Electronic fuse emulates fast- or slow-blow fuses

John A Hasse, Colorado State University, Fort Collins, CO -- EDN, 11/9/2000

The electronic-fuse circuit in Figure 1 combines the properties of a current transducer and a solid-state relay to disconnect low power at preset levels. Using this circuit lets you avoid the bother of stocking and replacing fusible links.

The circuit simulates fast- or slow-blow fuses as large as 10A in 1 or 2A increments using a convenient pushbutton reset. This device can bracket trip levels of functioning equipment or help locate chronic faults. The circuit full-wave-rectifies the output from the Lem current transducer and applies the result or with a variable delay to a window comparator. The reference steps are 600 mV/1A at Pin 7 as a high level. Signals greater than the H pin of the CA3098 set a flip-flop in the CA3098, which removes drive to the solid-state relay. Forcing Pin 1 of the CA3098 from –1V to 1V resets the flip-flop and restores load power. An offset current through the 15-kΩ, 1% resistor adds –300 mV to the set level, which is equivalent to –1 or –2A from the integral switch settings of 1 to 10 with standard 30° indexing. The circuit blocks two switch positions from use. You adjust the LF411CN with no ac load to zero voltage at Pin 6 relative to dc common.