Today, outdated safety elements used in electronic and electrical circuits are being replaced by better designs. Among them, self-healing fuses (SP) are of particular interest, with which we will get acquainted in more detail.
Device and principle of operation
Externally, the JV element looks like a round or oval plate with connecting branches for mounting using TNT technology. Moreover, he
manufactured in the form of a rectangular plate housing (if intended for SMT mounting). The specificity of the element under consideration is that in its characteristic there is a positive branch of the temperature coefficient of resistance (it increases with heating).
According to the principle of operation, the joint venture is more reminiscent of a well-known electronic element, a posistor or a bimetallic thermal fuse.
A feature of the SP device, which determines the principle of its operation, is the presence of a special non-conductive polymer in which carbon is integrated (in the form of conductive tracks). As long as a current with a nominal value flows through the tracks, nothing changes in the state of the joint venture.
But when its value begins to increase, the polymer substance heats up and gradually turns into an amorphous state. As a result of such transformations, the plate increases in size, which causes rupture of the conductive graphite tracks. The consequence of this process is an increase in resistance, which causes a rapid heating of the fuse base and the subsequent sharp interruption of the circuit.
After the base cools down, the reverse process begins, consisting in the crystallization of the polymer and the restoration of the conductive tracks. The fuse returns to its initial (working) state. If we represent the process of triggering the joint venture graphically, it will look like this:
From the figure above, it can be seen that the current flows through the circuit as long as the temperature indicator is between 20-120 degrees. Upon reaching 130-144 degrees, the resistance increases sharply (changes exponentially).
Performance and application
According to those attached to the device. documentation, its parameters are reflected by the following indicators:
- The maximum stress that the product can withstand until failure.
- Rated current in operating mode (up to the shutdown limit).
- Minimum operating current during heating.
- Resistances in the section before triggering.
- The same indicator, but one hour after the shutdown.
The last two parameters are estimated taking into account the air temperature. Moreover, the smaller they are, the better, since in this case less power is dissipated on the element. Despite all the disadvantages inherent in the joint venture (limited resource and relatively high cost), protective elements of this class are widely used in IPB.
As part of these devices, they are operated in conjunction with high-speed analogs.