Posted on May 23, 2012 9533
This circuit protects expensive portable equipment against all types of improper hookups and environmental hazards that could cause an overvoltage condition. It operates very quickly and does not latchup, that is, it recovers when the overvoltage condition is removed. In contrast, SCR overvoltage circuits can latch and do not recover, unless the power is removed.
Here, Ul senses an overvoltage condition when the drop across Rl exceeds 2.5 V. This causes Ul to apply a positive signal to the gate of Ql, turning it on and shorting the line going to the external circuits. Fuse 1 opens if the transient condition lasts long enough to exceed the i 2t rating.