Exercise: Power Switch

Objective

Test a simple circuit on a breadboard. Practice using a DMM to measure current and voltage.

We will use solderless breadboards a lot for prototyping circuits. The connection points accept component leads and solid wire. Each short row of five points is connected internally and is used for signal connections. The long rows along the edges are connected internally and are normally used for power distribution.

The easiest way to connect a bench supply to a breadboard is with solid core wire which can plug directly into the buss connections at top and bottom of the solderless breadboard. It’s a good habit to follow a consistent color code with red for positive and black for ground.

Steps and observations

  1. Wire and test the circuit as shown with a switch, resistor, and LED.
  2. Try operating the switch; if the LED doesn’t light, try reversing it. It is a polarized component and the direction of current flow matters.
  3. Measure the voltage across the LED in both on and off states. Voltage is measured between two points; the meter is placed in parallel with the LED.
  4. Current is measured through a conductor; measuring the LED current requires connecting the meter in series with the LED by disconnecting either wire leading to the LED and connecting the meter across the opened connection.
  5. Set the meter to DC current mode (mA) and move the positive probe to the connector for current.
  6. Open the LED circuit, connect the meter in-circuit, and observe the current in both on and off states.
  7. The LED acts as a resistor. What is the computed resistance in the on state? This can be calculated from the current measured for a given applied voltage.

Comments

The basic rules of current flow and voltage potentials in a circuit graph are captured by Kirchhoff’s circuit laws.

Schematic

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Other Files

  1. EAGLE file: power-switch.sch