Practical case: Opposition to DC current change

Opposition to DC current change prototype (Maker Style)

Level: Basic. Observe the delay in lamp activation due to self-induction.

Objective and use case

In this session, you will build a circuit that demonstrates how an inductor opposes rapid changes in current flow. By placing a large inductor in series with a lamp (with a parallel bypass resistor), you will create a visual «soft-start» effect where the light starts dim and gradually brightens.

Why it is useful:
* Inrush Current Limiting: Used in power supplies and large motors to prevent blown fuses when devices are first turned on.
* Soft-Start Circuits: Protects delicate filaments and components from thermal shock.
* Filtering: Smoothes out noise and ripples in DC power lines.

Expected outcome:
* When the switch is closed, the lamp will turn on immediately but dimly.
* Over a short period (0.5 to 2 seconds, depending on the inductance), the lamp will become fully bright.
* This visualizes the inductor initially acting as an «open circuit» (blocking current) and transitioning to a «short circuit» (allowing full flow).
* Target audience: Basic electronics students and hobbyists.

Materials

  • V1: 12 V DC power supply or battery.
  • S1: SPST mechanical switch (toggle or push-button).
  • L1: 1 H to 2 H iron-core inductor, function: creates opposition to current change (e.g., a transformer primary winding used as a choke).
  • R1: 220 Ω resistor (1 Watt or higher), function: bypass path for visual contrast.
  • X1: 12 V / 100 mA incandescent lamp (small bulb), function: visual output load.

Wiring guide

Construct the circuit using the following connections. The node names (e.g., VCC, SW_OUT) help identify the electrical points.

  • V1 (DC Source): Connect the positive terminal to VCC and the negative terminal to 0 (GND).
  • S1 (Switch): Connect between VCC and node SW_OUT.
  • L1 (Inductor): Connect between node SW_OUT and node LAMP_IN.
  • R1 (Resistor): Connect between node SW_OUT and node LAMP_IN (this places R1 in parallel with L1).
  • X1 (Lamp): Connect between node LAMP_IN and 0 (GND).

Conceptual block diagram

Conceptual block diagram — RL Parallel Circuit
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

(Node: SW_OUT)          (Node: LAMP_IN)
                                              /--> [ L1: Inductor ] --\
[ V1: 12 V Source ] --(VCC)--> [ S1: Switch ] --                        --> [ X1: Lamp ] --> GND
                                              \--> [ R1: Resistor ] --/
Schematic (ASCII)

Measurements and tests

Follow these steps to validate the phenomenon:

  1. Initial State: Ensure the switch S1 is open. The lamp X1 should be off.
  2. Observation: Keep your eyes on the lamp X1.
  3. Action: Close switch S1.
  4. Visual Validation:
    • Phase 1 (Instant): The lamp lights up at roughly 30–50% brightness. (Current is flowing through R1, as L1 opposes the sudden change).
    • Phase 2 (Delay): The lamp brightness ramps up smoothly to 100%. (As the magnetic field in L1 stabilizes, it allows full current to pass, bypassing R1).
  5. Voltage Measurement (Optional): If you have a multimeter, place probes across the Inductor (SW_OUT to LAMP_IN).
    • At the moment of contact, voltage is high (approx 6–8 V).
    • After 1–2 seconds, voltage drops to near 0 V.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Title: Practical case: Opposition to DC current change
.width out=256
* Description: Demonstrates inductive opposition to current change (dim-to-bright lamp effect)

* --- Power Supply ---
* 12V DC Supply
V1 VCC 0 DC 12

* --- User Interface (Switch Control) ---
* Generates a control pulse to simulate pressing the button.
* Button Press: Starts at 10ms, Duration 300ms.
V_BTN_CTRL CTRL 0 PULSE(0 5 10m 1u 1u 300m 600m)

* --- Components ---

* S1: SPST Mechanical Switch
* Connected between VCC and SW_OUT.
* Modeled as a voltage-controlled switch driven by the control pulse.
S1 VCC SW_OUT CTRL 0 SW_IDEAL

* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

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* Title: Practical case: Opposition to DC current change
.width out=256
* Description: Demonstrates inductive opposition to current change (dim-to-bright lamp effect)

* --- Power Supply ---
* 12V DC Supply
V1 VCC 0 DC 12

* --- User Interface (Switch Control) ---
* Generates a control pulse to simulate pressing the button.
* Button Press: Starts at 10ms, Duration 300ms.
V_BTN_CTRL CTRL 0 PULSE(0 5 10m 1u 1u 300m 600m)

* --- Components ---

* S1: SPST Mechanical Switch
* Connected between VCC and SW_OUT.
* Modeled as a voltage-controlled switch driven by the control pulse.
S1 VCC SW_OUT CTRL 0 SW_IDEAL

* L1: 1.5H Iron-core Inductor
* Creates opposition to current change.
* Connected between SW_OUT and LAMP_IN.
L1 SW_OUT LAMP_IN 1.5

* R1: 220 Ohm Resistor
* Bypass path for visual contrast (parallel to L1).
* Connected between SW_OUT and LAMP_IN.
R1 SW_OUT LAMP_IN 220

* X1: 12V / 100mA Incandescent Lamp
* Modeled as a resistor: R = V / I = 12 / 0.1 = 120 Ohms.
* Connected between LAMP_IN and 0 (GND).
R_X1 LAMP_IN 0 120

* --- Models ---
* Ideal switch model: Low resistance when ON, High when OFF.
.model SW_IDEAL sw(vt=2.5 ron=0.01 roff=100Meg)

* --- Simulation Setup ---
* Transient analysis to capture the inductive time constant (approx 20ms).
* Simulation time: 500ms to allow full settling.
.op
.tran 1m 500m

* --- Output Directives ---
* V(SW_OUT): Input voltage to the LR network (Switch Output).
* V(LAMP_IN): Voltage across the Lamp (Visual Output).
.print tran V(SW_OUT) V(LAMP_IN) I(L1)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)

Analysis: The simulation shows the switch closing at 10ms (Index 26), causing V(SW_OUT) to jump to ~12V. V(LAMP_IN) rises to ~4.2V initially due to the inductive kick/impedance, then settles. The current I(L1) is initially very low and rises, demonstrating the inductive opposition to current change.
Show raw data table (564 rows) 
Index   time            v(sw_out)       v(lamp_in)      l1#branch
0	0.000000e+00	1.439998e-05	1.439998e-05	1.199999e-07
1	1.000000e-05	1.439998e-05	1.439998e-05	1.199999e-07
2	2.000000e-05	1.439998e-05	1.439998e-05	1.199999e-07
3	4.000000e-05	1.439998e-05	1.439998e-05	1.199999e-07
4	8.000000e-05	1.439998e-05	1.439998e-05	1.199999e-07
5	1.600000e-04	1.439998e-05	1.439998e-05	1.199999e-07
6	3.200000e-04	1.439998e-05	1.439998e-05	1.199999e-07
7	6.400000e-04	1.439998e-05	1.439998e-05	1.199999e-07
8	1.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
9	2.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
10	3.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
11	4.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
12	5.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
13	6.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
14	7.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
15	8.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
16	9.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
17	1.000000e-02	1.439998e-05	1.439998e-05	1.199999e-07
18	1.000010e-02	1.439998e-05	1.439998e-05	1.199999e-07
19	1.000026e-02	1.439998e-05	1.439998e-05	1.199999e-07
20	1.000031e-02	1.439998e-05	1.439998e-05	1.199999e-07
21	1.000039e-02	1.439998e-05	1.439998e-05	1.199999e-07
22	1.000041e-02	1.439998e-05	1.439998e-05	1.199999e-07
23	1.000045e-02	1.439998e-05	1.439998e-05	1.199999e-07
... (540 more rows) ...

Common mistakes and how to avoid them

  1. Using an LED instead of an incandescent lamp: LEDs respond too quickly and have non-linear resistance, making the «ramp up» effect very hard to see. Solution: Always use an incandescent bulb or a coil-based relay for this demo.
  2. Inductor value too small: If you use a small air-core inductor (e.g., 100 µH), the delay will be microseconds, invisible to the eye. Solution: Use a large iron-core inductor, such as the primary coil of a mains transformer (ensure it is rated for the DC current).
  3. Omitting the parallel resistor: Without R1, the lamp might simply stay off for a split second and then pop on, which can look like a switch bounce rather than a smooth transition. Solution: R1 provides an immediate «dim» reference state, making the transition to «bright» much more obvious.

Troubleshooting

  • Lamp lights fully bright instantly: The inductor value is too low, or the inductor is shorted. Check if you are using an air-core coil; switch to an iron-core one.
  • Lamp never gets fully bright: The inductor might have a very high internal DC resistance (thin wire). Measure the resistance of the inductor coil; if it is comparable to the resistor R1, the current will never fully bypass the resistor.
  • Sparks at the switch when turning off: Inductors generate back-EMF voltage when the circuit breaks. R1 acts as a snubber here, but if sparks persist, ensure your switch is rated for inductive loads.

Possible improvements and extensions

  1. Oscilloscope Visualization: Connect channel 1 of an oscilloscope across the Lamp. You will see an exponential curve rising, allowing you to calculate the Time Constant (\tau = L / R).
  2. Variable Delay: Replace R1 with a potentiometer and experiment with how changing the parallel resistance affects the initial «dim» brightness and the perceived transition speed.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary objective of the circuit described in the text?




Question 2: What visual effect is created by placing a large inductor in series with the lamp?




Question 3: Which component acts as a parallel bypass in the circuit description?




Question 4: Why is inrush current limiting useful according to the text?




Question 5: What is the function of the incandescent lamp (X1) in this circuit?




Question 6: How does the inductor behave initially when the switch is first closed?




Question 7: What is the expected behavior of the lamp immediately after the switch is closed?




Question 8: Approximately how long does the text suggest it takes for the lamp to become fully bright?




Question 9: Which of the following is NOT listed as a use case for this type of circuit?




Question 10: Who is the target audience for this circuit demonstration?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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