When this article on component
(resistor, capacitor, and inductor) measurement was written, readily available,
inexpensive multimeters were not in existence. For about $20 you can now buy a brand
new handheld DMM that will make very accurate resistance measurements and reasonably
good capacitance measurements at frequencies up to a few MHz, where lead inductance
starts to be significant (test frequency is usually only a few kHz). Finding an
affordable, accurate inductance meter is another story. Cheap
LCR meters can be purchased on eBay, but don't
be surprised if the quality is not very good. The most accurate measurement method
uses a frequency in the realm of actual operation, and this article presents methods
that will allow you to do just that by using typical bench top instruments.
By N. H. Crowhurst
A discussion of various ways that circuit
components in radio and audio equipment can be checked without trouble.
Sometimes the simpler things one encounters in radio and audio work are apt to
get overlooked. For example, it would seem to be quite an easy matter to check the
inductance of a smoothing choke or the capacitance of an electrolytic capacitor,
with the correct polarizing current or voltage. However, when one looks around to
find a test instrument to make the measurement, it just isn't readily available,
so we are virtually forced into the routine of taking things for granted.
If we wish to check as to whether a certain component is functioning correctly
or not, the only available method seems to be by substitution, using another component
of the same type. Often this proves to be somewhat unsatisfactory, because the results
can be inconclusive. We really need to know how to check the various fundamental
components used in radio: resistance, inductance, and capacitance, to varying degrees
of accuracy, according to their purpose.
The simplest method of resistance checking is by means of a simple ohmmeter,
either an instrument built specifically for this purpose or an ohmmeter range on
a volt-ohm-milliammeter. Accuracy of this method of measuring resistance rarely
exceeds 10% and may not even be as good as this.
Assuming that the accuracy of the moving coil meter used for the instrument is
±2% and that the resistors used in the instrument are accurate to ±1 %, the accuracy
of the instrument as a perfect comparator between the internal and external resistances
cannot be better than ±1%. And the accuracy of comparison is only to within ±2%
of the full-scale current reading on the scale. If the scale reading, on a voltage
or current scale, is compared with the reading on the ohms scale, it will be found
that an error representing 2% of full scale in voltage or current reading may amount
to an 8% error in resistance value. This is at the point of maximum accuracy of
comparison, between the external resistance being measured and the internal resistance
of the instrument.
Thus it is seen that the best accuracy obtainable using an instrument with a
±2% movement and ±1 % internal resistance gives a guaranteed accuracy at center
scale reading of 9%. At readings between one-third and 3 times the resistance value,
which is the range one might expect to use before switching to the next scale, the
accuracy can reasonably be expected to stay within 10%. With an instrument using
lower accuracy components than those used for illustration, the accuracy of the
final reading in ohms will be considerably poorer than 10%.
From this it will be evident that an ohmmeter can only be used to make a rough
check as to whether a resistance is within the preferred value range for which it
is color coded - if it is of a ±10% or higher tolerance rating. To check that the
resistance is within ±10% of its rated value, the result is a little doubtful and
it is certainly impossible to rely on an ohmmeter reading to check to a tolerance
of ±5% or closer.
Although the ohmmeter readings cannot be trusted for checking to close tolerances,
it is possible to use an ohmmeter to check for reasonably good matching between
pairs of resistors, if this happens to be the requirement rather than close precision
in actual value.
As an example, in many push-pull amplifiers the resistors responsible for controlling
the gain in the two halves of the push-pull arrangement must be closely matched
to ensure balance. Production values may be specified to 5% or even closer tolerances,
to avoid the necessity of having to select matched pairs, but the essential feature
is that the value of the two corresponding resistors shall be within a close tolerance
of one another. It will not necessarily matter if both of them are, say, 10% or
15% from their nominal rating, as long as they are within 5% of each other. This
the ohmmeter is reasonably capable of checking, because it is quite possible to
read an ohmmeter scale to within 5%. Since the question as to whether the reading
is within 5% of its actual value is unimportant in this particular application,
the significance of the reading does not matter as much as whether the two resistors
which should be matched give readings within 5% of one another.
For some applications, however, such as calibrated attenuators or instruments
for use in radio it is necessary to check resistor values to closer limits such
as 5%, 2%, or even 1%, as the case may require. In these circumstances it is important
that the value shall really be within the specified percentage of its rated value.
The only method of making a measurement that is satisfactory for this purpose is
to use a Wheatstone type bridge, using calibrated elements whose accuracy is better
than the required component accuracy.
For most radio purposes the Leeds & Northrup bridge used for telephone line
work is quite accurate enough. In using a bridge there are two things that control
the accuracy of the reading obtained: (1) the accuracy of the resistance elements
of the bridge itself, and careful attention to see, that contact resistance does
not contribute an appreciable fraction under any circumstances; and (2) the sensitivity
of the null detector.
This second cause of inaccurate results can be checked by unbalancing the bridge
by a known percentage to see that an adequate off-balance reading is obtained. Suppose,
for example, the value required is 120,000 ohms, ±5%. Having balanced the bridge
and obtained a null at, say, 120,000 ohms, the resistance in the calibrated arm
should be altered by 5%, which represents a change of 6000 ohms.
If clicking in 6000 ohms additional in the calibrated arm shows appreciable deflection,
then the reading may be regarded as accurate; but if the addition of 6000 ohms does
not produce noticeable deflection from balance on the null detector, the result
is not reliable. To improve its reliability one can either use a larger battery
voltage or source of supply to the bridge, or else get a more sensitive null indicator.
Before leaving this discussion of resistance values it should perhaps be emphasized
that it is not wise to put absolute trust in the color coding on a resistor. Occasionally
even the best resistor will be found incorrectly color coded. If the error happens
to be in the third color of the code, then the discrepancy in resistance value will
be a matter of shifting the decimal point which can be quite serious. Also with
some sets of coding colors the difference between some of the colors is somewhat
difficult to determine, especially after the component has aged. For example, orange
and brown can get to look quite alike.
Usually the first and second colors in the code can be identified by the combination
used, from the recognized preferred value range. If the first color is blue, representing
6, the second color will most likely be either red, representing 2, or gray, representing
8, because 62 and 68 are the preferred values in the 60 to 70 range. But there is
no such ready clue as to the likely color of the third band: it could just as easily
be brown or orange. Thus a resistor in which this color looks at all doubtful could
be either 620 ohms or 62,000 ohms, which is a considerable difference!
This is where an ohmmeter check can easily determine which of the two values
Fig. 1 - Bridge configurations for measuring inductance.
(A) the "Hay" bridge. (B) the "Maxwell" bridge. Relative advantages of each type
are discussed in the article.
Fig. 2 - Modification of the "Hay" bridge to enable it to
measure inductance with polarizing current flowing. Care is necessary not to exceed
the dissipation rating of the various bridge elements. See text.
Fig. 3 - A simple inductance checker circuit for determining
inductance with the polarizing current flowing in the component.
Turning now to various kinds of inductance: the measurement of components not
intended for the passage of d.c. and without iron cores is a fairly simple matter,
with the aid of a conventional inductance bridge. Using such a bridge, employing
either the Hay or Maxwell configuration (see Fig. 1), the inductance can be
measured at a frequency suitable for the purpose, with a method quite similar to
the operation of a bridge for measuring resistance.
The principal difference is that two kinds of adjustment are usually necessary
to achieve null, because of the necessity for balancing the bridge in both amplitude
and phase. This enables the bridge to give a reading of both inductance value and
"Q" or loss factor. Bridges of this type are clearly marked to indicate the correct
setting of the controls for making each kind of measurement.
There is usually no difficulty in achieving a null with the air-core type of
coil, but if the inductance employs any kind of core, the null may not be quite
as definitive, because of the distortion of the injected test signal caused by the
core. Also, if the generator signal itself has any appreciable harmonic content,
a Hay bridge will never give a balance at both fundamental and harmonics at the
same setting. On the other hand, with an inductance where the only loss is due to
its resistance, such as occurs in an air-core coil, the Maxwell bridge will give
a fairly satisfactory balance for both fundamental and the lower harmonic frequencies
at the same setting.
When measuring an inductor that employs any kind of core to increase the permeability,
the magnetizing current is liable to distort so the inductor itself will generate
some harmonics not present in the input from the generator. When the bridge is balanced
to the fundamental generator input, there will be a residual harmonic present at
the null point, generated by the inductor itself.
This is a good reason for using earphones if the generator frequency is in the
audio range. Otherwise an oscilloscope with amplifier may be used as a null detector.
It is then possible, listening to the tone or looking at the trace, to determine
when the fundamental is balanced and the residue consists of harmonics.
But the conventional type of bridge is only suitable for measuring inductances
where there is no polarizing current. The usual variety of smoothing filter choke
has to provide a specified inductance when polarizing current is flowing and the
inductance in the absence of such polarizing current will be considerably higher
than the rated inductance of the choke with polarizing current. Unfortunately there
is no simple fixed relationship between these two values.
If the choke has been designed to provide its maximum inductance at the polarizing
current for which it is designed, the air gap will be adjusted so that, at this
value of polarizing current, either reduction or increase of the air gap would result
in a reduction of inductance value. However, in the absence of polarizing current,
increasing the air gap will always reduce inductance value, while reducing the air
gap will always increase inductance value.
From this simple fact it is evident that measuring an inductance with no polarizing
current flowing is no criterion of its performance with polarizing current. It can,
of course, provide a check that the inductance is not completely missing, due to
short-circuited turns, in which case the inductance might not even be adequate without
polarizing current flowing. But the fact that the inductance may measure twice its
required value with polarizing current is no evidence that the choke will give its
rated value with polarizing current.
Fortunately, with filter chokes of this nature close tolerances are not too important.
Usually a compliance with a minimum inductance value will suffice.
It is sometimes possible to use a modified Hay bridge, as shown at Fig. 2,
to inject a polarizing current so as to measure the inductance with the polarizing
current flowing. But this can be a dangerous procedure, because the polarizing current
may exceed the wattage rating of some of the internal components of the bridge and
cause permanent injury to it. It is, therefore, better to devise a simple checking
arrangement, as shown schematically in Fig. 3.
This does not employ a bridge method, but checks the inductance by injecting
a known frequency and comparing the a.c. voltage developed across the inductor with
that across the resistor in series with it. The relation between the a.c. components
of voltage developed will enable the approximate inductance value to be calculated.
This does not take into account the effect of the inductor distorting the waveform
of the a.c. signal component, which invariably occurs in this type of inductor and
is, in fact, another reason why any attempt to produce a precise figure of inductance
will be somewhat meaningless. A rough check of this nature is quite adequate for
If 60 cycles is the supply frequency for the a.c. component, dividing the calculated
impedance of the inductor by 377 will give the inductance value. For example, suppose
the series resistor used is 100 ohms (carefully checked in value), and the a.c.
voltages measured across the resistor and inductor are 2 and 30 volts, respectively:
then the impedance of the inductor at 60 cycles is 1500 ohms, representing approximately
Fig. 4 - The "Drysdale" bridge which is used for measuring
capacitance. Refer to text.
Fig. 5 - A simple bridge for capacitor checking that forms
the basis of a number of commercial units on the market. The null detector is usually
a "magic eye" tube.
Fig. 6 - Modification of a "Drysdale" bridge to permit the
measurement of electrolytic capacitors with polarizing voltage applied.
Fig. 7 - Modification of the simple bridge of Fig.
5 to enable polarizing voltage to be applied to the electrolytic capacitors.
For measuring all except electrolytic capacitors there are two methods, which
correspond in relative accuracy with the ohmmeter and bridge methods used for measuring
The Drysdale bridge (see Fig. 4) is a modified Wheatstone bridge, in which
resistance arms are used in the ratio positions, while a calibrated decade capacitor
is substituted for the calibrated resistance in the variable standard arm. This
type of instrument can give capacitance results comparable to those obtained with
the Wheatstone or Leeds & Northrup bridge for resistance, but its use involves
careful adjustment of a number of controls until a null is achieved.
The alternative method of capacitance measurement also uses a bridge, but one
in which the null is much more quickly achieved. In this bridge (see Fig. 5)
a standard capacitor is used in one arm, the unknown capacitor in another arm, and
a single potentiometer-type resistance for the other two arms. This resistance is
calibrated on the basis of the ratio between the unknown and standard capacitors
necessary to achieve null.
With this type of bridge the unknown capacitor is connected across the terminals
of the bridge and the one dial turned until null is indicated. The capacitance value
is then read off the dial. The accuracy of this type of instrument is usually comparable
to that of an ohmmeter, depending upon the accuracy with which the potentiometer
type resistance has been calibrated.
Neither of these methods is really satisfactory for the measurement of electrolytics.
This can better be understood by discussing a little further the behavior of electrolytic
capacitors under different conditions.
In the first place, electrolytic capacitors freshly formed ready for use, have
a dielectric film on the active plate of the correct thickness for the working voltage.
Under this condition the capacitor should have its rated capacitance.
But if the capacitor is operated consistently at a lower polarizing voltage,
the thickness of the formed film will gradually deteriorate with the result that
the effective capacitance will increase somewhat. This is not necessarily detrimental
to the performance of the capacitor, provided it is not subsequently required for
service at its nominal working voltage.
In much the same way electrolytic capacitors kept in storage also show a deterioration
in the dielectric film resulting in an increase in effective capacitance. This means
if a six-month-old capacitor is taken from the shelf and measured on a regular capacitance
bridge, without applying the necessary polarizing, it will probably show a value
considerably in excess of its nominal value. However, it will not be satisfactory
for operation until the electrolytic film has been formed up to the requisite thickness
for its working voltage.
This will have to be done with the aid of a limiting resistor connected in series
with the capacitor to limit the polarizing current while the film is forming. Only
when the film has formed up so the voltage appearing across the capacitor is at
its working value without excessive leakage current can its capacitance be measured
to give a reliable indication of its operating condition.
Also, if the capacitor is to be installed in a piece of equipment for operation
at its nominal working voltage, it is vital that this reforming of the capacitor
be performed before installation, so the capacitor does not take an abnormally high
leakage current when the power is switched on and possibly destroy itself before
it has had a chance to become correctly reformed.
The correct measurement of electrolytic capacitors with polarizing voltage applied
can be undertaken with either type of bridge, modified to a certain extent, as shown
in the schematics of Figs. 6 and 7. If the actual capacitance value of an electrolytic
capacitor is not vital, which often is the case, then all that is necessary in installing
a new one is to ensure that it is correctly formed to its working voltage before
connecting it in. This may be done with the aid of the circuit shown in Fig. 8,
which consists of a high resistance feeding the capacitor with a voltmeter across
it to indicate when working voltage has been reached. The resistor limits the leakage
current through the capacitor to well within the maximum leakage current allowed,
and when the capacitor has reached its nominal charged voltage, it can then be removed
from the charging arrangement. Then, after discharging the capacitor for the sake
of safety, the capacitor is ready for installation in its intended circuit. Discharge
should preferably be accomplished through a fairly large resistor. The common practice
of short-circuiting a fully charged capacitor results in a very high discharge current
that may damage the capacitor.
Sometimes a capacitor which has been in stock a long time will deteriorate in
the quantity of electrolyte present, so the capacitance will fall low in value,
even after it has been adequately reformed.
Fig. 8 - Details for constructing a simple jig for forming
electrolytic capacitors up to their working voltage.
If it is not convenient to build a capacitance measuring arrangement incorporating
the polarizing supply, a fairly legitimate result can usually be achieved by ensuring
that the capacitor is correctly formed using the polarizing jig of Fig. 8,
then discharging the capacitor and finally measuring it immediately with the aid
of one of the conventional capacitance bridges without polarizing voltage.
If the electrolytic capacitor is reasonably stable, a null will be obtained which
will not vary at a perceptible rate. If the capacitor is not sufficiently stable
to be reliable in use, the null may be observed to vary perceptibly while the measurement
is being taken. If the capacitance varies at a rate that can be noticed while making
the measurement, then the capacitor should be discarded as insufficiently stable
for reliable operation.
The foregoing discussion has covered the more common measurements necessary on
resistance, inductance, and capacitance. Sometimes much more precise methods of
measurement are necessary, especially where the equipment is for some kind of standard
operation such as a precision oscillator. In this kind of application it is often
necessary to make measurements, not only as to the precise value at room or ambient
temperature, but to determine the effect of temperature on the component. To make
such measurements, only precision bridge apparatus is satisfactory, and the component
should be measured under carefully controlled conditions of temperature and the
measurements repeated at different temperatures, to discover what temperature coefficient
the component possesses. Fig. 8. Details for constructing a simple jig for
forming electrolytic capacitors up to their working voltage. See article.
Posted November 12, 2019(original 7/8/2013)