July 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Deciding which fuse to use
as a replacement for a blown fuse is usually a simple matter - read the part number
or electrical parameters (current, voltage) off the package and make sure of its
type (fast or slow blow). Of course whenever a fuse blows, you usually have more
or a problem than just a bad fuse. Deciding which fuse to use when designing
a circuit which requires overcurrent protection requires a lot more consideration.
This article from fuse making company Bussmann's Charles James gives a brief introduction
to the kinds of parameters you need to factor into a selection. There's more to
it in most cases - especially for a shippable product - than measuring the current
under normal operating conditions and then adding some arbitrary buffer like 20%.
Extreme ambient temperature requires derating, mechanical conditions during operation
like high vibration or impact, time delay for more than a short-lived transient
event, available space for mounting, whether to make it easily replaceable or soldered
in, etc., requires careful study.
Selecting the Proper Fuse from the August 1965 issue of Electronics World.
Types of fuses and where they are used.
How to select the right fuse for your circuit.
By Charles W. James
Special Applications Engineer
Bussmann Mfg. Div., McGraw-Edison Co.
Fig. 1 - Operating characteristics of three fuse types described
From a strictly mechanical point of view, fuses may be placed into two general
categories. The first category is the "clip-in" fuse, which must be placed into
some kind of fuseholder or a pair of clips to perform its normal function. The other
category includes those fuses that have leads soldered to the end-caps and are generally
referred to as "pigtail" fuses. Pigtail fuses can be soldered directly into an electronic
circuit or printed circuit board, without a fuse-holding device.
Time-Delay Fuses. One of the most popular fuses in use is the
so-called "time delay" fuse (sometimes referred to as "slow-blow"). This is a general-purpose
fuse with the ability to pass harmless transient currents and yet blow with sustained
overloads or short circuits. It is usually constructed with a solder-alloy heat
sink that can dissipate the heat generated by momentary transient currents and is
spring operated when the current lasts long enough to cause the solder alloy to
melt. This type of fuse is sensitive to ambient temperature and must be de-rated
when applied in an extremely warm location in order to carry the load current.
Fast-Acting Fuses. Another very popular fuse is the "Fast-acting"
(or "normal blow") fuse. This is usually applied in circuits where there are no
transient or surge currents to hamper its operation. This fuse generally has a single-element,
wire link construction, without any heat sinks to absorb momentary overcurrents.
Fast-acting fuses thus blow very quickly on overloads and must be applied very carefully
with regard to the amount of full load current. Quite frequently these fuses are
used to provide short-circuit protection only and, therefore can be sized at approximately
250-300% of the full load current. Ambient temperature has very little affect on
the performance of these fuses.
"Very fast-acting" fuses are becoming increasingly popular for use in circuits
that require extremely fast operation to protect critical components, such as meters
or semiconductor rectifiers. Electronic equipment that has very little ability to
withstand over currents requires this kind of protection. This fuse is constructed
similarly to the "fast-acting" fuse except that the link is usually surrounded by
a special filler material and the fuse body is made of ceramic or phenolic material.
The very fast-acting fuse is essentially insensitive to ambient temperature.
Comparing Fuse Characteristics. Figure 1 shows the operating
characteristics of the three types of fuses mentioned above. Consider that all three
types carry a one-ampere full load rating but, as can be observed, the blowing time
for each is considerably different for a given overload current. For example, when
the overcurrent is 200% (2 amperes), the time-delay fuse takes 18 seconds to blow
while the fast-acting fuse opens in approximately 1.4 seconds. A 2-ampere current,
through a very fast-acting one-ampere fuse, causes the fuse to blow in 0.13 second.
Table I - Effect of Ambient Temperature on Current-Carrying Ability.
It can be seen from the above that a knowledge of the circuit in which a fuse
is applied is important. Will the circuit develop transient currents? How fast must
the fuse operate when a short occurs? These and many other questions should be considered
when initial circuit design is undertaken.
There are many fuses today which have been developed to meet special needs. Usually,
the fuse dimensions or physical construction have been altered so that a special
mounting means can be employed or so that an indicator can be built into the fuse
to signal when it has blown.
These fuses have particular applications and are not considered to be general-purpose
fuses. The fuses covered here are general-purpose types readily available on the
Criteria for Selecting Fuses.
There are many considerations that should be given to fuse selection. Voltage and
current ratings are the two most popular (often the only) parameters that are investigated
when selecting a fuse. Other criteria that must be examined include short-circuit
current rating, fuse characteristics, application temperature, fuseholders and mechanical
dimensions of the fuse.
Voltage Rating. Select a fuse with a voltage rating equal to
or greater than the voltage of the circuit. The standard fuse voltage ratings which
are available for electronic fuses are 32, 125, and 250 volts. Keep in mind that
a fuse with a higher voltage rating can always be used on a lower voltage circuit.
For example: a 250-volt fuse can be used in a 125-volt circuit. The reverse procedure,
however, can be very dangerous and should always be avoided. All 125- and 250-volt
fuses have the voltage rating stamped on the end caps. If there is no voltage rating
stamped on the cap, then it should be considered to be a 32-volt fuse unless reference
to its symbol can be made elsewhere.
Automotive circuits use 32-volt fuses, while 125-volt fuses are often applied
in the input circuit of power supplies. Fuses rated at 250 volts, for example, may
be applied in the B+ circuit of a TV receiver.
Current Rating. Once the voltage rating is determined, a fuse
with an ampere rating greater than the expected circuit full load current should
be selected. The generally accepted procedure is to choose a rating about 25% greater
than the full-load current of the circuit, because fuses are built to carry their
rated current in open air at room ambient; whereas they are usually applied in some
type of enclosure and the enclosure temperature is often higher than room ambient.
Table II - Electronic Fuse Selection Chart.
An important point to remember is that the voltage rating described above does
not in any way affect the ampere rating. A one-ampere, 125-volt fuse and a one-ampere
250-volt fuses have identical current-carrying capacities. Only the ability of the
fuse to open a short-circuit current is affected by its voltage rating.
Another frequent mistake made in selecting the ampere rating of a fuse concerns
the current waveshape. Many electronic circuits have unusual waveshapes, such as
those in rectifier circuits. The object of a rectifier circuit is to produce a dc
voltage from an ac source; thus, the normal thought would be to select a fuse for
the de circuit on the basis of the dc current that is flowing. This would be acceptable
if the rectified wave were perfect; however, we know that in practical circuits,
we do not need a perfect dc current and it is difficult to produce. Since the dc
wave is not perfect, there is an rms value of that wave which, in many cases, exceeds
the dc current value. Consequently, the fuse must be selected for the rms value.
An example of this is the case of a simple half-wave rectifier with a one-ampere
dc output and an rms value of the wave shape of 1.57 amperes.
The general rule to follow is to select a current rating based on the rms value
of the current. Only when the rms value equals the dc value is it acceptable to
pick the fuse size based on dc current.
Short-Circuit Current Rating. Should a severe short circuit
occur in an electronic circuit, it is mandatory from a safety standpoint that the
fuse clear the fault without rupturing. It is for this reason that fuses are given
a short-circuit rating that goes along with their voltage rating and must never
A normal 125-volt circuit load current could be two amperes full load but, when
a short occurs in the circuit wiring, the current might increase to 1000 or 2000
amperes. The fuse, in turn, must be able to open the circuit safely under this condition.
Generally, short-circuit currents with magnitudes in the thousands of amperes are
the exception rather than the rule in the case of low-energy electronic equipment.
For most electronic devices, if a fuse of the proper voltage rating is selected,
it will have an adequate short-circuit rating.
Temperature. How many times have you checked a troublesome circuit
and found that the current was less than the fuse rating? Did you happen to check
the temperature to which the fuse was being subjected as well? The effect of ambient
temperature on fuse performance can be appreciable, especially where time-delay
fuses are involved.
Table I shows the effect of temperature on the current-carrying ability of the
various types of fuses previously discussed. If a time-delay fuse were to be selected
for operation in an 80°C ambient and the circuit current were 375 milliamperes,
then the ampere rating of the fuse should be at least 1/2 ampere. If the same temperature
and current conditions were to be imposed on a fast-acting fuse, the fuse rating
should be at least 4/10 ampere.
There are many applications where operating temperatures can be considerably
higher than room temperature, especially in circuits where the components are enclosed
by a cabinet or case, as in radios, TV's, power supplies, and amplifiers.
Time-Current Characteristic. Once the voltage and current ratings
are decided upon, a major consideration is the time-current characteristic of the
fuse. The circuit determines to a great extent whether a time-delay, fast-acting,
or very fast-acting fuse is the correct choice. If harmless transient currents might
occur, a time-delay fuse would be needed. If the circuit is a bridge rectifier,
a very fast-acting fuse would be recommended.
Dimensions. Fuse dimensions are usually considered in initial
design and can be critical when space is a factor. The most common electronic fuse
dimensions are 1/4" x 1", 1/4" x 1 1/4" and 13/32" x 1 1/2". A wide variety of mountings
with a number of special features (if desired) are made for these fuse sizes.
Fuseholders. The most popular fuseholder for mounting on a chassis
inside an enclosure is the ordinary Bakelite (phenolic) fuseblock which has fuse
clips and wire terminals attached. For mounting the fuse in an enclosure or panel,
the "panel-mounted fuseholder" is extensively used. This fuse-holder has the advantage
of being accessible from outside the enclosure.
Panel-mounted fuseholders with lamps to indicate a blown fuse are also available.
These are particularly helpful where many fuses are used in the same area.
The pigtail fuse is, of course, the least expensive from a fuseholder point of
view. However, a blown pigtail fuse is more difficult to remove from the circuit.
Table II is a quick reference chart of fuses giving their voltage and current
ratings, operating characteristics, dimensions, and some typical applications.
Posted October 18, 2017