In the design of the circuit, the safety of the circuit is a problem that needs to be considered. That is to say, the protection of the circuit must be considered, and the control of the circuit, as well as the matching of each component in the whole circuit. The following focuses on the selection of fuse design.

In the process of circuit design of the whole vehicle, the determination of fuse capacity needs to be based on the load size and electrical performance characteristics of each electrical system of the whole vehicle, and the determination of fuse type needs to be based on the electrical characteristics of each electrical system and the design of the central electrical box.

One, the determination of fuse capacity

After determining the circuit current, select the appropriate specifications of the fuse.At room temperature (25℃), 75% of the fuse capacity is selected as the working current of the electrical load. When the ambient temperature is high, the current carrying capacity of the fuse will decrease. The following two empirical formulas can be referred to, which are described as follows:

A. Formula 1:

I F – Ideal rated current for fuses

RR- temperature reduction rate

I N – Normal operating current

Where I n=P/Un, can be calculated according to the load power and rated working voltage, and then through the temperature reduction rate curve of the fuse to get the temperature reduction rate value under the working environment temperature of the fuse.I F value can be obtained by plugging In In and RR into the formula.Select the current rating of the fuse which is the most identical to or larger than I F in the current specification.

B. Formula 2:

I protection is equal to I 0 over kT

kt =1- (t- t0 ) *k

I is equal to P over U

Among them:

I insurance – Calculates the current of the fuse

I 0 – refers to the working current of an electrical appliance at room temperature (25℃). Generally, I 0=I, 0.75

I – The rated current of the electrical appliance

P – The rated power of the electrical appliance

U – Rated voltage of electrical appliance

Kt – Correction factor for maximum operating temperature of fuse

K – Temperature coefficient K =0.0015/ ℃

T – Maximum operating temperature where the fuse is located

T 0- The temperature at room temperature, t 0=25℃

In general, the specifications of the fuse end with the nearest specification greater than the calculated value;For shorter working hours, not often

You can choose a fuse of a relatively small size for the electrical components you use.

Design input:

The maximum indoor working temperature is 85℃

The maximum working temperature of forward compartment is 120℃

For the calculation formula of the above two fuses, formula one is preferred

In the case of common insurance, the insurance size should be calculated according to the total power of the load, but the corresponding relationship between the insurance and the conductor must be selected according to the above table.However, for the load on the ECU and its sensors, airbags, ABS and other electrical components that affect the safety of the vehicle, it is not allowed to share the insurance.Each large current load must have its own separate fuse.

Second, the selection of fuse type

Choose the type of fuse (slow fuse or fast fuse) according to the electrical load characteristics in the circuit.Fuses can be divided into slow fuses and fast fuses according to different fuses.Fast fuses are commonly used in resistive circuits to protect some components that are particularly sensitive to current changes.The slow fuse is often used to disconnect the circuit quickly when the circuit state changes due to the fault of large inrush current.

Finally, according to the inductive/capacitive circuit (e.g., fan motor circuit), it can withstand the impact of the surge pulse when the switch machine, and the assembly location of the real risk wire is selected suitable structure of the fuse type.Priority is given to fuses that take up little space after installation.

Three, the setting of fuse position

The position of fuse in the circuit should be taken into account when designing the circuit in order to give full play to the protection of fuse in the circuit.

When A short circuit occurs at A or B as shown in the circuit diagram, the fuse can be blown out in time to protect the circuit and switch;

As shown in the circuit diagram, the switch will burn out in case of short circuit at A, and the fuse cannot be blown out in time, so the front line and switch cannot be protected.If a short circuit fuse occurs at B, it can be timely fuses, which can play the role of protecting the circuit.

As shown in the circuit diagram in A short circuit when the switch will still burn out, and fuse will not fuses, that is: this way of the fuse can not play the protection of the circuit and electrical components.So the location of the fuse in the circuit design is more reasonable is the circuit diagram.

Note: for the case that the relay and fuse are integrated in one electrical box, the fuse connection method is not relevant as shown in figure or Figure (because the possibility of short circuit in the same electrical box is not likely), on the contrary, the most reasonable connection method is referred to figure.

Iv. Other matters needing attention

After determining the specifications of the fuses, priority is given to the used fuses to reduce the development cycle (reduce product performance test verification).