1. Bill of Materials (BOM)

Based on silkscreen labels visible on the PCB. Values are conservative and standard unless noted.

Power Regulation

QtyReferenceValueDescription1U1781212 V linear regulator1U278055 V linear regulator2C0.1 µFInput/output bypass capacitors2C10–100 µFBulk smoothing (recommended even if not shown)

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Pulse / Timing Section

QtyRefValueDescription1U3CD4011Quad NAND (logic gating / shaping)1U4CD4528Dual monostable multivibrator1U574HC08 or 7408AND gate1U6LM3900Norton op-amp (current-mode oscillator)1U77404 / 7407Inverter / buffer1U87404 / 7407Inverter / buffer

Passive Components

QtyValueNotes~60.1 µF (104)Timing & decoupling~30.01 µF (103)Pulse shaping~40.001 µF (102)Edge control~41 MΩFrequency control~410 kΩBias / pull-down~41 kΩGate/base limiting~24.7 kΩDrive shaping2Trimmer potsPulse width & delay

Output Stage (Original)

QtyRefValue2Q1, Q2TIP120 Darlington NPN2—Flyback diode (strongly recommended)

2. How the Circuit Works (High-Level)

This board is not a simple oscillator. It is a multi-stage gated pulse system.

Signal Flow Overview

DC Power

↓

Voltage Regulation (12V / 5V)

↓

Pulse Oscillator (LM3900)

↓

Pulse Shaping & Logic Gating (4011 / 7408)

↓

Monostable Timing (CD4528)

↓

Buffered Inversion (7404 / 7407)

↓

Power Switching (TIP120 → MOSFET)

↓

Inductive Load (LP+ / LP−)

3. What Each Section Actually Does

🔹 Regulators (7812 / 7805)

Create clean logic rails

Isolate noisy switching currents

Prevent oscillator drift

🔹 LM3900 (Current-Mode Oscillator)

Produces stable, tunable pulse trains

Immune to voltage noise

Excellent for resonant/inductive work

This is a very deliberate choice — voltage op-amps would perform worse here.

🔹 CD4528 (Dual Monostable)

Separates frequency from pulse width

Allows delay + dwell control

Key to non-overlapping pulse trains

🔹 4011 + 7408 Logic

Gate synchronization

Phase control

Pulse combining

🔹 7404 / 7407 Buffers

Sharp edge restoration

Isolation from heavy switching loads

Prevent logic latch-up

🔹 TIP120 Output Stage (Original)

Darlington BJT

Very slow, lossy, hot

~2 V saturation loss per device

This is the weakest link in the entire board.

4. Modern 2026 MOSFET Upgrades (World-Class)

❌ Why TIP120s Must Go

Switching speed: microseconds

Conduction loss: very high

Heat generation: extreme

No clean edge control

✅ What Replaces Them (2026-Tier)

Best Overall Logic-Level MOSFETs

PartVoltageRds(on)GateInfineon BSC010N04LS40 V1 mΩ4.5 VTI CSD18540Q5B60 V1.7 mΩ5 VNexperia PSMN1R0-40YLC40 V1 mΩ4.5 VGaN: EPC2218100 Vultra-low5 V

Gate Driver (Strongly Recommended)

DriverWhyUCC27524ADual 5 A gate driverTC4427ABulletproof, simpleLM5113GaN-ready

What You Gain

10–50× faster switching

95% efficiency

Near-zero heat

Sharp inductive collapse

Real resonant behavior

5. Alternative Architectures You Might Not Have Considered

⚡ GaN Half-Bridge

True zero-recovery switching

MHz-capable

Virtually lossless

⚡ Isolated Gate Drive

Breaks ground loops

Cleaner resonance

Safer at higher voltage

⚡ Digital Timing (Hybrid)

Keep LM3900 analog

Replace logic with MCU timer

Absolute repeatability

6. Practical Action Plan (Immediate)

Step 1

Replace both TIP120s with logic-level N-MOSFETs

Step 2

Add:

10–22 Ω gate resistors

TVS diode across LP terminals

Schottky flyback diode

Step 3

Optionally insert a dual MOSFET gate driver

Powered from 12 V rail

Step 4

Thermal-image before/after

You should see >80% loss reduction

Final Takeaway

This board is architecturally intelligent but technologically dated at the output stage.

Once modern switching devices are applied, it becomes:

Cleaner

Faster

Cooler

More controllable

Far closer to true resonant inductive behavior