Fig.3. Chip fuse construction The main parameters of a fuse. Current rating , A nominal value of current. It is maximal safe current that fuse can pass – through for unlimited time at particular ambient temperature. Commonly a fuse is derated 25% regarding normal load. For example 10 A fuse is typically used for load of 7.5 A. Fuse actuation is a thermal process therefore maximal safe current depends on ambient temperature: the lower is a temperature the higher is a maximal safe current. According to recommendations of manufacturer the fuse has to be in addition derated or uprated depending on the circuit temperature. Voltage rating , V maximal voltage that can be applied to the terminals of actuated (burnt out) – fuse. , A maximum current that a fuse can safely interrupt at rated voltage (and Breaking capacity – any voltage below ). Time/current characteristics graphically specified functional relationship between current – value and (interval of time between the instant of current starting and the instant of clearing time current interruption). According to relationship between clearing time and current a fuse may be fast acting or slow blow (time delay). Particular fuse standard specifies some points in this curve. For example IEC 60127-4 standard specifies time/current characteristics as the following: Current Time 1.25 1 hour 2 < 2 min 10 <0.001 s (FF) t 0.001 0.01 (F) t 0.01< 0.1 (T) t 0.1< 1.0 (TT) t Designations: FF - very quick acting, F - quick acting, T - time-lag, TT - long time-lag. Fig.4. Typical time/current characteristics of fuse. Thermistors. A thermistor is a thermally sensitive resistor. There are two basic types of thermistors: negative temperature coefficient (NTC) thermistors, positive temperature coefficient (PTC) thermistors.
Fig.3. Chip fuse construction The main parameters of a fuse. Current rating , A – nominal value of current. It is maximal safe current that fuse can pass through for unlimited time at particular ambient temperature. Commonly a fuse is derated 25% regarding normal load. For example 10 A fuse is typically used for load of 7.5 A. Fuse actuation is a thermal process therefore maximal safe current depends on ambient temperature: the lower is a temperature the higher is a maximal safe current. According to recommendations of manufacturer the fuse has to be in addition derated or uprated depending on the circuit temperature. Voltage rating , V – maximal voltage that can be applied to the terminals of actuated (burnt out) fuse. Breaking capacity , A – maximum current that a fuse can safely interrupt at rated voltage (and any voltage below ). Time/current characteristics – graphically specified functional relationship between current value and clearing time (interval of time between the instant of current starting and the instant of current interruption). According to relationship between clearing time and current a fuse may be fast acting or slow blow (time delay). Particular fuse standard specifies some points in this curve. For example IEC 60127-4 standard specifies time/current characteristics as the following: Current Time 1.25 1 hour 2 < 2 min 10 t <0.001 s (FF) 0.001 t 0.01 (F) 0.01< t 0.1 (T) 0.1< t 1.0 (TT) Designations: FF - very quick acting, F - quick acting, T - time-lag, TT - long time-lag. Fig.4. Typical time/current characteristics of fuse. Thermistors. A thermistor is a thermally sensitive resistor. There are two basic types of thermistors: negative temperature coefficient (NTC) thermistors, positive temperature coefficient (PTC) thermistors.