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Current fuse, small fuse, tubular fuse application selection guide part 2

. Ten elements to choose a fuse

---- Rated current;
---- Rated voltage;
---- Ambient temperature;
---- Voltage drop / cold resistance;
---- Fuse characteristics: overload capacity, time / current characteristics;
---- Breaking capacity;
---- Melting heat value;
---- Durability (lifetime);
---- Structural features: shape/dimension, installation form;
---- safety certificate.
1. Rated current---In
The rated current of a fuse refers to its nominal rated current, which is usually the maximum current value at which the circuit can work.
* To correctly select the rated current value of the fuse, the following considerations must be taken:
For example: the working current of the circuit: Ir = 1.5 A, the rated current of the fuse should be: In = Ir/Of = 1.5/0.75 = 2A; Of here is the reduction rate of the UL specification fuse, so you should choose a 2A fuse

There is no reduction rate requirement for IEC specification fuse, namely: Ir = In
* If the special rated current is not universal, the nearest higher value should be selected.
* Wrong selection: use the current value of the fuse that you want to blow as the rated current value.

2. Rated voltage---Un
The rated voltage of a fuse refers to its nominal rated voltage, which is usually the maximum voltage that the fuse can withstand after it is disconnected.
* When the fuse is energized, the voltage at both ends is much less than its rated voltage, so the rated voltage is basically irrelevant.
* Correctly choose the fuse rated voltage should be equal to or greater than the circuit voltage

For example: a 250V fuse can be used in a 125V circuit

* For low-voltage electronic applications, an AC rated fuse can be used in a DC circuit.
* Regarding the rated voltage of the fuse, you should mainly consider: When the circuit voltage does not exceed the rated voltage of the fuse, whether the fuse has the ability to break the maximum current given

3. Ambient temperature
* Ambient temperature or known operating temperature has an impact on the operation of the fuse
* The higher the ambient temperature, the hotter the fuse will be during operation, and the shorter its life will be
* Regardless of UL specifications or IEC specifications, the requirements of the fuse refer to 250C at room temperature. If the environment or operating temperature is high, the temperature reduction rate of the fuse must be considered.
For example: A fast-acting fuse works at 900C and operates at 1.5A. Refer to the figure below, and its reduction rate (Tf) is 95%.

The figure above shows the results that affect the time-current characteristics (typical graph)

Picture solution: curve A: traditional slow-blow fuse; curve B: very fast-blow, fast-blow and spiral-wound fuse; curve C: recoverable PTC

If you choose an IEC fuse, the rated current is:
In 1.5A
In = ---- = ------ = 1,58 A
Tf 0.95
In this case, 1.6 A or 2 A fuse is recommended

If you choose a UL rated fuse, the rated current is:
In 1.5 A
In = --------- = ------------- = 2.1 A
Of X Tf 0.75 X 0.95
A fuse with a rated current of 2.5 A should be used

4. Voltage drop/cold resistance--Ud/R

* Under normal circumstances, the resistance value of the fuse is inversely proportional to its rated current value.
* In the protection circuit, the resistance of the fuse is required to be as small as possible, so that its power loss is also small; therefore, its maximum voltage drop value is specified in the fuse parameters.
* Voltage drop of the fuse: the reading obtained after passing the rated DC current to make the fuse reach thermal equilibrium.
* The cold resistance of the fuse: the reading measured under the condition of less than 10% of the rated current
* The voltage drop of the fuse and the cold resistance can be converted to each other.
* The voltage drop of a small-size fuse has a greater impact on the low-voltage circuit, so be careful! In extreme cases, the operating current cannot be output.

5. Time-current characteristics---I-T characteristics or ampere-second characteristics

* When the current flowing through the fuse exceeds the rated current, the temperature of the melt gradually rises, and finally the fuse is blown. We classify this as an overload state.
* Time/current characteristic is the most important electrical performance index of the fuse, it shows the time range of the fuse fusing under different overload current loads.
* The fuse requires a certain overload capacity:
The maximum non-fusing current of UL standard fuse is 110%In;
The maximum non-fusing current of the IEC standard fuse is 150%In or 120%In
* The fuse is also required to blow out in time when the overload current exceeds the limit:
The minimum fusing current of UL standard fuse is around 130%In;
The minimum fusing current of the IEC standard fuse is around 180%In
* The time/current characteristic curve best describes the overload performance of the fuse. It is usually specified that several key points in the curve are used to assess the overload performance of the fuse.
* UL standard fuses stipulate test points such as 110%In, 135%In and 200%In;
* The IEC standard fuse stipulates test points such as 150% (120%) In, 210% In (200%) 275% In, 400% In and 1000% In
* According to different fusing characteristics, fuses can be divided into fast type and time delay type, etc.:
* Fast fuses are commonly used in resistive circuits to protect some components that are particularly sensitive to current changes;
* Time-delay fuse is commonly used in inductive/capacitive circuits with a large surge current when the circuit status changes. It can withstand the impact of the surge pulse when the machine is switched on and off, and it can still open the circuit faster when a fault occurs.
* According to IEC 127, DC power is provided to determine the fusing time of the fuse, and the time-current curve can be obtained. If AC power is provided, the fusing time will change, especially when the fuse is blown in a short period of time, it Will vary with the phase angle of the AC sine wave when the circuit is closed.
* The typical time-current characteristic curve is shown in the figure below:

Each curve in the figure represents the fusing characteristics of a specification fuse, and its fusing time can be found for each load current.

* Different types of fuses have different shapes of characteristic curves.

6. Breaking capacity---Ir

* When the current flowing through the fuse is so large that it is short-circuited, it is still required that the fuse can safely break the circuit without causing any destructiveness.
* The breaking capacity is the most important safety index of the fuse. It indicates the maximum current that the fuse can safely cut off under the specified voltage. The breaking capacity is also called the maximum breaking capacity or short-circuit breaking capacity.
* When the rated breaking current value is exceeded, the fuse may break, explode, splash, and cause unsafe phenomena such as burning and destruction of surrounding people or other components.
* We also know that the rated breaking capacity (in the UL file) is directly related to the rated voltage of the fuse. The larger the rated current, the smaller the rated breaking capacity.
* The breaking capacity of the fuse depends on the structure of the fuse and the material used. Generally speaking, most of the low breaking capacity fuses are glass shells, and the high breaking capacity fuses usually have ceramic shells, many of which are also filled with pure particles. Quartz material
* UL198-G specifications have different breaking capacities:
Under the AC 125V condition, the fuse must be able to cut off 10000 A. Under the AC 250V condition, the current that the fuse must be able to cut off is:

Rated current of the fuse Rated breaking capacity
0-1 A 35 A
1.1-3.5 A 100 A
3.6-19 A 200 A
10.1-15 A 750 A
15.1-30 A 1500 A

* IEC 127 specifies the breaking capacity under AC 250V conditions:
The low breaking capacity fuse (LBC) must pass 35 A or 10 In, whichever is greater
High breaking capacity fuse (HBC) must pass 1500 A
The enhanced breaking capacity fuse (MBC) must pass 150 A.
* According to the convention, when the protected system is directly connected to the power input circuit and the fuse is placed in the power input part, a high breaking capacity fuse must be used.
* In most secondary circuits, especially when the voltage is lower than the power supply voltage, a low breaking capacity fuse is sufficient.

7. Melting heat energy value—I2 t instantaneous current and pulse

* The internal instantaneous current comes from the switching operation of the capacitive and inductive energy storage elements in the protected circuit,
* External instantaneous current refers to the inrush current that is injected into the system like a surge from the outside and has a short duration.
* The impulse current or instantaneous current that lasts less than 10 milliseconds is called pulse current. Pulse is harmful. It may damage the fuse and cause the fuse to fail.
* In most cases, slow blow fuses are most suitable for circuit protection with pulses

I2t value and its application
* The I2t value is a direct measurement of the energy value required to cut off the fuse
* Total I2t (Clear I2t) = Melting I2t + Flashover I2t; Clearing I2t refers to the full thermal energy melting I2t during the complete disconnection of the fuse (equivalent to pre-arcing I2t in the IEC standard) refers to melting from the melt The energy arcing time required to start the arcing is from the moment the arcing starts to the arcing finally extinguished.For low-voltage fuses, the arcing time is very short and is usually ignored.

Examples of utilization of I2t value:
Pulse I2t
The I2t value of this pulse = 1/2 X 552 X 0.0005 = 0.75625 A2Sec, we need to select a fuse with a melting I2t value greater than this pulse

b. Pulsating current overload
Sometimes the applied current is not a stable DC, but a pulsating current like the following:
In this case, it is essentially necessary to determine the effective value (Ieff) or square root value (Irms) of the application current. The specific calculation steps are as follows:

① Determine the I2t value of each pulse
Pulse I2t value = (1.5A X 1.5A X 0.2m) + (1A X 1A X 0.8mS)
= 0.00045 + 0.0008 = 0.00125 A2Sec

② Determine the square root value of each pulse
Irms = (0.00125 A2Sec / 0.0013 Sec) = 0.98 (rms)
Therefore, we can choose a 1A fuse for this application.
In any case, the I2t value of the fuse must be greater than the I2t value of each pulse.

③ Determine the resistance of the pulse period
The I2t value of Littelfuse 429001 is 0.035 A2Sec
(The data can be obtained from the sample), see Chart II to get:

Pulse I2t 0.00125
---------- = ---------- = 3.5%
Melting I2t 0.035

In this application, the fuse can withstand more than 100,000 pulses.

Chart 1 Chart 2

* The I2t calculation formula listed in Chart 1 can be used to calculate the energy of different types of pulse current waveforms, and the closest waveform can be selected for approximate calculation.
* The I2t of the fuse must be added with the necessary margin on the basis of the calculation.

8. Durability/Life
* The life of the fuse is very long, it can be almost synchronized with the life of the device under the condition of no failure
* Method to test the life of a small fuse of IEC specification: under the condition of DC power supply, conduct a current of 1.20 In (or 1.05 In) for one hour, disconnect for 15 minutes, continue for 100 cycles, and finally use 1.5 In (or 1.15 In) ) When the current is on for one hour, there should be no fusing or other abnormal phenomena during this period.
* The storage period of the fuse is not less than two years under normal conditions, and it can be stored again after passing the re-inspection after the expiration date.

9. Structural features and installation form


* Tubular: glass tube-low breaking capacity, ceramic tube-high breaking capacity; filled with fine quartz sand-used for arc extinguishing, glass tube discoloration-fusing indication; internal welding and external welding; plus lead cap-for Soldering (sometimes it is necessary to shape the lead first);
* Miniature: resistive type, transistor type, thin film type...
* Others: insert type, bolt type, sealed type, alarm type...
* Melt structure: round wire, flat wire, monofilament, double wire, composite wire; straight, wavy, zigzag; flake melt (with one or more bottlenecks)
* Combined melt: fuse winding, tin ball, metal sheet, resistance, etc.

Installation form

* Panel installation: fuse box, fuse socket ... ...
* Bottom plate installation: fuse clip, fuse clip holder ... ...
* Printed circuit board installation: plug-in installation (wave soldering): radial lead, axial lead...Surface mounting (infrared welding): traditional type, film type... sometimes it is necessary to heat outside the tube Shrink the sleeve to insulate the fuse from the surrounding components
* Hanging installation: fuse cover

10. Full certification

* The safety certification and requirements of the fuse will be discussed in detail in the sixth part of this book.
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