October 1960 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Back in the early 1990s I had
my first need as a product design engineer to select a fuse type; it was for protecting
a downconverter chassis. Having already worked in the electronics and electrical
equipment installation and maintenance field for twenty years, I have had to replace
many fuses of many different types in many types of assemblies ranging from AC motor
controls to microprocessor boards. Slo−Blo, Fast−Blo, Standard−Blo, high voltage
and low voltage, high current and low current, glass bodied, plastic bodied, and
even metal explosion-proof bodied fuses were in the realm my experience. However,
some other engineer had already decided what the appropriate fuse was for the application.
I assume each instance had been determined from a selection process based on knowledge
of both the device to be protected and the characteristics of the fuses. Not just
any fuse will do the job. Sometimes you get lucky and the one used will work so
long as the current and voltage ratings are within the normal operational range
of the product being protected, but if that was generally the case, there wouldn't
be such a large variety of fuse types, n'est−ce pas? This "Fuses Are Not for Confusion"
article from a 1960 issue of Popular Electronics magazine is a great primer
on the art of fuse selection.
Fuses Are Not for Confusion
By George D. Philpott
Wrong replacements cause annoyance - or harm. Know which types to use, when,
and how to identify them.
In recent years, manufacturers of TV receivers and of fuses as well have done
much to alleviate a set of common complaints made by set owners and service technicians.
"It's that fuse again," is heard less often than in the past. So is the wail of
woe over a burned-out part that was not adequately protected. Special requirements
have been analyzed and met. New fuses and fuse holders have been developed. However,
one ominous factor remains to becloud an otherwise clearing fuse picture - correct
An undo-it-yourself set owner - sometime even a service technician - doesn't
happen to have a satisfactory replacement for a blown fuse or cannot identify it
properly because the brand he happens to have on hand uses a method of coding that
differs from that of the original. The replacement used is one thought to be a reasonably
acceptable equivalent. The result may be annoying and unnecessary repeated fuse
failure or the burn-out of an expensive component while the fuse remains intact.
Sometimes, on the other hand, a replacement with slightly different characteristics
is desirable. In any case, the operator should know what he is doing. This means
he must know something about fuses and their applications.
Fig. 1 - Fuse in transformer primary (A) protects more than (B)
The speed with which a fuse blows is important. Broadly, there are three general
categories, the quick-acting, medium-lag, and slow-acting (or "Slo-Blo," as Littelfuse
calls these types). Actually there are more shades of difference than these three
types would indicate. High-speed instrument fuses, like those used to protect meter
movements, generally act faster than the more commonly encountered fast-acting types.
They are used because some types of instantaneous overloads could damage a meter.
Types N and C, especially suited to television receiver protection, irrespective
of voltage rating or manufacturer, are directly interchangeable.
* The ampere rating applying to individual voltage ratings may be determined
from manufacturer's literature.
Medium-lag fuses are the most popular, being widely used in aircraft applications,
amplifiers, car radios, TV receivers, and other equipment. Ac-cording to brand,
these are found in the AGC types (Bussmann) and the 3AG or 5AG types (Littelfuse).
Slow-blowing fuses are most useful where harmless transient currents or voltage
surges are likely to occur that would ordinarily cause quicker acting fuses to blow
unnecessarily. Most fuses, whatever delay they may show under partial overload,
will blow rather quickly under heavy overload.
Slow-blowing types often find specific applications in TV receivers. The transient
surges caused by the action of certain LG networks in the horizontal section of
a TV set might cause even a medium-lag fuse to open unnecessarily. The inductances
used in many TV circuits (yokes, flyback transformers, peaking coils, others) often
create reverse e.m.f.'s for short periods during set warm-up that may double the
current drain in a circuit.
Table 1 - Common TV and other fuses in the lines of two leading
manufacturers. Note that code designations assigned to various fuse types relate
to such differences as physical size and blowing characteristics, but not to current
A common example of apparent fuse unreliability occurs in the horizontal output
stage. Low drive voltage from the horizontal oscillator to the grid of the output
tube, especially during warm-up, may permit the latter tube to overload.
A frequently asked question is, "What does the fuse's voltage rating mean?" Some
sets come from the factory with 125-volt fuses installed. Others may use fuses with
a maximum rating of 250 volts. Furthermore, the latter may appear in 500-volt circuits.
Does the excess voltage make the fuse blow?
Essentially, fuses respond to changes in current without respect to the applied
voltage, within certain limits. The maximum voltage rating, according to Litteltuse,
is that voltage up to which a fuse, when subjected to a current overload (defined
as a 10,000-ampere d.c. short circuit), will interrupt safely, without shattering
or burning up. The most severe short likely to occur, say, in a TV receiver will
produce a current that is only a small fraction of this test rating. Thus fuses
rated at 125 or 250 volts can safely be used at much higher voltages. However current
ratings must be considered very carefully, taking into account the normal current
range likely to be found in the protected circuit and the possible magnitude of
Table 2 - Contents of service-maintenance fuse kit recommended
Occasionally a technician wonders why a fuse of any type may, for no reason that
can be found, fail during operation. He may say facetiously that the fuse just got
tired. He may also be correct. In circuits where a constantly interrupted current
is flowing through the fuse (vibrator applications, TV horizontal sections, frequency
choppers, high-current multivibrators, and the like), such cycling may produce constant
expansion and contraction of the fuse element, eventually wearing out this slender,
metal strand. This "cyclic fatigue" is just one of many problems confronting a fuse
manufacturer endeavoring to supply a reliable product to users. The slow-blowing
fuses usually stand up better under cyclic fatigue.
One of the problems likely to confuse a technician attempting replacement is
the fact that different manufacturers do not code identical or similar fuses in
the same way. Table 1 is a handy cross-listing of fuses manufactured by Littelfuse
and Bussmann. The boxes in which fuses by the latter manufacturer are packed usually
carry cross-identification, although the fuses themselves are not so marked. For
example, a pack of Bussmann AGC fuses may carry such information as "formerly called
3AG." For some similar types, there may be slight differences in certain characteristics
between one manufacturer and another. While these do not usually affect use much,
it is a good idea to learn what they are and what effect they might have in specific
Some TV Applications
The problem of fuse failure and replacement are of above-average concern in the
case of TV receivers, where more than one fuse is likely to be used, where there
may be conflicts between over-all and single-section protection, and where the fuses
used on a single set may be quite different from each other. A brief examination
of common configurations will help show how and where fusing is provided and help
in choosing replacements to meet special problems.
The partial schematic of Fig. 1A is of a commonly encountered full-wave rectifier
circuit. The fuse, in the primary lead of the transformer, gives over-all protection.
A disadvantage, if no other fusing is used, lies in the fact that it takes a rather
healthy overload to pop the unit. Thus, if no other protection is used, excess current
due to a short in a particular circuit may be enough to cause circuit damage but
under the amount required to blow the fuse. An advantage is that the expensive power
transformer is fully safeguarded.
Fig. 2 - Half-wave rectifier supply, fused in d.c. line going
Fig. 3 - Separate fusing for "B+" supply in TV's horizontal-sweep
Table 3 - Ratings of fuses generally used with popular sizes
of copper wire.
Suppose that the envelope on a rectifier tube should crack. When it does - and
every technician has probably seen this happen several times-the filament of a heavy-duty
rectifier like the 5U4 may, in the process of melting as it is exposed to air, form
a dead short across the 5-volt winding. This could cook a transformer to death.
Standard cartridge fuses, rated at a few amperes, are used here.
An alternate method of fusing the low-voltage section is shown in Fig. 1B. Here
the center-tap on the transformer's high-voltage secondary is fused to ground or
"B-." The transformer is protected except in the case of filament-circuit overloads.
Since the receiver's many tubes consume quite a bit of filament power in relation
to total drain, and current for the filaments does not pass through the fused winding,
the current rating of the fuse is substantially reduced. The type 3AB shown is a
special, arc-quenching variety often found here. It prevents element flashing, a
condition related to the inductance of the transformer. The fuse is a medium-lag
Fig. 2 shows a common half-wave rectifier using a semiconductor. Sometimes a
voltage doubler is used without the transformer. Here a standard pigtail fuse is
likely to be found wired into the circuit, but it will be the slow-blowing type
too. A surge resistor is necessary to protect the rectifier from the heavy initial
surges that would otherwise charge the filter capacitors. Because of these surges,
a faster acting fuse might open needlessly.
Fig. 3 shows the damper portion of a horizontal-output and high-voltage circuit.
Again a slow-acting fuse, but at a reduced current rating, is used. Although the
location of the fuse in the horizontal-output circuit may vary, the fuse characteristics
are likely to remain the same.
Many recently made TV receivers use other protective devices, such as resettable,
thermal-overload cut-outs, fusible resistors, and thermistors, instead of fuses.
These do not act rapidly, as a rule. Where fuses are used instead, they correspondingly
tend to be slow-blowing. Radios and amplifiers usually have medium-acting fuses.
Instruments and aircraft equipment tend to use fast-acting fuses, although many
aircraft applications call for medium-speed types.
Bayonet-type fuses, becoming increasingly popular, were originally designed for
the TV industry. These are made in different sizes depending on current rating,
as are the corresponding fuse holders. Thus only the right fuse can be inserted
and locked into the fuse holder. These type C and N fuses are made by both leading
manufacturers, but the two brands may be interchanged.
If your fuse kit happens to be short of some types when you need them, you might
check your supply against the list of most frequently used types for service dealers
and maintenance engineers in Table 2. Some common holders and mountings are listed
as well. Also helpful is Table 3, which is a short list of fuse sizes generally
used to protect insulated copper wiring of various sizes.
Just one more thought before closing. Fuses are used for good reasons - protection
and safety, both to equipment and personnel. This good purpose should not be thrown
away with a thoughtless or indifferent replacement.
Posted June 2, 2023