Resettable fuse

"PPTC" redirects here. For information about the Pacific Paramedic Training Center, see Pacific Open Learning Health Net.

Recoverable fuses - PolySwitch devices
A resettable fuse

or
Polymer Positive Temperature Coefficient
(
PPTC
) device is a passive electronic component used to protect against overcurrent errors in electronic circuits.
The device is also known as a multifunctional
or
polyfuse
or
polyswitch
. They are similar in function to PTC thermistors in certain situations, but work with mechanical changes instead of charge carrier effects in semiconductors. These devices were first discovered and described by Gerald Pearson at Bell Labs in 1939 and are described in US Patent #2,258,958.[1]

Operation

The PTC polymer device consists of a non-conducting crystalline organic polymer matrix that is loaded with black carbon particles[2] to make it conductive. When cold, the polymer is in a crystalline state, with carbon penetrating into the areas between the crystals to form many conducting chains. Because it is conductive ("initial resistance"),[3] it will allow current to pass. If too much current is passed through the device, it will begin to heat up. As the device heats up, the polymer will expand, turning from crystalline to solid. amorphous state.[4] The expansion separates the carbon particles and disrupts the conductive paths, causing the device to heat up faster and expand further, further increasing resistance.[5] This increase in resistance significantly reduces the current in the circuit. There is still a small current flowing through the device (leakage), which is enough to maintain the temperature at a level that will keep it in a high resistance state. Leakage current can vary from less than one hundred mA at rated voltage to several hundred mA at lower voltages. The device can be said to have a locking function.[6] Holding current is the maximum current at which the device is guaranteed not to operate. The tripping current is the current at which the device is guaranteed to turn off.[7]

When the power is turned off, heating due to leakage current stops and the PPTC device cools down. As the device cools, it regains its original crystalline structure and returns to a low-resistance state where it can hold the current specified for the device.[6] This cooling usually takes a few seconds, although a tripped device will maintain a slightly higher resistance for several hours unless the power in it is weaker or is used frequently, slowly approaching the initial resistance value. Reset often does not occur even if only the fault is cleared while power is still supplied because the operating current may be higher than the PPTC holding current. The device may not return to its original resistance value; it will most likely stabilize at a much higher resistance (up to 4 times the initial value). It may take hours, days, weeks, or even years for the device to return to a resistance value similar to its original value, if it returns at all.[8]

The PPTC device has a current rating and voltage rating.[9]

Self-resetting fuse for headlight wiper motor

I rebuilt the right headlight wiper drive. Still, a fuse is needed there, although the article on SVR is useful, the advice to throw out the fuse and solder the contacts turned out to be harmful. As a result, under unclear circumstances, the electric motor itself burned out (perhaps the brush froze, it seemed like it was winter).

I picked up this fuse: FUSE FUSB250F/RUSB-250 - PolySwitch (2.5A/16V). It fit perfectly into the niche in the case and was soldered to the copper contacts with POM-3 lead-free solder.

The result is that both headlight cleaners work perfectly, which is very important in autumn weather.

One of the bulbs in the headlight also burned out, I bought some H4 +50% from Lenta with a platinum-coated bulb (I don’t remember the manufacturer), but the standard power was 60/55W. Although I am skeptical about the additional light appearing out of nowhere in a standard power halogen lamp, they shine really well. But wet asphalt is still not really visible; in other conditions the light is quite normal.

Applications

These devices are often used in computer power supplies, largely due to the PC 97 standard (which recommends a sealed PC that the user should never open) and in aerospace/nuclear applications where replacement is difficult.[ citation needed

] Another use for such devices is audio protection. music speakers, especially tweeters, from being damaged by over-drive: by connecting a resistor or light bulb in parallel with the PPTC device, a circuit can be designed that limits the total current through the tweeter to a safe value, rather than turning it off, allowing the speaker to continue operating without damage when The power amplifier provides more power than the tweeter can handle. Although a fuse may also offer similar protection, if the fuse is blown, the tweeter will not be able to operate until the fuse is replaced.[10]

Everything about the electrical fuse

I think there is no need to explain what an electrical fuse is. The fuse itself is a switching device, inherently protection. In the event of a short circuit or network overload, the fuse cuts off the electrical circuit by opening. It follows that the electrical fuse protects the equipment from short circuits and prevents possible breakdowns. In this article we will talk about the types of fuses and tell you how to use them correctly.

Operating principle of electrical fuse

Almost each of us has witnessed a situation in which home appliances operating from the network suffered from a short circuit. Why is this happening? It is probably clear to everyone that the operation of household electrical equipment directly depends on the stability of the voltage in the network and the quality of the supplied electricity. Accordingly, if the voltage in the electrical network changes urgently, then this, of course, can “kill” not the most durable equipment in the house. I'll give you an example. Why are iPhone owners not advised to buy Chinese chargers for 100 rubles? No, not just because they will break quickly. The fact is that cheap chargers can charge your phone, but they do it intermittently. This process greatly affects the phone’s battery and even if it can charge, it will equally or later lead to its disposal, since it will hold less charge. Lack of voltage stability with a sharp, even gradual increase, why minimize this voltage, can in rare cases lead to ignition. I'll step back for a second and say about cheap chargers, don't use them if you love your equipment. On average, a decent charger will cost from 500 rubles, good ones, for example, from Rexant.

As for large household appliances, a sudden power surge can cause serious damage, for example a refrigerator. This is, of course, a million-dollar situation, but if the moment when the device’s compressor turns on coincides with a surge, then a breakdown cannot be avoided.

How does the fuse work? This protective device operates in series with the equipment that consumes this current and only breaks the circuit when the rated current is exceeded.

Fuse classification

This device can be divided into four classes based on the principle of operation during overvoltage. They can be fusible, electromechanical, electronic and self-healing.

Fuses

During a short circuit, the conductive element of the fuse melts or evaporates. In other words, he burns out. Fuses are considered obsolete, but they are considered quite reliable. But an obvious drawback can be considered its response speed, which is why the efficiency, accordingly, decreases. The fact is that it takes some time to melt, even just a fraction of seconds. Because of this, the equipment or person is left without protection in the short time the fuse operates. To compensate for this drawback, manufacturers make it in such a shape that it is as thin as possible in acceptable places. For this, tin is used, which melts quickly and does not interfere with current conductivity.

Electromechanical fuses

Such fuses are called circuit breakers or circuit breakers; the latter option is used most often. Such a device consists of a dielectric housing with movable and fixed contacts located inside. When the rated current value is exceeded, the switch sensor actuates a mechanical lever that breaks the circuit. The moving contact contains a spring that releases the contact as quickly as possible. The release mechanism is usually activated by two releases: thermal or electromagnetic.

Two types of sensors in circuit breakers:

1. A heat sensor is a metal plate that heats up as the current increases, causing the spring to fire.
2. Electromagnetic sensor - is an inductance coil with a movable core. During a power surge, this core is retracted and the spring is released.

Electronic fuses

Such devices are intended only for low-voltage electrical circuits. They can be observed in home appliances or, say, in computer equipment, e.g. They are a microcircuit that, when the current increases above the rated value, breaks the circuit using a semiconductor gate. This type of fuse has a significant advantage - it is the speed of operation, but the disadvantage can be considered a limitation in the scope of possible application.

Self-resetting fuses

This type of fuses is used in household and electrical appliances. When the current becomes higher than normal, the resistance in the fuse conductor increases, which leads to a break in the circuit. Such devices are made from polymers with dielectric properties and conductive carbon. As the current increases, the carbon heats up and is released, at which time the fuse becomes a dielectric. As the current decreases, the carbon cools and crystallizes. Thus, the device returns to normal.

Trade names of devices

These devices are sold by different companies under different brand names, including Multifuse

(Bourns),[11]
PolySwitch
(TE connection)[
quote needed
], and
Polyfuse
(Littelfuse)[
quote needed
].
PolySwitch is the earliest product of this type[ citation needed
], having been invented at Raychem Corporation (now TE Connectivity) and introduced in the early 1980s[
citation needed
].
Thanks to the general availability, electronic engineers and technicians[ WHO?
] this device is often referred to as a "multi-switch" or "polyfusor" in a generic sense, regardless of the actual brand.[
citation needed
].

If it blows out in the wilderness: how to safely restore a car fuse?

The fuse is one of the oldest electrical devices. The inventor of the fuse is believed to be Louis Francois Clément Breguet, a French electrical engineer. In 1847, he conducted a series of experiments and found out that inserts made of thin wire of a fixed cross-section are capable of burning out when the current in the electrical circuit exceeds a certain strength proportional to the cross-section. The practical use of fuses as devices began approximately in the middle of the 19th century - first in industrial and household electrical engineering, and with the widespread use of “steel horses” - in automotive applications. Despite the invention of various types of automatically reset fuses - spring, bimetallic, semiconductor - the most common disposable fuse by Louis Breguet is still the most common and effective device for protecting electrical circuits in any car from current overload and short circuit.

In both budget and premium cars, almost all electrical consumers are protected by fuses. And in the manual for any car, and even more so in the maintenance and repair manual, there is always the following (or very similar) phrase:

It is prohibited, even temporarily, to install jumper wires in place of the corresponding fuses, as this may damage the electrical wiring and cause a fire!

It is said correctly, and we do not intend to argue with it. However, we note that there are all sorts of circumstances, including not quite ordinary ones. And any owner of a car that is middle-aged and has seen life may find itself in a situation where some important fuse for driving has burned out - in the circuit of the fuel pump, ignition, windshield wipers in the rain, headlights at night, etc., and there is nothing to replace it with. This can happen not only in the city, where there are a lot of car shops, sympathetic citizens, as well as car services and tow trucks, and finally, but in the wilderness, when there are no spare fuses in the stash, there are no shops or just kind people nearby, but you still need to go...

So it is better to have knowledge “for a case that will not happen” than to be helpless in an unlikely, but still extreme situation! What is the supposed Japanese proverb that people love to write in statuses on social networks? “A samurai carries a sword all his life, even if he needs it only once in his life”... Or something like that...

When the bug... was the norm

Young car owners are well acquainted with modern U-shaped blade fuses, and, as a rule, are familiar with their predecessors, common on Soviet cars - rod-type fuses with their always lousy contacts. But what happened EVEN EARLIER, most likely, they no longer remember...

But until the 70s, cars (by the way, not only Soviet ones!) used fuses that were very similar to modern U-shaped blade fuses. They were a textolite plate with riveted springy contact legs and 15-20 centimeters of spare wire wound around it. If the wire between the contacts burned out (the fuse tripped), the driver simply unwound a few centimeters from the reel and reconnected the contacts. The fuse has restored its properties!

Not to say that this principle was ideal - the contact in the wire fuse often deteriorated over time, since it depended on the strength and accuracy of winding the wire. However, the system still existed for many years on a wide variety of cars and trucks and was considered quite functional. And pay attention: by modern standards, this is a natural “bug”!

"Bug" - but not a "bug"! Such a fuse would be considered a “bug”, dangerous and prohibited for use, if RANDOM wire was used in it! But the wire in it was strictly calibrated, with a cross-section corresponding to the required protection current, the value of which was printed on the fuse body! Therefore, if you take a modern knife-type fuse (burnt out) and connect its legs with a wire, whose combustion cross-section approximately corresponds to the current marked on the fuse initially, then such a “bug” will not pose a danger to the car’s electrical wiring. But how do you know what kind of wire is needed?

Wire diameter and combustion current

But for this there is a special calculation formula. But more often, for simplicity, a lookup table filled in with this formula is used! At one time, when technical creativity was considered entertainment, and not degradation on social networks, every schoolchild who attended a radio club knew this sign. Because making a fuse for a homemade design with your own hands was the norm! In the reference plate, for convenience, the cross-section of a single-core copper wire has already been converted to diameter.

For the most common car fuse ratings, the table looks like this. If you are interested in other current values ​​(and these tables usually include wire diameters for making fuses from 0.5 amperes to 200-300 amperes), then it will not be difficult to google the full version.

For example. In the common “twisted pair” LAN cable used to provide Internet access to apartments, one core has a diameter of about 0.5 mm - a piece of such wire will act as a fuse at a current of about 30 amperes. In stranded wires of the ShVVP type, used to connect electrical appliances to an outlet, cores with a diameter of 0.2 mm are often used - for 7 amperes... If you fold it in half, you get a fuse of 14-15 amperes. Well, etc.

Measuring wire without calipers and micrometer

OK, now we know how to make a relatively safe “bug” for the current we need... And in principle, finding a wire and unbraiding it into strands is not difficult - in extreme cases, you can cut and clean the insulation from the wire of a carrying lamp, a USB cord, or it is also possible to temporarily “ampute” some standard electrical circuits in the car by cutting off the wire from something not very necessary, such as a rear fog light... In the end, extreme situations sometimes require extreme solutions.

But how can we understand what current a wire is designed for if we don’t know its diameter? After all, it is impossible to determine the thickness by eye! But, as a rule, there is neither a caliper nor a micrometer in the trunk... And again, old-fashioned tricks will come to the rescue - simple but reliable.

We take any cylindrical object - a pencil, a screwdriver, a match, a twig - and wrap the wire around it tightly, turn to turn. The more turns, the higher the measurement accuracy will be, but usually 15-20 turns are enough. Wound - measure the total length of the winding with a ruler and divide the resulting number of millimeters by the number of turns. The result is the wire diameter!

The bores will mutter: “Well, yes, I don’t have spare fuses with me, but I found a ruler in the car, you see!” Especially for bores: well, the 21st century is just around the corner... Many, for example, have a ruler in their smartphone!

Making a bug fuse

...well, the last step - if the wire is found, its diameter is measured, and it suits us. We wrap a piece of copper wire tightly and tightly around the legs of the burnt fuse and insert it into place. Ready!

And finally, we remind you once again that it is necessary to carry out such repairs, even far from civilization and help, very, very carefully! Make a “bug” only after finally making sure that the factory fuse similar to the burnt one cannot be, at least temporarily, pulled out of the circuit where it protects something that you can temporarily do without, for example, a heated rear window or something similar. And if there are no spare ones, there is nowhere to pull it out from and there is no one to turn to for help - then just wind the “bug”... But do not forget to replace it with a normal fuse at the first opportunity and find out the cause of the burnout!

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Specifications

Table 1 shows the electrical parameters of the joint venture.

Table 1. Electrical characteristics of Bourns Multifuse self-resetting fuses

Maximum operating voltage (Vmax)

- this is the maximum permissible voltage that the joint venture can withstand without destruction at the rated current.

Maximum permissible current (Imax)

- this is the maximum current that the joint venture can withstand without destruction.

Rated operating current (Ihold) is the maximum current that the SP can conduct without tripping, i.e. without opening the load circuit.

Minimum trip current (Itrip)

- this is the minimum current through the SC, leading to a transition from a conducting state to a non-conducting one, i.e. to trigger.

Initial resistance (Rmin–Rmax)

— this is the resistance of the joint venture before the first operation (upon receipt from the manufacturer).

Since SPs are devices with a pronounced positive TCR, their characteristics depend on the ambient temperature. Table 2 shows the dependence of the normal operating current and the minimum operating current on the ambient temperature.

Table 2. Dependence of normal operating current and minimum operating current on ambient temperature

As you know, any heating takes some time. Due to the fact that the joint ventures heat up, they do not switch instantly, but require some time, which depends not only on the ambient temperature, but also on the overload current flowing through them.

Table 1 shows the operating time at a current 5 times the normal operating current (Ihold).

The dependence of the response time on the overload current is shown in the graphs (Fig. 7).