Electronics guide > Measuring current and voltage > Ohms per volt
Ohms per voltA meter’s internal resistance is stated as a number of ohms per volt. For example,
the resistance of the meter we use in this book is 20,000 ohms per volt (written
20 kΩV-1) on d.c. voltage scales so when it’s to read 4.5 V, it’s resistance is
90 k. This resistance, in parallel with resistor R2 forms an equivalent resistance,
given by the law of parallel resistors, of:
This resistance is now the new value of R2 in the circuit so, applying the voltage
divider rule, the measured voltage will be:
which is roughly what you measured (hopefully).
Any difference between the actual measurement and this calculated value may be
accounted for because this lower voltage causes a lower multi-meter resistance which,
in turn, affects the voltage which, in turn, affects the resistance, and so on until
a balance is reached. All of this occurs instantly, as soon as the multi-meter is
connected to the circuit.
What this tells us is that you must be careful when using your multi-meter to
measure voltage. If the circuitunder- test’s resistance is high, the multi-meter
resistance will affect the circuit operation, causing an incorrect reading.
Any multi-meter will affect operation of any circuit to a greater or lesser extent
— but the higher the multimeter resistance compared with the circuit resistance,
the more accurate the reading.
Hint:
A good rule-of-thumb to ensure reasonably accurate results is to make sure
that the multi-meter resistance is at least ten times the circuit’s resistance.
Fortunately, because of their very nature, digital multi-meters usually have
an internal resistance in the order of millions of ohms; a fact which allows it
to accurately measure voltages in circuits with even high resistances.
Well. We’ve reached the end of this chapter, so how about giving the quiz over
the page a go to see if you’ve really learned what you’ve been reading about.
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