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Nov 1 2019

The Selection Techniques of Fuse Resistors

 Fuse Resistor

Resistors and fuses are similar in material and construction, and fuse-type resistors have both functions. They can be used as resistors. Once the current is abnormal, they act as fuses to protect the equipment. The cost is reduced due to the dual function. The fuse resistor can be divided into metal film fuse resistor, fuse type wirewound resistor and fuse type cement resistor. The power is 1/4W, 1/2W, 1W and 2W. As the power increases, the product size will continue to increase. The resistance of the fuse resistor is generally small, and most of it is less than 1 ohm. It often acts as a sampling resistor in the circuit. When a surge occurs, or other large currents are generated, it is broken to make the circuit open and protect. At the same time, most of the fuse resistors are surface mount type, and the performance of them is relatively stable. It is generally called the Jetbe fuse resistor, and the Jetbe surface mount type fuse resistor. The product has fast-broken one and slow broken one.

The Selection Techniques of Fuses

A good or suitable fuse should meet at least three requirements: be broken when needed, not be broken when isn't needed and the break must be safe.

The first function of the fuse is the protection function, that is, the fuse should function when protection is required, which is the first consideration when choosing a fuse. Under normal circumstances, the rated current of the fuse must be greater than the normal working current of the circuit, and has a certain overload capacity. But if the margin is too large, it will reduce or weaken its protection function. The fuse doesn't work when needed and it may cause damage to the protected components or even more serious dangerous consequences. The primary reference tool for designers when selecting fuses is the “time-current characteristic curve” in the product specification provided by the fuse manufacturer. Since the breaking time reflected on the curve is under normal atmospheric conditions, we need to properly consider the influence of ambient temperature and so on if necessary. The protection function of the fuse can be met only when selecting the fuse with the proper breaking characteristics and the appropriate rated current specifications.

The second function of the fuse is the load-bearing function, which is commonly referred to as the pulse-resistant capability. This is an important issue that we must consider when choosing a fuse. In the process of using the fuse, the chance of the appearance of normal current fluctuation or transient pulse is much more than the fault overcurrent. So, in a sense, this consideration is particularly important and practical for the use of the fuse. As long as the fuse's melting heat value I2t is greater than the energy of the circuit pulse, the fuse can withstand. The "time-melting heat curve" is a tool for designers to choose the fuse with the ability to withstand the pulse (again, the current-melting heat curve can also be used). Furthermore, it can be seen that the fuse will be damaged even if it is not blown when subjected to pulse shock. In other words, the I2t of the fuse will be reduced, that is, the ability to withstand the pulse is reduced. Therefore, the attenuation factor must also be considered when selecting the fuse. A typical simple calculation requires a 3-5 times the margin to ensure that the fuse has sufficient pulse resistance. There is a contradiction between the pulse resistance of the fuse and its protection performance. In these two aspects, we must find a reasonable balance and find the best combination point. The load-bearing function (pulsation resistance ability) can be met only when selecting a fuse with the proper melting heat value and a sufficient and reasonable safety margin.

The third function of the fuse is the safety function. High-quality and reliable fuses should guarantee the safety before, during and after the operation, that is, safely turned on and safely broken. The main technical indicator that guarantees the fuse's requirement is the breaking capacity. The breaking capacity is the maximum current that the fuse can safely cut off the circuit. In general, it refers to the short-circuit current, that is, the fuse must be able to break the circuit absolutely safely when it encounters a short-circuit current, which means that no unsafe factors occur during the breaking process, such as continuous arcing, multiple conduction, crushing, splashing, burning, and even explosion. The breaking capacity of each fuse must be greater than or equal to the maximum short-circuit current of the circuit being protected. The rated voltage of the fuse determines its withstand voltage and this is another indicator of the safety of the fuse. It can only be used in circuits where the operating voltage is less than or equal to the rated voltage of the fuse. Safety components are safely certified in countries and regions around the world. Safety certification of fuses is also essential for its safety functions. The safety function of the fuse can be satisfied by selecting a fuse type that has sufficient breaking capacity and rated voltage and obtains the safety certification of the required application area.

In summary, proper breaking characteristics and rated current, adequate and reasonable melting heat value, appropriate breaking capacity, rated voltage and safety certification are necessary to ensure the main function of the fuse. Only on the basis of these three conditions, while coordinating the constraints between protection performance and pulse resistance, and obtaining the most reasonable balance, we can make a judgment. That is, such a fuse is a good and reliable fuse.

2.1 Rated Current — In

The rated current of a fuse is its nominal rated current, which is usually the maximum current that the circuit can operate.

Correct selection of the rated current of the fuse:

For example: Operating current of the circuit: Ir = 1.5 A, the rated current of the L fuse should be: In = Ir/Of = 1.5/0.75 = 2A

Here, Ir is the circuit operating current, and Of is the reduction factor of the UL fuse, so the fuse of 2A should be selected.

There is no reduction rate requirement for IEC fuses, which means that Ir = In

If the special rated current is not universal, the nearest higher value should be selected.

Wrong choice: regard the current value that you wants the fuse to be broken as the rated current value.

2.2 Rated Voltage — Un

The rated voltage of a fuse is its nominal voltage rating, which is usually the maximum voltage that the fuse can withstand when 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.

The correct selection of the rated voltage of the fuse: it should be equal to or greater than the circuit voltage. For example: 250V fuse can be used for 125V circuit.

For low voltage electronic applications, an AC rated fuse can be used in DC circuit.

About the rated voltage of the fuse, you should mainly consider that whether the fuse has the ability to break the maximum current given when the circuit voltage does not exceed the rated voltage of the fuse.

Misunderstanding: The rated voltage of the fuse must be consistent with the circuit voltage.

2.3 Ambient Temperature

The ambient temperature of where the fuse is or the known operating temperature has an effect on the operation of the fuse: the higher the ambient temperature, the hotter the fuse is, and the shorter its life is.

Regardless of whether it is UL or IEC, the indicators of the fuse refer to the temperature as 25 °C. If the operating temperature is high in a small environment, for example: 

Use fast-broken fuse to work in a small environment of 90 °C and under 1.5A current. If IEC fuse is used, the rated current is: In = In/ Tf = 1.5A/0.95 = 1,58 A, 1.6 A or 2 A fuse is recommended.

If a UL fuse is used then the rated current is: In = In/OfxTf = 1.5A/0.75x0.95 = 2.1 A, 2.5 A fuse should be selected.

2.4 Voltage Drop / Cold Resistance — Ud / R

In general, the resistance of a fuse is inversely proportional to its rated current.

In the protection circuit, the smaller resistance value of the fuse will be better because the loss power will be less. Therefore, the maximum voltage drop value or the cold resistance value is specified in the fuse technical parameters. However, it is not used as the product acceptance basis.

Voltage drop of the fuse: The value after the fuse reach the heat balance under the DC rated current.

Cold resistance of the fuse: the value measured at less than 10% of the rated current

The values of the voltage drop and cold resistance of the fuse can be converted to each other.

The voltage drop of the specification fuse has a great influence on the low-voltage circuit, so be careful! In extreme cases, the required operating current cannot be output due to the large resistance.

2.5 Breaking Characteristic

It is also known as the time-current characteristic or I-T characteristic or the amperage characteristic of the fuse. It is the main electrical performance index of the fuse, which indicates the time range in which the fuse is broken under different overload current loads. When the current flowing through the fuse exceeds the rated current, the melt temperature gradually rises, and finally the fuse is broken, and we attribute all of these to an overload condition.

The fuse needs to have a certain overload capacity:

The maximum non-broken current of the UL fuse is 110% In; the maximum non-broken current of the IEC fuse is 150% In or 120% In

The fuse also requires a timely broken when a limited amount of overload current is exceeded:

The minimum breaking current of UL fuses is around 130% In; the minimum breaking current of IEC fuses is around 180% In

According to the different fuse characteristics, the fuse can be divided into fast type and time delay type:

Fast fuses are commonly used in resistive circuits to protect components that are particularly sensitive to current variations. Time-delay fuses are commonly used in inductive or capacitive circuits with large inrush currents when the circuit state changes. It can withstand the impact of surge pulses when switching on and off, and can still open the circuit faster when a fault occurs.

Each curve represents the breaking characteristics of a specification fuse, and its breaking time can be found for each load current. Different types of fuses have characteristic curves of different shapes.

The time/current characteristic curve best describes the overload performance of the fuse and is the primary reference for the designer to select the fuse.

 Figure 1. Time-current Characteristics Table

Figure 1. Time-current Characteristics Table

It is usually stipulated that several key points in the curve are used to evaluate the overload performance of the fuse. It is the main basis for quality evaluation or acceptance of fuses.

2.6 Breaking Ability — Ir

The breaking capacity is also called the maximum breaking capacity or the short-circuit breaking capacity or the breaking current. Breaking capacity is the most important safety indicator for fuses. It shows the maximum current that the fuse can safely cut off at the specified voltage. When the current flowing through the fuse is so large that it is short-circuited, the fuse is still required to safely break the circuit without causing any damage. When the rated breaking current value is exceeded, the fuse may be broken, exploded, splashed, causing unsafe phenomena such as burning and destruction of surrounding people or other components. Conventionally, when the protected system is directly connected to the power input circuit and the fuse is placed in the power input section, a fuse with high breaking capability must be used. In most secondary circuits, especially when the voltage is lower than the supply voltage, a low-breaking fuse is sufficient.

2.7 Melting Heat Value — I2t

The fuse's melting heat value (If2t) refers to the energy value required for the melt to be broken. It is usually used as a technical specification for the fuse to withstand surge capability, where I is the overload current and t is the breaking time.

Principle:

When selecting a fuse, you must consider If2t>Ir2t, that is, the melting heat of fuse should be greater than the heat released by the surge current.

The fuse's breaking time is related to the heat generated by the current, the heat dissipation condition and the heat capacity characteristics of the fuse. Many factors will affect the fuse's breaking time, so the fuse will have different If2t at dThe fuse's fusing time is related to the heat generated by the current, the heat dissipation condition and the heat capacity characteristics of the fuse. Many factors will affect the fuse's fusing time, so the fuse will have different If2t at different breaking time or under different breaking current, that is, If2t is not a constant.

The energy/time curve best describes the melting heat changes of the fuse and it the the primary reference for the designer to select the pulse withstand capability of the fuse.

Frequency of pulse impulse resistance:

When If2t > Ir2t, the fuse should be able to withstand the impact of the pulse. And it will not be broken but will suffer some damage, thus slightly reducing its If2t. By calculating and selecting the relationship between If2t and Ir2t, you can know the frequency of pulse impulse resistance that the fuse can withstand. Conversely, the frequency of surges impulse that the fuse can withstand depends on the relationship between If2t of the fuse and Ir2t of the circuit pulse that you choose.

The approximate relationship between If2t and Ir2t of AEM fuses:

Ir2t ≤ 30% If2t: 100,000 times

Ir2t ≤ 38% If2t: 10,000 times

Ir2t ≤ 48% If2t: 1,000 times

The approximate relationship between If2t and Ir2t of Littelfuse fuses:

Ir2t ≤ 22% If2t: 100,000 times

Ir2t ≤ 29% If2t: 10,000 times

Ir2t ≤ 38% If2t: 1,000 times

2.8 Durability / Life

The life of the fuse is very long and can be synchronized with the life of the device in the absence of faults.

Method for testing the life of a small tubular fuse of IEC: under DC power conditions, a current of 1.20 In (or 1.05 In) is conducted for one hour and disconnected for 15 minutes and last for 100 consecutive cycles. Finally, a current of 1.5 In (or 1.15 In) is conducted for one hour without being broken or other abnormalities.

The storage period of the fuse is not less than two years under normal conditions, and can be stored again after passing the re-inspection.

2.9 Structural Features and Installation Forms

Tubular:

Glass tube - low breaking capacity, ceramic tube - high breaking capacity;

Filled with fine-grained quartz sand - used for arc extinguishing, glass tube discoloration - breaking indication;

Internal welding and external welding;

Add lead caps - for soldering (sometimes the leads needs to be formed first)

Miniature: Resistive, Transistor, Thin Film

Slice: film type, multi-layer monolith, resistive

Other: insert type, bolt type, sealed type, alarm type

Melt structure: round wire, flat wire, monofilament, double wire, composite wire

 

The Selection Techniques of Resistors

3.1 Summary of the Normalized Selection Direction of Resistors

This principle of normalization selection is only a "contour" for resistance selection. According to the selection experience of engineers in the past, it has a popular selection meaning. In the demanding circuit design, it is necessary to further consider the selection of the resistor according to the electrical requirements in the specific circuit design.

A. "contour" of resistor selection

1. Metal film resistor: The power of 1W or less is preferably a metal film resistor, and the power of 1W or more is preferably a metal oxide film resistor.

2, Carbon film resistor: It is the category for the phone-specific. The preferred rating information is marked with a "T".

3. Fusible resistor: Not recommended. The reaction rate is slow and cannot be recovered. Fast, recoverable devices are recommended to protect the circuit and reduce maintenance costs.

4, Wirewound resistor: high power resistors.

5, Integrated resistor: SMT. The plug-in project only retains the parallel type, and the plug-in independent project will be phased out, replacing it with a chip integrated resistor of the same classification.

6. Chip thick film resistors: In the direction of miniaturization and high power, the preferred library will be dynamically adjusted as the direction of adaptation develops. This type of resistor is the preferred object for low power resistors.

7. Chip thin film resistors: It is recommended to use a higher precision category.

B. Pairing table of selection and application requirements

1. Performance requirements - optional types

Figure 2. Performance Requirements and Optional Category Selection Table 

Figure 2. Performance Requirements and Optional Category Selection Table

2. Rated power - range of resistance value

Figure 3. Rated Power and Resistance Range Selection Table 

Figure 3. Rated Power and Resistance Range Selection Table

3.2 Summary of the Selection Principle of the Characteristic Parameters of the Resistor

In the first part, the characteristic parameters of the resistor are explained in detail. Among the many concepts, there are two concepts that are particularly important for the selection of the resistor - the nominal resistance and the resistance tolerance.

The nominal resistance is determined by the resistor design and is typically the resistance value marked on the resistor. The resistance obtained by measuring the resistor under specified conditions is called the actual resistance. In order to facilitate production and use, the state has stipulated a series of resistance values as the standard values of the products. These resistance values are called the nominal resistance series of resistors. In general, the precision is related to the nominal resistance series. The higher the precision, the denser the selected nominal series; the lower the precision, the thinner the selected nominal series. Due to the needs of the commercial production of the factory, the specifications of the reactive component products are provided in a specific series. Considering the technical and economic rationality, the E series is currently mainly used as the reactive component specification. Commonly used series are E6, E12, E24, E96 series.

There may be a deviation between the actual resistance of the resistor and the nominal resistance. The maximum allowable range of this deviation is called the resistance tolerance, also known as precision. It is usually expressed as a percentage of the nominal resistance.

After understanding the nominal resistance and resistance tolerances, let's take a look at the selection of resistors.

General characteristics parameter selection requirements for resistors:

1) Precision

Do not blindly pursue the precision of the resistor itself in the design, even if the high-precision resistor is affected by the environment and it may exceed its range. Therefore, we should pay more attention to the indicators of reliability testing. At present, the precision of selecting resistors is not recommended to exceed 0.1%. The precision of commonly used thick film resistors is 5%. The precision of 1% requires to select thick film resistors. The precision requirement of 1% or less is recommended to use thin film resistors.

2) Do not use the limit and edge specifications

The limit specifications of each class of resistors are not used such as the edge specification of the maximum and minimum resistance values in a specific series of resistors.

3) Derating

Derating is the most important means to improve the reliability and life of resistors. The power of the resistor depends on the size of the package. The power of the thin film resistor is very small, generally less than 1W. When the resistor is used, the power must be derated.

Different types of resistors have different insulating media and self-healing mechanisms. The requirements for the degree of derating of withstand stress (mainly operating voltage, power consumption and working environment temperature) are different, but they are generally used under 0.6 times of rated withstand stress and no more than 0.75 times. It is recommended to reduce the derating curve by another 80%. The winding resistor has a large power characteristic.

The calculation method of the rated power of the resistor:

When the resistance is less than the rated resistance, the rated voltage:

 The Calculation of the Rated Power

The Calculation of the Rated Power

When the resistance is less than the rated resistance, the rated voltage is equal to the highest voltage.

4) The Changes of Resistance Value

The resistance value of the resistor in actual operation is different from the nominal resistance value and is related to the following factors:

Resistance deviation: In actual production, the resistance of the resistor will deviate from the nominal resistance, and this deviation should be within the tolerance of the resistance.

Working temperature: The resistance of the resistor changes with temperature. This characteristic is measured by the T.C.R value, which is the temperature coefficient of resistor.

Voltage effect: The resistance of the resistor is related to the voltage applied, and the change can be expressed by the voltage coefficient. The voltage coefficient is the relative change in the resistance of the resistor when the applied voltage changes by 1 V.

Frequency effect. As the operating frequency increases, the distributed capacitance and inductance of the resistor itself play an increasingly important role.

 Time dissipation effect. The resistor gradually ages as the working time increases, and the resistance value gradually changes (in general, it increases).

The resistance value drift under external stress should be within the range required by the circuit, and the aging factor should also be considered. Design margins should be given (typically half the range of circuit requirements), such as circuit requires that it can vary within ±10%, resistors that vary within ±5% should be selected.

5) Rated Working Temperature

Each specific type of resistor has a specified rated ambient operating temperature range and should not exceed the specified ambient operating temperature range in actual use.

At present, the resistor with small TCR is only thin film resistor. In general, the carbon film and the ceramic resistor TCR are negative, and for the low TCR design, 10 ppm is recommended. The TCR of the resistor of different materials varies greatly. The approximate range can be seen from the following table:

Figure 4. 不同材料电阻的TCR.jpg 

Figure 4. TCR of the Resistance of Different Materials

6) The Power Derating Curve

When the working environment temperature is higher than 70 °C, the derating should be carried out on the basis of the original use. The derating curve is shown in figure 5:

 Figure 5. The Power Derating Curve of the Resistor

Figure 5. The Power Derating Curve of the Resistor

7) Surface Metal of Pin

The surface metal of the pin is Sn, Pb or Sn. The soldering performance is good and the price is low. Try to avoid the use of precious metal pins or external electrode resistors. (For special types of resistors, if the industry commonly use precious metal as the surface metal of the pin, then you should adopt the industry's general standards).

8) Installation

Use surface mount resistors whenever possible. Surface mount is not only efficient in production, small in size, but also low in price due to heavy use. To save space, surface mount integrated resistors (which are chip thick film resistor arrays, also known as resistor banks or resistor networks) can be used.

3.3 Case Analysis of Power Meter Resistor Selection

Now take the voltage resistor sampling on the power meter as an example to explain how to select the resistor. The specific circuit requirements are:

1. Applied to sampling circuit

2. The voltage across the resistor is 500VAC

3. The resistor is less affected by the ambient temperature

4. The resistance value is around 1.5M

5. High precision requirements for resistor

· This resistor is used in the sampling circuit. The power requirements are not high and the precision requirements are relatively high. Film resistors (metal film, chip thick film, chip thin film) can be used. Considering the price, actual resistor package, and circuit installation, the film resistor are chosen.

· This circuit is less affected by temperature (TCR value is small, generally less than 100ppm).

· Parameter selection:

(1) According to the common component series E24, we choose 1.5M

(2) According to the actual use, the resistor precision is 1%.

(3) The TCR is 100ppm and its precision is in the range of three thousandths.

(4) Power = U * U * ÷ 1.5M = 0.3W, derate 70% and select 1W resistor

(5) Rated voltage = 500 * 1.414 = 700, select 1KV withstand voltage

The preliminary selection results are:

Metal film resistor, 1.5M, 1% precision, TCR of 100ppm, 1W power.

After the preliminary selection, the circuit needs to be tested after the sample is produced, the specific parameters of the resistor are adjusted, and the selection test is repeated to achieve stable operation of the system, which is a complete selection of the resistor.

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