Ohm Meter
OHM METERS
The resistance of a wire or a circuit is measured by an ohm
meter. An ohm meter aids the troubleshooter in determining if a ground or a short
exists in a circuit.
Here is a great article about ohm meters. SOURCE
Ohmmeter design
Though mechanical ohmmeter (resistance meter) designs are rarely used today,
having largely been superseded by digital instruments, their operation is
nonetheless intriguing and worthy of study.
The purpose of an ohmmeter, of course, is to measure the resistance placed
between its leads. This resistance reading is indicated through a mechanical
meter movement which operates on electric current. The ohmmeter must then have
an internal source of voltage to create the necessary current to operate the
movement, and also have appropriate ranging resistors to allow just the right
amount of current through the movement at any given resistance.
Starting with a simple movement and battery circuit, let's see how it would
function as an ohmmeter:
When there is infinite resistance (no continuity between test leads), there
is zero current through the meter movement, and the needle points toward the far
left of the scale. In this regard, the ohmmeter indication is
"backwards" because maximum indication (infinity) is on the left of
the scale, while voltage and current meters have zero at the left of their
scales.
If the test leads of this ohmmeter are directly shorted together (measuring
zero Ω), the meter movement will have a maximum amount of current through
it, limited only by the battery voltage and the movement's internal resistance:
With 9 volts of battery potential and only 500 Ω of movement
resistance, our circuit current will be 18 mA, which is far beyond the
full-scale rating of the movement. Such an excess of current will likely damage
the meter.
Not only that, but having such a condition limits the usefulness of the
device. If full left-of-scale on the meter face represents an infinite amount of
resistance, then full right-of-scale should represent zero. Currently, our
design "pegs" the meter movement hard to the right when zero
resistance is attached between the leads. We need a way to make it so that the
movement just registers full-scale when the test leads are shorted together.
This is accomplished by adding a series resistance to the meter's circuit:
To determine the proper value for R, we calculate the total circuit
resistance needed to limit current to 1 mA (full-scale deflection on the
movement) with 9 volts of potential from the battery, then subtract the
movement's internal resistance from that figure:
Now that the right value for R has been calculated, we're still left with a
problem of meter range. On the left side of the scale we have
"infinity" and on the right side we have zero. Besides being
"backwards" from the scales of voltmeters and ammeters, this scale is
strange because it goes from nothing to everything, rather than from nothing to
a finite value (such as 10 volts, 1 amp, etc.). One might pause to wonder,
"what does middle-of-scale represent? What figure lies exactly between zero
and infinity?" Infinity is more than just a very big amount: it is
an incalculable quantity, larger than any definite number ever could be. If
half-scale indication on any other type of meter represents 1/2 of the
full-scale range value, then what is half of infinity on an ohmmeter scale?
The answer to this paradox is a logarithmic scale. Simply put, the
scale of an ohmmeter does not smoothly progress from zero to infinity as the
needle sweeps from right to left. Rather, the scale starts out
"expanded" at the right-hand side, with the successive resistance
values growing closer and closer to each other toward the left side of the
scale:
Infinity cannot be approached in a linear (even) fashion, because the scale
would never get there! With a logarithmic scale, the amount of resistance
spanned for any given distance on the scale increases as the scale progresses
toward infinity, making infinity an attainable goal.
We still have a question of range for our ohmmeter, though. What value of
resistance between the test leads will cause exactly 1/2 scale deflection of the
needle? If we know that the movement has a full-scale rating of 1 mA, then 0.5
mA (500 ľA) must be the value needed for half-scale deflection. Following our
design with the 9 volt battery as a source we get:
With an internal movement resistance of 500 Ω and a series range
resistor of 8.5 kΩ, this leaves 9 kΩ for an external (lead-to-lead)
test resistance at 1/2 scale. In other words, the test resistance giving 1/2
scale deflection in an ohmmeter is equal in value to the (internal) series total
resistance of the meter circuit.
Using Ohm's Law a few more times, we can determine the test resistance value
for 1/4 and 3/4 scale deflection as well:
1/4 scale deflection (0.25 mA of meter current):
3/4 scale deflection (0.75 mA of meter current):
So, the scale for this ohmmeter looks something like this:
One major problem with this design is its reliance upon a stable battery
voltage for accurate resistance reading. If the battery voltage decreases (as
all chemical batteries do with age and use), the ohmmeter scale will lose
accuracy. With the series range resistor at a constant value of 8.5 kΩ and
the battery voltage decreasing, the meter will no longer deflect full-scale to
the right when the test leads are shorted together (0 Ω). Likewise, a test
resistance of 9 kΩ will fail to deflect the needle to exactly 1/2 scale
with a lesser battery voltage.
There are design techniques used to compensate for varying battery voltage,
but they do not completely take care of the problem and are to be considered
approximations at best. For this reason, and for the fact of the logarithmic
scale, this type of ohmmeter is never considered to be a precision instrument.
One final caveat needs to be mentioned with regard to ohmmeters: they only
function correctly when measuring resistance that is not being powered by a
voltage or current source. In other words, you cannot measure resistance with an
ohmmeter on a "live" circuit! The reason for this is simple: the
ohmmeter's accurate indication depends on the only source of voltage being its
internal battery. The presence of any voltage across the component to be
measured will interfere with the ohmmeter's operation. If the voltage is large
enough, it may even damage the ohmmeter.
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REVIEW:
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Ohmmeters contain internal sources of voltage to supply power in taking
resistance measurements.
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An analog ohmmeter scale is "backwards" from that of a
voltmeter or ammeter, the movement needle reading zero resistance at
full-scale and infinite resistance at rest.
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Analog ohmmeters also have logarithmic scales, "expanded" at
the low end of the scale and "compressed" at the high end to be
able to span from zero to infinite resistance.
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Analog ohmmeters are not precision instruments.
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Ohmmeters should never be connected to an energized circuit (that
is, a circuit with its own source of voltage). Any voltage applied to the
test leads of an ohmmeter will invalidate its reading.
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