Zener Diode

Electricity and Magnetism Level 3

Assume:

  • Zener diode forward bias 0.7V
  • Zener breakdown region 5V
  • Inductance is 1 mH and resistance is zero

If the switch is open :

  • What is the voltage over resistor R0?

If the switch is closed :

  • The circuit is left for enough time to reach steady state, what will be the measured current passing through R1?

  • Theoretically,​ what current will be passing through the inductor branch?

Answer the questions consecutively.

0.7 and very small current and infinite 5 and very small current and infinite 0.7 and 0 and infinite 5 and 0 and infinite

Brandon Louw by Brandon Louw , 29, South Africa

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2 solutions

Brandon Louw
Jun 19, 2016

Open Switch

The voltage measured over that branch is 0.7V, the zener is forward biased and in parallel with the resistors

Closed Switch

The second answer could be explained with this circuit diagram after closing. Assuming that the inductor is fully charged, it shorts out and forms a single piece of wire, also shorting out the entire network

Thus for the third solution, ohms law shows that

  • I = V R \frac{V}{R} and R = 0
  • I = infinity

Theoretically this is true, but in practice, resistance always exists and losses are not avoidable.

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why the second question answered 0 A 0 A ?

Mbah Abal - 4 years, 11 months ago

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After an inductor reaches steady state in DC circuits, it essentially becomes a short, by keeping this in mind (when switch is closed), you will see that it shorts out the source and no current will flow through the rest of the system and all the current will pass through the short instead.

Brandon Louw - 4 years, 11 months ago
Veeresh Pandey
May 27, 2019

s w i t c h o p e n switch \ \ open : zener diode forward bias - voltage across 15k resistor =0.7V

s w i t c h c l o s e d : switch \ \ closed: when steady state reached - inductor shorted -whole current bypass through it because current follow least resistance path -as current through both resistors is zero there will be no drop across both of them........my intution said i should mark option D but against it i do purely mechanical solution and marked C but i still think very small current will pass through 1 ohm resistor .

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