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Old 07-09-2003, 07:33 AM
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Electrical quiz for my MB friends.

Dear friends:

1) If you take a look at multimeters, you will notice that most of them only support a maximum of 1mA to 10A for DC, and 1A to 1000A for AC, even though they support voltage range of 1V to 1000V for both DC and AC. The question is: If multimeters can only handle low current (A) loads for DC, why it can handle much larger loads for AC ?

Diesel afficionados have always wanted a multimeter that can handle at least 30 A for DC so that they can efficiently test their glow plugs easily (measuring resistance only is not deterministic of a actual condition of a glow plug). Unfortunately, such multimeters are very expensive (high end Flukes...)

2) What are the proper terms to describe the opposition to a DC current and an AC current? Do they use the same unit?

Good Luck :-)

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Old 07-09-2003, 07:57 AM
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I think you're comparing oranges to apples. The same meter that measures 10A DC will not measure 1000A AC. Current is a series measurement. 1000A AC would fry most anything that does not have a cable that is less than a half inch in diameter.

For measuring higher currents you simply use a high wattage, low tolerance, 1 Ohm resistor wired in series with the load. You then take a voltage measurement across the resistor and using Ohms law determine the current through the resistor. Inside the DMM, that's what they are doing, it's just that they can't fit a very high wattage resistor into the case and provide large enough wires to handle the current.

Have a great day,
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Old 07-09-2003, 08:26 AM
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Just to bypass the whole technical "thing" (thanks Larry), I have a 0-60 analog ammeter (it's old, it's a Mac ET 1008, made by Prime Instruments in Cleveland, OH. Bought it in the early 80's.

I tried checking around a little, couldn't come up with anything comparable on the net, but I'm sure they're out there somewhere.

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Old 07-09-2003, 09:03 AM
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Location: Newcastle upon Tyne, UK
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What are the proper terms to describe the opposition to a DC current and an AC current? Do they use the same unit?
DC is resistance and is measured in Ohms.

AC is not quite so simple, but you probably are thinking about inductance which is measured in Henrys. Inductance occurs in coils. Straight forward resistance is relevant to AC, too.
Cheers, Neil
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Old 07-09-2003, 09:16 AM
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Measurement of voitage and resistance is different for AC and DC. AC voltage is measured as rms because of the sinusoidal nature of the waveform. Look on a scope; the trace crosses above and below the zero line.. DC is measured in straight volts. DC appears as a straight line on a scope.
In AC work, resistance is sometimes expressed as impedance; i.e. in measuring the “resistance” of a speaker coil, also expressed in ohms (usually 4, 8 or 16 ohms.)
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Old 07-09-2003, 11:09 AM
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Resistance to AC Current Flow

This "resistance" to AC current flow is called impedance and can be capacitive or inductive or purely resistive in nature. Think of impedance as being the long side (hypotenuse) of a right triangle with the resistance as the base and the capacitive or inductive component at a right angle to it. The magnitude if the capacitive component is inversely proportional to the frequency of the voltage while the magnitude of the inductive component is proportional to it.
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