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
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 do do just that by using typical bench top
By N. H. Crowhurst
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.
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 is correct. Inductance
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.
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
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 the purpose.
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 4 henrys.
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
Fig. 7. Modification of the simple bridge of Fig. 5 to
enable polarizing voltage to be applied to the electrolytic
For measuring all except electrolytic capacitors there are two methods,
which correspond in relative accuracy with the ohmmeter and bridge methods
used for measuring resistance.
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
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.
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.
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.
Fig. 8. Details for constructing a simple jig for forming electrolytic
capacitors up to their working voltage.