Objective and use case

• What you will build: A circuit that utilizes two DPDT (Double Pole Double Throw) electro-mechanical relays to simultaneously demonstrate basic Boolean logic operations (AND and OR gates).
• Why it is useful:
  • Forms the historical foundation of industrial automation and ladder logic programming.
  • Demonstrates how to handle logic for high-voltage or high-current systems where standard silicon ICs are unsuitable.
  • Provides complete electrical isolation between the control inputs (coils) and the logic outputs (contacts).
  • Illustrates the fundamental fail-safe interlock principles used in heavy machinery and safety circuits.
• Expected outcome:
  • The AND output (Green LED) will only illuminate when both relay coils are energized (series contacts).
  • The OR output (Red LED) will illuminate when either relay coil is energized (parallel contacts).
  • Successful measurement of control voltages confirming the activation of specific logic paths.
• Target audience and level: Intermediate electronics students exploring automated control systems and electromechanical switching.

Materials

• V1: 5 V DC supply, function: power for coils and logic
• SW1: SPST switch, function: Input A control
• SW2: SPST switch, function: Input B control
• K1: 5 V DPDT relay, function: Logic gate element A
• K2: 5 V DPDT relay, function: Logic gate element B
• D1: Green LED, function: AND logic output indicator
• D2: Red LED, function: OR logic output indicator
• D3: 1N4148 diode, function: K1 flyback protection
• D4: 1N4148 diode, function: K2 flyback protection
• R1: 330 Ω resistor, function: D1 current limiting
• R2: 330 Ω resistor, function: D2 current limiting

Conceptual block diagram

Measurements and tests

1. Input Verification: Power on V1. Using a multimeter, measure the voltage at nodes VA and VB with respect to 0 (GND). Ensure it reads 0 V when the corresponding switch is open, and 5 V when closed.
2. Mechanical Operation: Toggle SW1 and SW2 individually. You should hear a distinct mechanical «click» from K1 and K2, confirming coil activation.
3. OR Gate Testing: Close SW1 only. Measure the voltage at OUT_OR (should be ~5 V) and ensure the Red LED lights up. Repeat this step for SW2 only.
4. AND Gate Testing: Ensure both SW1 and SW2 are closed. Measure the voltage at OUT_AND (should be ~5 V) and confirm the Green LED lights up. If either switch is opened, the Green LED must turn off.

Common mistakes and how to avoid them

• Omitting flyback diodes: Failing to include D3 and D4 can cause high voltage spikes when the switches are opened, which can damage the switches or surrounding sensitive electronics. Always wire them in reverse-bias across the coils.
• Confusing NO and NC contacts: Accidentally wiring to the Normally Closed (NC) pin instead of the Normally Open (NO) pin will invert the logic, effectively creating NAND/NOR conditions instead of AND/OR. Double-check your relay’s datasheet or pinout.
• Insufficient power supply current: Relays consume significantly more current than digital ICs (often 50–100 mA per coil). Ensure your 5 V supply can comfortably deliver at least 300 mA to prevent voltage dips when both relays are energized.

Troubleshooting

• Symptom: Neither LED lights up under any switch combination, and no «clicks» are heard.
  • Cause: Power supply V1 is disconnected, dead, or current-limited.
  • Fix: Verify VCC and 0 connections to the main supply and check the supply limits.
• Symptom: Relays click when switches are pressed, but LEDs never turn on.
  • Cause: The control side (coils) is working, but the logic side (contacts) is unpowered.
  • Fix: Verify that VCC is properly routed to the Common pins of both poles on K1 and K2.
• Symptom: Power supply resets or dips severely when a switch is pressed.
  • Cause: A flyback diode (D3 or D4) is installed backward, creating a direct short circuit to ground when the switch closes.
  • Fix: Check diode orientation; the cathode (striped side) must face the positive incoming voltage (VA or VB).
• Symptom: The AND logic acts like an OR logic.
  • Cause: The contacts for the AND gate were wired in parallel instead of series.
  • Fix: Inspect the node AND_MID. VCC should strictly flow through K1 into K2, not directly to both.

Objective and use case

In this practical case, you will build a power management circuit using a Single Pole Double Throw (SPDT) relay. The circuit will automatically switch a connected load to a backup battery whenever the main power supply fails.

This automated switching topology is highly useful in several real-world applications:
* Uninterruptible Power Supplies (UPS) for routers and critical network equipment.
* Alarm and security systems that require continuous operation during grid power outages.
* Medical monitoring devices that must remain functional during patient transport.
* Solar power systems that automatically switch to battery power after sunset.

Expected outcome:
* When the main supply (V_MAIN) is active, the relay coil energizes, and the load connects to the Normally Open (NO) contact powered by the main supply.
* When the main supply fails (drops to 0 V), the relay de-energizes, seamlessly transferring the load to the Normally Closed (NC) contact powered by the backup battery.
* The load voltage (V_LOAD_OUT) remains continuous, save for a minor mechanical switching delay.
* A visual LED indicator successfully reports the presence of the main power supply.

Target audience: Hobbyists and intermediate electronics students learning about electromechanical relays and power redundancy.

Materials

• V1: 12 V DC supply, function: main power source
• V2: 9 V DC supply, function: backup battery source
• K1: 12 V SPDT relay, function: automatic power switch
• D1: 1N4007 diode, function: flyback diode to protect against relay coil voltage spikes
• D2: Red LED, function: main power indicator
• R1: 1 kΩ resistor, function: LED current limiting
• R2: 100 Ω resistor, function: simulated system load

Wiring guide

• V1: Connect positive terminal to V_MAIN and negative terminal to 0 (GND).
• V2: Connect positive terminal to V_BACKUP and negative terminal to 0 (GND).
• K1 (Coil): Connect one side to V_MAIN and the other side to 0 (GND).
• D1: Connect parallel to the K1 coil. Connect the cathode to V_MAIN and the anode to 0 (GND).
• K1 (NO Contact): Connect the Normally Open terminal to V_MAIN.
• K1 (NC Contact): Connect the Normally Closed terminal to V_BACKUP.
• K1 (COM Contact): Connect the Common terminal to V_LOAD_OUT.
• R2: Connect between V_LOAD_OUT and 0 (GND).
• R1: Connect between V_MAIN and NODE_LED.
• D2: Connect the anode to NODE_LED and the cathode to 0 (GND).

Conceptual block diagram

Measurements and tests

1. Connect and activate both V1 (12 V) and V2 (9 V).
2. Measure the voltage at V_MAIN and V_BACKUP with a multimeter to verify both sources are stable.
3. Measure the voltage at V_LOAD_OUT. It should read approximately 12 V. The relay should emit an audible «click» upon power-up, and the indicator LED (D2) should be brightly lit.
4. Disconnect V1 to simulate a power outage (V_MAIN drops to 0 V).
5. Measure the voltage at V_LOAD_OUT again. It should now read approximately 9 V, confirming the load has successfully transferred to the backup battery. The LED should turn off.
6. Reconnect V1. Observe the relay clicking again as V_LOAD_OUT returns to 12 V.

Common mistakes and how to avoid them

• Relay coil voltage mismatch: Using a 5 V relay on a 12 V line will cause the coil to overheat and fail quickly. Always ensure the relay’s rated coil voltage matches the main supply voltage exactly.
• Omitting the flyback diode: Failing to install the reverse-biased diode across the relay coil can result in high-voltage spikes when the main power is abruptly disconnected, potentially damaging parallel components on the main power bus.
• Reversing NO and NC contacts: Wiring the backup battery to the NO contact and the main supply to the NC contact will result in a dead system when the main power fails. Verify the relay pinout before soldering or powering the circuit.

Troubleshooting

• Symptom: The load completely loses power when the main supply drops.
  • Cause: The backup battery is either dead or connected to the Normally Open (NO) terminal instead of the Normally Closed (NC) terminal.
  • Fix: Measure the battery voltage independently, then verify its connection to the NC terminal of the relay.
• Symptom: The relay chatters or buzzes continuously instead of switching cleanly.
  • Cause: The main power supply cannot provide enough current for both the relay coil and the load, causing the voltage to repeatedly dip below the relay’s hold threshold.
  • Fix: Upgrade the main power supply to a higher current rating, or add a large smoothing capacitor across the V_MAIN line.
• Symptom: The indicator LED does not light up, but the switching works.
  • Cause: The LED is inserted with reverse polarity, or the current limiting resistor is disconnected.
  • Fix: Verify that the flat side (cathode) of the LED is connected to ground.

Objective and use case

You will construct an LR low-pass filter using an RF choke to isolate a DC power line from high-frequency AC noise. By superimposing an AC signal onto a DC voltage supply, you will observe how the inductor’s frequency-dependent reactance permits DC to pass while heavily attenuating high-frequency noise before it reaches the load.

This circuit concept is highly useful in the real world for:
* Preventing high-frequency switching noise from entering sensitive analog sensor circuits.
* Filtering out radio frequency interference (RFI) from long power supply lines.
* Isolating different functional blocks that share a common power rail on a PCB.
* Protecting automotive audio and communication electronics from alternator whine.

Expected outcome:
* The mixed input signal (V_IN_MIX) will display a steady DC offset combined with significant high-frequency ripples.
* The output voltage (V_OUT_CLEAN) across the load will show a stable DC level with the AC noise vastly reduced.
* An FFT (Fast Fourier Transform) analysis of the input will reveal a large 0 Hz (DC) component and a prominent high-frequency peak.
* An FFT analysis of the output will show the high-frequency peak almost completely suppressed, confirming the choke’s blocking action.

Target audience: Intermediate electronics students learning about reactive components and AC/DC superimposition.

Materials

• V1: 5 V DC source, function: main DC power supply
• V2: 500 mV peak sine wave AC source at 100 kHz, function: high-frequency noise simulator
• L1: 1 mH inductor, function: RF choke to block high-frequency noise
• R1: 100 Ω resistor, function: load simulation

Wiring guide

• V1: connects between V_DC and 0
• V2: connects between V_IN_MIX and V_DC
• L1: connects between V_IN_MIX and V_OUT_CLEAN
• R1: connects between V_OUT_CLEAN and 0

Conceptual block diagram

Measurements and tests

1. Connect an oscilloscope probe to V_IN_MIX with the ground clip attached to node 0. Set the channel coupling to DC. You should observe a 5 V DC baseline with a 1 V peak-to-peak 100 kHz sine wave riding on top of it.
2. Connect a second oscilloscope probe to V_OUT_CLEAN. Observe that the DC voltage remains at approximately 5 V, but the high-frequency 100 kHz ripple is drastically attenuated due to the high inductive reactance (XL = 2\pi fL) of the choke.
3. Activate the FFT (Fast Fourier Transform) math function on the oscilloscope for the V_IN_MIX channel. Note the massive spike at 0 Hz (representing the 5 V DC component) and the distinct noise spike at 100 kHz.
4. Apply the FFT function to the V_OUT_CLEAN channel. Compare the magnitude of the 100 kHz spike against the input measurement; it should be significantly reduced, successfully proving the inductor’s high-frequency blocking capabilities.

Common mistakes and how to avoid them

• Using an inductor with a low self-resonant frequency (SRF): All inductors have parasitic winding capacitance. If the noise frequency exceeds the inductor’s SRF, the component behaves like a capacitor and allows high-frequency noise to pass straight through. Always verify the SRF is well above your target noise frequency.
• Neglecting the inductor’s DC resistance (DCR): Inductors are made of coiled wire which naturally possesses resistance. High load currents passing through an inductor with high DCR will cause an unacceptable DC voltage drop. Choose a choke with an appropriately low DCR for your load.
• Core saturation due to high DC current: If the load draws more continuous current than the inductor’s saturation rating (Isat), the core’s magnetic flux saturates. This causes the inductance to drop sharply, destroying its filtering capability. Always check the saturation current rating.

Troubleshooting

• Symptom: High-frequency noise is still heavily present at V_OUT_CLEAN.
• Cause: The inductor value is too low to provide significant reactance at the simulated noise frequency, or its SRF has been exceeded.
• Fix: Increase the inductance value (e.g., scale from 10 µH to 1 mH) or verify the frequency limits of the specific choke being used.
• Symptom: Significant DC voltage drop at V_OUT_CLEAN under load (e.g., reading 4 V instead of 5 V).
• Cause: The inductor’s internal DC resistance (DCR) is too high relative to the load resistor R1.
• Fix: Replace the inductor with a physically larger one that uses thicker wire, which lowers the DCR, or increase the load resistance if it’s drawing more current than intended.
• Symptom: The choke gets excessively hot during operation.
• Cause: The DC current drawn by the load exceeds the continuous thermal current rating (Irms) of the inductor.
• Fix: Select a higher-rated power inductor capable of safely handling the steady-state load current.

Objective and use case

In this practical case, you will build a parallel LC tank circuit driven by an AC sine wave source through a series resistor. By sweeping the input frequency, you will observe the precise point where inductive and capacitive reactances cancel out, maximizing the circuit’s impedance.

Understanding LC resonance is essential in modern electronics because these circuits are the fundamental building blocks of frequency selection. Real-world applications include:
* Radio frequency (RF) tuning: Selecting a specific station’s frequency while rejecting others.
* Audio and signal filtering: Creating band-pass or band-stop (notch) filters to eliminate noise.
* Wireless power transfer: Maximizing the efficiency of inductive coupling between transmitter and receiver coils.
* Oscillator circuits: Generating stable clock signals for microcontrollers and transceivers.

Expected outcome:
* You will calculate the theoretical resonant frequency based on the chosen $L$ and $C$ values.
* The total current drawn from the source (Itotal) will drop to its minimum value at resonance.
* The voltage across the LC tank (VLC) will peak at the resonant frequency.
* You will observe how energy continuously sloshes back and forth between the capacitor’s electric field and the inductor’s magnetic field.

Target audience: Intermediate electronics students transitioning from DC basics to AC reactive circuits.

Materials

• V1: 5 V peak-to-peak AC voltage source, function: sine wave generator for frequency sweep
• R1: 1 kΩ resistor, function: source impedance to allow voltage variations across the tank
• L1: 10 mH inductor, function: magnetic energy storage
• C1: 100 nF ceramic or film capacitor, function: electric energy storage

Wiring guide

• V1: Connect the positive terminal to node IN and the negative terminal to node 0 (GND).
• R1: Connect one pin to node IN and the other pin to node TANK.
• L1: Connect one pin to node TANK and the other pin to node 0 (GND).
• C1: Connect one pin to node TANK and the other pin to node 0 (GND).

Conceptual block diagram

Measurements and tests

1. Calculate the theoretical resonant frequency (fr):
   Use the formula fr = (1 / 2\pi\sqrtLC). With L = 10 mH and C = 100 nF, the expected resonant frequency is approximately 5032 Hz.
2. Set up the frequency sweep:
   Configure V1 to output a 5 V peak-to-peak sine wave. Begin with a frequency of 1 kHz and gradually increase it up to 10 kHz.
3. Measure VLC (Tank Voltage):
   Monitor the voltage amplitude at node TANK relative to node 0 (GND) using an oscilloscope or an AC voltmeter. As you approach 5 kHz, the voltage amplitude will rise steadily, hitting a sharp maximum exactly at resonance, and then fall as the frequency increases further.
4. Measure Itotal (Source Current):
   Measure the current flowing through R1 (this can be done by observing the voltage difference between IN and TANK and applying Ohm’s law: Itotal = ((VIN – VTANK) / R1)). Note that at resonance, the parallel LC tank exhibits maximum impedance, meaning Itotal will drop to its minimum.
5. Calculate the circuit’s Q-factor:
   Identify the -3dB (half-power) frequencies above and below the resonance peak to find the bandwidth ($BW$). The Quality Factor is Q = (fr / BW).

Common mistakes and how to avoid them

• Using a polarized capacitor in an AC circuit: Electrolytic capacitors are generally polarized and can fail or explode if subjected to reversing AC voltages. Always use non-polarized capacitors (like ceramic or film) for an LC tank.
• Ignoring the inductor’s Equivalent Series Resistance (ESR): Real inductors consist of long coils of wire, adding parasitic DC resistance to the tank. If the measured Q-factor is much lower than expected (resulting in a wider, flatter peak), inductor ESR is usually the culprit.
• Confusing angular frequency (\omega) with standard frequency ($f$): Remember that \omega = (1 / \sqrtLC) yields results in radians per second. You must divide by 2\pi to get the frequency in Hertz.

Troubleshooting

• Symptom: The measured resonant frequency is significantly higher or lower than the calculated 5032 Hz.
  • Cause: Component tolerances. Standard ceramic capacitors can have a ±20% tolerance, and inductors often have ±10%.
  • Fix: Measure the exact values of L1 and C1 using an LCR meter and recalculate the expected frequency.
• Symptom: VLC shows no noticeable peak during the sweep; the voltage remains relatively flat.
  • Cause: The chosen frequency sweep range does not cover the resonant point, or R1 is too small, effectively shorting the tank to the rigid voltage source.
  • Fix: Double-check the math for your specific $L$ and $C$ values to ensure the sweep range encompasses fr. Ensure R1 is adequately sized (1 kΩ is a good starting point).
• Symptom: Signal distortion or clipping is observed at node TANK.
  • Cause: The AC source might be overdriving the circuit, or core saturation is occurring in the inductor (if using a very small ferrite core at high currents).
  • Fix: Reduce the amplitude of V1 from 5 V to 1 V peak-to-peak and check if the sine wave becomes clean again.

Objective and use case

In this practical case, you will build a basic open-loop Boost converter to demonstrate how an inductor stores and releases magnetic energy to step up a DC voltage.

Why it is useful:
* Allows battery-powered devices to operate at higher voltages (e.g., generating 5 V from a single 3.7 V Li-ion cell).
* Drives strings of LEDs that require a constant, high forward voltage.
* Captures and steps up voltage in energy harvesting and regenerative braking systems.
* Provides versatile power rails in compact portable electronics without requiring multiple batteries.

Expected outcome:
* You will observe the inductor current (I_inductor) ramping up when the switch is closed and ramping down when it opens.
* The output voltage (V_out) will be demonstrably higher than the input voltage source.
* You will record the direct relationship between the switch’s Duty Cycle and the resulting V_out magnitude.

Target audience and level:
Intermediate electronics students learning the fundamentals of switch-mode power supplies.

Materials

• V1: 5 V DC source, function: main power input
• V2: Pulse voltage source (0-5 V, 100kHz, 50% duty cycle), function: PWM signal for the switch
• L1: 100 µH inductor, function: magnetic energy storage
• M1: N-channel MOSFET (e.g., IRLZ44N), function: main switching element
• D1: Schottky diode (e.g., 1N5819), function: prevents reverse current from capacitor
• C1: 47 µF capacitor, function: output voltage smoothing
• R1: 100 Ω resistor, function: basic load to discharge capacitor

Wiring guide

• V1: connects between VIN and 0 (GND).
• V2: connects between GATE_PWM and 0 (GND).
• L1: connects between VIN and SW_NODE.
• M1: Drain connects to SW_NODE, Gate connects to GATE_PWM, Source connects to 0 (GND).
• D1: Anode connects to SW_NODE, Cathode connects to VOUT.
• C1: connects between VOUT and 0 (GND).
• R1: connects between VOUT and 0 (GND).

Conceptual block diagram

Measurements and tests

1. Initial state check: Apply V1 (5 V) with V2 turned off (0% duty cycle). Measure VOUT. The voltage should be roughly 4.7 V (the 5 V input minus the forward voltage drop of the Schottky diode).
2. Switching activation: Activate V2 to supply a 100kHz square wave at a 50% duty cycle. Measure VOUT across R1. The voltage should rise to approximately 9 V-10 V, demonstrating the step-up action.
3. Inductor current observation: Probe the current flowing through L1 (I_inductor). You will observe a triangular waveform. The upward slope occurs while M1 is ON (energy storage), and the downward slope occurs while M1 is OFF (energy release to VOUT).
4. Duty Cycle mapping: Adjust the Duty Cycle of V2 from 30% to 70% in 10% increments. Record VOUT at each step to verify that a higher duty cycle yields a higher output voltage.

Common mistakes and how to avoid them

• Using a standard rectifier diode (e.g., 1N4007): Standard diodes are too slow to turn off at 100kHz, leading to massive switching losses and poor voltage conversion. Always use a fast-recovery or Schottky diode like the 1N5819.
• Inductor core saturation: If the inductor’s maximum current rating is lower than the peak switching current, the magnetic core will saturate. The inductor will then act as a short circuit, potentially destroying the MOSFET. Always verify the inductor’s saturation current rating.
• Operating without a load: Running a boost converter with no load resistor (R1) can cause the output voltage to continuously rise with every switching cycle, theoretically reaching infinity and destroying the output capacitor or MOSFET. Always include a minimum load.

Troubleshooting

• Symptom: Output voltage equals the input voltage (minus diode drop).
• Cause: The MOSFET is not switching. V2 might be disconnected or the voltage level is too low to surpass the MOSFET’s gate threshold.
• Fix: Check the GATE_PWM signal with an oscilloscope. Use a logic-level MOSFET if your PWM signal is limited to 3.3 V or 5 V.
• Symptom: MOSFET becomes extremely hot very quickly.
• Cause: The inductor is saturating, or the MOSFET has a high ON-resistance (RDS(on)) and is experiencing high conduction losses.
• Fix: Swap the inductor for one with a higher current rating. Ensure the gate drive voltage is sufficient to turn the MOSFET completely ON.
• Symptom: Unstable or highly rippled output voltage.
• Cause: The output capacitor C1 is too small for the load or has a high Equivalent Series Resistance (ESR).
• Fix: Increase the capacitance of C1 or place a ceramic capacitor in parallel with the electrolytic capacitor to lower the overall ESR.

Objective and use case

In this practical case, you will build a switched inductor circuit monitored by an oscilloscope to observe the destructive voltage spike (inductive kickback) that occurs when current is abruptly interrupted. You will then install a flyback diode in parallel with the inductive load to safely clamp this transient voltage.

Why it is useful:
* Prevents catastrophic overvoltage damage to sensitive switching components such as transistors, MOSFETs, and microcontroller pins.
* Significantly reduces electromagnetic interference (EMI) and radio frequency interference (RFI) caused by high-voltage arcing across mechanical switch contacts.
* Increases the reliability, safety, and lifespan of power supply systems, motor controllers, and relay-driven circuits.

Expected outcome:
* Without the diode, opening the switch will produce a massive negative voltage spike on the oscilloscope, often reaching hundreds of volts.
* With the flyback diode installed, the transient spike will be immediately clamped to a safe level of approximately -0.7 V.
* The stored magnetic energy will safely dissipate as a steadily decaying circulating current through the inductor-resistor-diode loop.

Target audience and level: Intermediate electronics students learning about reactive components, energy storage, and circuit protection techniques.

Materials

• V1: 12 V DC supply, function: main power source
• SW1: SPST toggle or push-button switch, function: circuit connection control
• L1: 100 mH inductor, function: magnetic energy storage
• R1: 100 Ω resistor, function: limits steady-state current to 120 mA
• D1: 1N4007 rectifier diode, function: flyback protection

Wiring guide

• V1: connects between node VCC (positive) and node 0 (ground).
• SW1: connects between node VCC and node SW_OUT.
• L1: connects between node SW_OUT and node L_MID.
• R1: connects between node L_MID and node 0.
• D1: connects between node 0 (Anode) and node SW_OUT (Cathode) for reverse bias during normal closed-switch operation.

Conceptual block diagram

Measurements and tests

1. Connect the oscilloscope probe to node SW_OUT and attach the ground clip to node 0. Set the oscilloscope trigger to a falling edge, single-shot mode.
2. Begin with the flyback diode (D1) completely disconnected from the circuit.
3. Close the switch (SW1) to allow current to flow. Wait a moment for the magnetic field in the inductor to fully build up.
4. Quickly open the switch (SW1). Observe the oscilloscope capture; you will see a massive negative voltage transient as the inductor acts as a current source, forcing current across the open switch gap.
5. Connect the flyback diode (D1), verifying that the cathode (striped end) connects to node SW_OUT and the anode connects to node 0.
6. Repeat the switching process. The oscilloscope trace will now show the negative transient safely clamped at roughly -0.7 V as the diode forward-biases to provide a safe discharge path.

Common mistakes and how to avoid them

• Reversing the diode polarity: Placing the diode with the anode pointing to the positive voltage node creates a direct short circuit to ground when the switch is closed. This will destroy the diode or trigger the power supply’s overcurrent protection. Always ensure the cathode faces the higher potential.
• Using a diode with inadequate current rating: The flyback diode must safely handle a peak forward current equal to the steady-state current of the inductor just before switching. Always use properly rated rectifier, Schottky, or fast-recovery diodes.
• Omitting the series resistor: Connecting a pure inductor directly across a high-current DC source acts as a near short-circuit once the magnetic field is fully established. Always include a current-limiting series resistor, or ensure the inductor (such as a relay coil) has sufficient internal DC resistance.

Troubleshooting

• Symptom: The power supply shuts down or its current limit LED turns on immediately upon closing the switch.
  • Cause: The flyback diode is installed backwards, creating a short circuit from the power source to ground.
  • Fix: Disconnect power immediately and flip the diode so its striped end (cathode) faces the switch node.
• Symptom: A massive voltage spike still appears on the oscilloscope even with the diode supposedly installed.
  • Cause: The diode may have blown open due to a previous overcurrent event, or the breadboard connection is loose.
  • Fix: Verify diode continuity using a multimeter’s diode mode, and check the physical seating of the pins at the switch and ground nodes.
• Symptom: The oscilloscope trace shows high-frequency ringing instead of a clean clamp.
  • Cause: Parasitic capacitance in the switch, wiring, or oscilloscope probes interacting with the inductor.
  • Fix: Ensure the oscilloscope probe is properly compensated (x10 mode recommended for high voltage spikes) and keep ground leads as short as physically possible.