Coil resistor wire testing....?
Tech Contributor
Joined: Jun 2004
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
But for amusement here's what happens when you hook a digital voltmeter to the disconnected C+ line as mentioned earlier. The ballast resistance is usually about 1.5 ohm. The input impedance/resistance of a DVM is usually more than one megOhm. You are correct that there is a current path through the meter, and you are also somewhat correct in assuming there would be some kind of voltage drop across the ballast wire. Let's do some simple math. The voltage divider circuit formed by the 1.5 ohm resistor, the 1000000 ohm meter, and the 12v battery gives us (using Ohm's Law): 1000000/(1.5 + 1000000 =1000001.5) times 12v = 11.999982v. A cheap meter like this example would load (draw current) the circuit enough to cause the reading to be 18 millionth of a volt in error (assuming the readout display had enough digits
). Higher quality (higher input impedance) meters would have less error, and I would also expect the internal A/D circuits to show this voltage to be rounded up to 12.000v anyway.
