Well, Happy Santa’s Day to you, and I hope you got more than an lump of coal in your stocking. 🙂
Measure With a Cheap Meter When you wind the Joule Thief core, there are a few things that you should try to avoid. The most important is to keep the resistance of the coil down to a low value, less than 1/4 ohm. How do I know that the resistance is? I can measure the resistance and if it’s more than 1/4 ohm, use heavier wire or less turns. Yes, I know. When I measure with my cheap DMM, the test leads shorted together have more than 1/4 ohm (really cheap, thin wire in the test leads). I have to subtract the ohm value I get when I short the leads together from the reading of the winding wire.
I got one cheap DMM meter from Harbor Freight, and plugged the test leads in, and one of the test leads was open so I couldn’t even make a measurement. I had to open the meter up and solder some wire onto the jacks inside. I ran the wire out through the hole where the plugs plug in. The great thing about this is that now when I measure s very low resistance, it doesn’t jump around as much. That’s because the cheap test lead plugs seem to make poor contact with the meter jacks. Soldering the leads directly to the jacks eliminates those poor contacts.
Wire Tables Or else instead of measuring with all its uncertainty, I use the wire tables to find out how much resistance per foot the wire has. First off I have to know what the actual wire gauge is. For enamelo covered magnet wire, the wire gauge is marked on the reel. Then I can go online and do a search for wire table and find one that has the columns of numbers. One column is marked ohms per 1000 feet or ohms per kFt (or for metric, ohms per meter). I just have to remember that 1 ohm per kFt is equal to 1 milliohm per foot – a milliohm is 1/1000 of an ohm. If I’m using 30 AWG wire, it has 104 milliohms per foot or about 1/10 ohm per foot. If I use 24 AWG, it has .0257 ohms per foot. If my coil uses 18 inches or 1.5 feet of wire, then I multiply the milliohms per foot by 1.5.
Another Way There is another way to find the very low resistance of a length of wire. If you have a power supply with adjustable current up to 1 amp and a low volt meter, it’s easy. Connect the wire up to the power supply and set the current for 1 amp. But remember that if the wire is very thin, it can overheat. For example, when the wire has 1 amp going through it, and the voltage measured from one end to the other is 1/4 volt, then the wire has 1/4 ohm resistance. You could also adjust the current to 1/10 amp, then multiply your measurement by ten.
And Another way Use a decent quality ohmmeter. I have an HP (Agilent) DMM that has a four wire connection on the back. I can make the measurement with test leads that are any length and have that bothersome internal resistance. But it takes four wires. Two of the wires furnish the current at the point where the test leads connect to the resistance. The other two sense the reading at that same point so there is no measurement of the resistance of the test leads, only the resistance of what I’m measuring. I can accurately measure much less than a tenth of an ohm this way.
You can now go back to unwrapping your presents, including that new show shovel. 😉