Most analogue
multimeters are capable of measuring resistance over quite a wide range
of values, but are rather inconvenient in use due to the reverse reading
scale which is also non-linear. This can also give poor accuracy due to
cramping of the scale that occurs at the high value end of each range.
This resistance meter has 5 ranges and it has a forward reading linear
scale on each range.The full-scale values of the 5 ranges are 1K, 10K,
100K, 1M &10M respectively and the unit is therefore capable of
reasonably accurate measurements from a few tens of ohms to ten Megohms.

*
The Circuit*

Most linear scale resistance meters
including the present design, work on the principle that
if a resistance is fed from a constant current source the
voltage developed across that resistance is proportional
to its value. For example, if a 1K resistor is fed from a
1 mA current source from Ohm’s Law it can be calculated
that 1 volt will be developed across the resistor (1000
Ohms divided by 0.001 amps = 1 volt). Using the same
current and resistance values of 100 ohms and 10K gives
voltages of 0.1volts (100 ohms / 0.001amps = 0.1volts)
and 10 volts (10000 ohms / 0.001amps = 10 volts).

Thus the voltage developed across the
resistor is indeed proportional to its value, and a
voltmeter used to measure this voltage can in fact be
calibrated in resistance, and will have the desired
forward reading linear scale. One slight complication is
that the voltmeter must not take a significant current or
this will alter the current fed to the test resistor and
impair linearity. It is therefore necessary to use a high
impedance voltmeter circuit.

The full circuit diagram of the Linear
Resistance Meter is given in Figure 1. The constant
current generator is based on IC1a and Q1. R1, D1 and D2
form a simple form a simple voltage regulator circuit,
which feeds a potential of just over 1.2 volts to the
non-inverting input of IC1a. There is 100% negative
feedback from the emitter of Q1 to the inverting input of
IC1a so that Q1’s emitter is stabilised at the same
potential as IC1a’s non-inverting input. In other words
it is stabilised a little over 1.2 volts below the
positive supply rail potential. S3a gives 5 switched
emitter resistances for Q1, and therefore 5 switched
emitter currents. S3b provides 5 reference resistors
across T1 & T2 via S2 to set full-scale deflection on
each range using VR1.

As the emitter and collector currents
of a high gain transistor such as a BC179 device used in
the Q1 are virtually identical, this also gives 5
switched collector currents. By having 5 output currents,
and the current reduced by a factor of 10 each time S3a
is moved one step in a clockwise direction, the 5
required measuring ranges are obtained. R2 to R6 must be
close tolerance types to ensure good accuracy on all
ranges. The high impedance voltmeter section uses IC1b
with 100% negative feedback from the output to the
inverting input so that there is unity voltage gain from
the non-inverting input to the output. The output of IC1b
drives a simple voltmeter circuit using VR1 and M1, and
the former is adjusted to give the correct full-scale
resistance values.

The CA3240E device used for IC1 is a
dual op-amp having a MOS input stage and a class A output
stage. These enable the device to operate with the inputs
and outputs right down to the negative supply rail
voltage. This is a very helpful feature in many circuits,
including the present one as it enables a single supply
rail to be used where a dual balanced supply would
otherwise be needed. In many applications the negative
supply is needed simply in order to permit the output of
the op-amp to reach the 0volt rail. In applications of
this type the CA3240E device normally enables the
negative supply to be dispensed with.

As the CA3240E has a MOS input stage
for each section the input impedance is very high (about
1.5 million Megohms!) and obviously no significant input
current flows into the device. This, together with the
high quality of the constant current source, and the
practically non-existent distortion through IC1b due to
the high feedback level gives this circuit excellent
linearity.

With no resistor connected across T1 &
T2 M1 will be taken beyond full-scale deflection and
overloaded by about 100 or 200%. This is unlikely to
damage the meter, but to be on the safe side a
push-to-test on/off switch (S1) is used. Thus the power
is only applied to the circuit when a test resistor is
connected to the unit, and prolonged meter overloads are
thus avoided.

A small (PP3 size) 9 volt battery is a
suitable power source for this project which has a
current consumption of around 5mA and does not require a
stabilised supply.

Photos showing inside and outside of the completed
Linear Resistance Meter.