Tricky Circuit

D + R

A one-way gate for current

Reverse a diode and discover why polarity matters.

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D + R · Diode
Diode / anode / cathode. A diode conducts mainly in one direction. D marks the diode, R the current-limiting resistor. The bar on its symbol is the cathode; the other terminal is the anode.
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EXPERIMENT BENCH

Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.

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A one-way gate for current

Fixed 0.7 V forward drop; reverse leakage, breakdown and temperature are omitted.

Forward current is (5 − 0.7)/R. Reversing the diode blocks current in this model. V(D) is measured from anode to cathode, so it becomes negative in reverse. A real diode has a reverse-voltage limit.

Difficulty

  1. Build · A one-way gate for current

    Power on with R = 1000 Ω and the diode forward. Observe 4.30 mA.

    Need a hint?

    R = 1000 Ω

  2. Explore · A one-way gate for current

    Reverse the diode with power on. Observe zero current.

    Need a hint?

    I = 0 mA

  3. Solve it · A one-way gate for current

    Return the diode forward and set a current between 9 and 11 mA.

    Need a hint?

    R = 430 Ω → 10 mA

Extra reading and circuit recipes

A one-way gate for current

0.7 V is a model, not a magic switch

The simulator assigns the forward diode a fixed 0.7 V drop. A real diode changes gradually, and its drop depends on current, temperature and construction. Do not treat 0.7 V as a universal exact turn-on voltage.

Reverse does not mean indestructible

The model reports zero reverse current. Real diodes can leak a small current and can break down above their reverse-voltage limit. Breakdown means a large rise in reverse current; the component data sheet defines its permitted operation.

With the diode forward, what happens when R changes from 1000 Ω to 500 Ω?

  1. Current halves
  2. Current doubles
  3. Current stays unchanged
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The model leaves 5 − 0.7 = 4.3 V across the resistor. Halving its resistance doubles current from 4.3 mA to 8.6 mA.

Predict before reversing

Power on with R = 500 Ω. Predict the reading, then reverse the diode: current becomes 0 mA in this model. Restore forward direction and confirm about 8.60 mA.