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Useful information about miniature fuses
Electronics systems are getting smaller: Functions are performed by highly integrated ICs or microcontrollers, and external components are available in miniaturized designs. This means that the boards of modern electronic modules can handle much less surface area. Because voltage sources, loads, peripherals and functional units must also be effectively protected against overload in such systems, fuses must be installed in the critical circuits. However, with 5 x 20 mm², the fine fuses or device fuses (G fuses) that are actually quite small would still be too large for such applications. This is why micro-fuses have been developed for this purpose, which are sometimes referred to as subminiature fuses. Miniature fuses are used in power supplies of household and consumer electronics devices as well as in professional electronic systems.
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How are micro-fuses structured and what versions are available?
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How are miniature fuses specified?
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What was to be taken into account when selecting and operating micro-fuses?
How are micro-fuses structured and what versions are available?
As with conventional fine fuses, miniature fuses also contain a conductor that melts when the nominal current is exceeded for a longer period of time and thus interrupts the circuit. Unlike the usual device fuses, whose fuse conductors are installed in a glass or ceramic tube, these are in miniature fuses, depending on the design, e.g. in cylindrical or angular plastic housings, which are radially wired, in an axial-wired tube or are designed as SMD fuse, i.e. as a surface-mounted component without connecting wires, Which is designed for mounting directly on the circuit board.
In addition to fuses, circuits can also be protected with PTC fuses. These are resistors with a positive temperature coefficient. If the current exceeds a certain value, the component heats up and the resistance increases, was reduced. After cooling down, it returns to its original value. In contrast to fuses, however, PTC fuses do not completely interrupt the circuit.
How are miniature fuses specified?
The properties of fuses are defined in the European standard IEC 60127 or DIN EN 60127-1, VDE 0820-1:2015-12. Like other device fuses, miniature fuses are designed for a nominal current, the rated current, when it is clearly exceeded, e.g. by approximately 1.5 times the value, they trigger and interrupt the circuit. Due to the small dimensions, the values for the rated currents of miniature fuses range from a few mA to a few amps.
The reaction time to tripping, i.e. the tripping behavior, varies depending on the type of fuse: The versions of low-blow fuses "T", medium-blow "M", quick-blow "F" and super-quick-blow "FF" are available for micro-fuses as with other device fuses. During slow-blow and medium-blow fuses that do not immediately trigger short-term current peaks, e.g. when switching on a device, fast-blow and super-fast-blow start immediately. The current-time characteristic of a fuse is described as the most important characteristic in the data sheet on the basis of a diagram.
Another important feature of a fuse is the switching capacity. This is indicated in Ampere at a maximum voltage value. It describes the current permitted at the maximum rated voltage for the fuse, which can switch off a fuse without causing damage to it or to surrounding parts of the circuit when it is switched off. If this value is exceeded, there is a risk that the fuse element will produce an arc that, if the housing bursts, can cause smudder or fire. Due to the lower volume, the values of the switching capacity of miniature fuses are usually lower than with comparable fuses with larger mechanical dimensions. For example, a typical value for miniature fuses is 50 A at 250 VAC, while for larger fuse types it can be several hundred amps.
What was to be taken into account when selecting and operating micro-fuses?
The circuit parts of an electronics or electrical system to be protected provide the rated current and the tripping behavior of the fuse to be installed. If these are areas in which higher currents occur when switching on than during operation, e.g. when capacitors with higher capacity are charged or motors start up, then inert fuses must be used here. If micro-electronics components that could be destroyed within a short time in the event of overload must be protected, fast-acting or super-fast fuses must be selected.
The switching capacity must be dimensioned in such a way that the current, which results in a short circuit directly behind the fuse due to the voltage and the internal resistance of the circuit supplying the circuit, does not exceed. The fuse must also be designed for the maximum occurring voltage in the triggered state.
Because a certain electrical power is required to reach the melting temperature of the metal in the fuse, these components have ohmic resistance. The is higher the lower the rated current of a fuse. For example, with a miniature fuse for 50 mA, this is 12.5 Ω, was the result of a 0.625 volt drop in the rated current, which should be taken into account when designing the circuit to be protected.
Defective fuses may only be replaced by fuses with the same rated current, shutdown behavior and switching capacity. If the fuse is triggered again after replacement, the cause of the fault is obviously not eliminated. Use of a fuse for higher rated current or bridging is not permitted, because in the event of a fault, the voltage source and components in the protected circuit can be endangered, destroyed or even fires can be triggered.Like all other fuses, micro fuses become unusable after tripping and must be replaced after the fault has been rectified. To make this easier, there are fuse holders for some types, also known as "sockets" or "sockets", in which the miniature fuses can be easily inserted. Other versions must be soldered in and out.
The commercially available miniature fuses are RoHS compliant.