic. When batteries are rated in ‘amp hours’ though, or ‘milliamp hours’ it means that if you have a load that draws 1 amp, the battery should last this that many hours. In theory this is somewhat linear, in that if you have a 5 amp hour battery, that giving it a load of 5 amps will provide a useful life of 1 hour. It’s a nice theory, but the reality is that as the amperage load goes up, the battery life drops at a rate that is not linear. Take that same 5 amp hour battery and give it a 10 amp load, it will die in a bit less than half an hour. Give it a 20 amp load, and you’re probably going to be lucky to get 10 minutes out of the battery, much less 15 min.

So what sort of lifetime will a dead short give you? That’s an interesting question. In this case there was a stretch of wire, an estimate being about 12 inches, or about 2 feet of wire, that was actually producing a very low but existing resistance. I’m sure it was less than an ohm, but there was some resistance. More resistance than the leads that were directly attached to the battery. It was enough resistance that the flow of electrons generated heat. As noted above.

So approximately how many ohms of resistance? Or how many amps was the circuit drawing? Well, given that the resistance of the wire was less than an ohm, we can do some rough back of the envelope calculations. According to Ohms law, Voltage is the product of resistance and amperage. (Product means you multiply the other two elements.) In this case I happen to know that the voltage of the battery is 12, because that’s the variety of battery I was using.

So to get amperage, or resistance, I need to take what I ‘know’ and divide 12 by that to get the other. Since resistance can’t therefore be zero, or we would have a divide by zero problem, let’s use some sample loads and see what happens. Let’s say we have a 1 amp fuse in the circuit, and we turn a variable resistor until the 1 amp fuse blows. What is the resistance on the variable resistor? Presuming that the actual load that the fuse blows at is 1 amp, to determine the resistance, we divide the voltage, 12 by the amperage, 1 and we end up with a resistance of 12 ohms. If we get a 12 amp fuse, and adjust the resistor until it blows the fuse again, what do we get? Well, 12 volts divided by 12 amps, gives us 1 ohm. If you cause a 60 amp fuse to blow, the resistance has to be about 0.2 ohms.

If you want, you can use the power law now to determine how many watts are being generated. Power in Watts is equivalent to voltage times amperage. So the 60 amp draw on a 12 volt battery gives you a power draw of 720 watt load. Considering that a light bulb that draws 200 watts is hot enough to melt and ignite nylon that is not in contact with the light bulb, that’s theoretically close enough to what I saw for the purpose of this blog. If you want to try the experiment, might I suggest doing so at a good electronics and electrical bench, on isolated circuits. So let’s say that 2 feet of this wire adds approximately .2 ohms of resistance to the load.

Be careful doing this stuff. I actually had fomal training in soldering, and electronics. If you’ve followed the entire blog to this point, you’re well on your way to being a geek. You will probably do the same thing that I just demonstrated that someone with formal training in this stuff, should have known not to do. I had the fortune of doing it with something that theoretically should not cause a problem. However 60 amps is nothing to play around with. For that matter it takes less than .2 amps to stop your heart. I would certainly much rather not have your doing something like that on my conscience. So let your Power Shorts be the ones that leave you feeling you can run 10 miles without breaking a sweat. Not leaving you concerned about the safety of your home.