Read Fuse-and-RL Circuits: Follow the Current in Time
An ideal fuse is a threshold device: it has zero resistance until its branch current reaches the rated value. First find that current as a function of time while the fuse is intact.
Electricity questions depend on identifying charge, a conducting path and the condition that controls current or magnetic effect.
Start with the condition
For the RL branch use i(t) = I-infinity[1 - exp(-tR/L)]. Use the circuit topology to express the fuse current, then set it equal to the 4 A threshold.
Build the governing relation
The solution must be positive and the limiting current must exceed the fuse rating; otherwise the fuse can never blow.
Finish with a physical check
Check the limiting case before selecting an option: remove the special feature in the diagram or make the relevant parameter very large or very small. The result should reduce to a familiar relation.
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- ✓State the physical condition before choosing a formula.
- ✓Keep direction, sign and constraint information explicit.
- ✓Test one limiting case before committing to an option.
Analysing the circuit after the fuse opens cannot tell when it first opens; use the intact-circuit current up to the threshold.
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