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Self-induction on opening the circuit

Curricula: APC, Sek II

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Aim

Observe the self-induced EMF when a circuit containing an inductor is opened.

The bench

A 6 V source, a switch (closed) and a 20 H inductor with a winding resistance of 50 Ω; in parallel with the inductor — a 6 V / 0.5 W lamp. The inductor builds up its current in a couple of seconds after the bench is opened: about 0.12 A, 1.4 times the lamp current (0.083 A).

Procedure

  1. Wait a couple of seconds until the inductor current settles.
  2. Set the speed chip in the top panel to ×0.1 so that the flash is easier to see, open the switch and watch the lamp.
  3. Return the speed to ×1, close the switch, wait a couple of seconds again and open it.
Expected result

After the switch is opened, the lamp flashes three times brighter than usual and goes out within tenths of a second (at ×0.1, within a couple of seconds). The inductor maintains its current, and it flows through the lamp — about 0.12 A, 1.4 times its rated current; the voltage across the lamp briefly rises to about 9.3 V — higher than that of the source. The inductor is chosen so that its current is only slightly greater than the lamp current: with an inductor that builds up three times the current, the lamp would burn out the first time the switch is opened.

Questions

  1. Where does the energy of the flash come from?
  2. Why is a diode connected in parallel with the windings of a relay?
Answers
  1. From the magnetic field of the inductor. At 0.12 A it stores \(W = L \cdot I^2 / 2\) = 20 · 0.12² / 2 ≈ 0.14 J; after the switch is opened, this energy keeps the current flowing through the lamp and turns into heat and light in the filament and heat in the 50 Ω winding.
  2. When a relay is switched off, its coil keeps its current flowing, as the inductor on this bench does. With no path for that current, the self-induced voltage rises high enough to spark across the switch contacts or break down the transistor that drives the coil. The diode, reverse-biased in normal operation, gives the current a closed loop round the coil, the job the lamp does on this bench, so the switch sees little more than the supply voltage.