Resettable fuse, 5 amps. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network
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A diagram of such a "fuse" with a maximum load current of 5 A is shown in fig. 7.19. Its efficiency exceeds 90% in more than a tenfold range of load current changes. The current consumed in the absence of load is less than 0,5 mA.
To reduce the voltage drop across the "fuse", a germanium transistor is used as VT4. When the load current is less than the allowable, this transistor is on the verge of saturation. This state is supported by a negative feedback loop, which, when the transistor VT2 is open and saturated, is formed by transistors VT1 and VT3. The voltage drop in the collector-emitter section of the transistor VT4 does not exceed 0,5 V at a load current of 1 A and 0,6 V at 5 A.
When the load current is less than the protection trip current, the VT3 transistor is in active mode and the voltage between its collector and emitter is sufficient to open the VT6 transistor, which ensures the saturated state of the VT2 transistor and, ultimately, the conductive state of the VT4 key. With an increase in load current, the base current of the transistor VT3 is negative. The OS increases, and the voltage on its collector decreases until the transistor VT6 closes. At this moment, the protection is activated. The most unfavorable load for the "fuse" is a powerful incandescent lamp, in which the resistance of a cold filament is several times less than that of a heated one.
A test carried out with a 12 V (32 + 6) W car lamp showed that 0,06 s for warming up is enough and the "fuse" after it is turned on reliably enters the operating mode. But for more inertial lamps, the duration and repetition period of the pulses may have to be increased by installing a larger capacitor C2 (but not oxide). The GT806A transistor can be replaced with another from the same series or a powerful germanium one, for example, P210 with any letter index. If there are no germanium transistors or it is necessary to work at elevated temperatures, silicon transistors with h2t3\u40e 818 can also be used, for example, KT8101 or KT5 with any letter indices, increasing the value of the resistor R10 to 4 kOhm. After such a replacement, the voltage measured between the collector and emitter of the VT0,8 transistor did not exceed 5 V at a load current of XNUMX A.
When making a "fuse", the VT4 transistor must be installed on a heat sink, for example, an aluminum plate with dimensions of 80x50x5 mm. A heat sink with an area of 1,5 ... 2 cm2 is also needed for the VT3 transistor.
Author: Semyan A.P.
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