# Handheld Pulse Welder — System Architecture ## Overview A benchtop capacitor-discharge micro-bonding tool targeting solder reflow and aluminum foil welding on PCB work. MCU-controlled pulse delivery, dual power input with auto-switching, detachable pen handpiece. --- ## System Block Diagram ``` ┌─────────────────────────────────────────────────────────┐ │ BASE UNIT │ │ │ │ ┌──────────┐ ┌──────────────┐ ┌───────────────┐ │ │ │ USB-C PD │ │ DC Barrel │ │ Power Mux / │ │ │ │ 20V/5A │───▶│ 12–24V in │───▶│ Auto-Switch │ │ │ └──────────┘ └──────────────┘ └──────┬────────┘ │ │ │ │ │ ┌───────▼────────┐ │ │ │ Charge Circuit │ │ │ │ (boost/buck │ │ │ │ to ~20-40V) │ │ │ └───────┬────────┘ │ │ │ │ │ ┌───────▼────────┐ │ │ │ Capacitor Bank │ │ │ │ (main energy │ │ │ │ storage) │ │ │ └───────┬────────┘ │ │ │ │ │ ┌──────────────────────┐ ┌────────▼────────┐ │ │ │ MCU (STM32 / Nano) │──────────│ Discharge FET │ │ │ │ - Pulse timing │ trigger │ (MOSFET switch)│ │ │ │ - Energy presets │ └────────┬────────┘ │ │ │ - UI / display │ │ │ │ │ - Safety logic │ │ │ │ └──────────────────────┘ │ │ │ │ │ │ │ ┌───────▼──────┐ │ │ │ │ UI Panel │ ┌──────▼────────┐ │ │ │ - OLED display │ Output Jack │ │ │ │ - Encoder knob │ (4mm banana │ │ │ │ - Ready LED │ │ or XLR) │ │ │ │ - Trigger in │ └──────┬────────┘ │ │ └──────────────┘ │ │ └─────────────────────────────────────────────┼───────────┘ │ ┌───────────────▼──────────┐ │ PEN HANDPIECE │ │ │ │ ┌────────┐ ┌──────────┐ │ │ │Trigger │ │ Tips │ │ │ │Button │ │(swap out)│ │ │ └────────┘ └──────────┘ │ │ │ │ Electrode A Electrode B │ │ │ │ │ └──────┼─────────────┼──────┘ │ │ [workpiece — solder/foil joint] ``` --- ## Subsystems ### 1. Power Input & Auto-Switching **Goal:** Accept USB-C PD (20V) or DC barrel (12–24V), use whichever is present, prefer higher voltage. **Approach:** - USB-C PD negotiation via small PD trigger board (e.g. IP2721 or HUSB238 module) — pulls 20V from any PD charger - Barrel jack input with reverse-polarity protection (P-channel MOSFET) - Auto-switch via ideal diode controller (e.g. LTC4412 or simple MOSFET OR circuit) — highest voltage source wins, no relay needed - Bulk capacitance on the rail (1000–2200µF) for transient handling **Parts:** - HUSB238 USB-C PD sink module (~$2) - LTC4412 ideal diode controller or discrete equivalent - Schottky diodes as fallback OR circuit --- ### 2. Capacitor Bank & Charge Circuit **Goal:** Store enough energy for a single pulse, recharge in <2 seconds between pulses. **Energy math (target):** - Solder reflow joint: ~1–5 joules - Foil weld nugget: ~2–8 joules - Target bank: 10J usable (gives headroom and some margin for losses) **Capacitor sizing:** ``` E = ½CV² At 40V: C = 2E/V² = 2×10/1600 = 12,500µF → ~10,000–15,000µF bank At 20V: C = 2×10/400 = 50,000µF → impractically large ``` → **Boost to 40V for charging is the right call.** Keeps cap bank to ~10,000µF (2–3 × 4700µF electrolytics). **Charge circuit:** - Boost converter from 20V input to adjustable 20–40V charge voltage - MCU controls charge voltage via DAC or PWM + RC filter on feedback pin - Charge current limited to ~2A (keeps charge time to ~1–2s) - Voltage sense on cap bank feeds ADC on MCU — stops charging at target voltage **Parts:** - XL6009 or LM2587 boost module (adjustable, up to 35V) — or a proper boost IC like LM3478 - 3× 4700µF 50V electrolytic caps in parallel (Nichicon or Panasonic, low ESR) - Voltage divider + MCU ADC for bank voltage monitoring --- ### 3. Discharge Switch (MOSFET) **Goal:** Dump capacitor bank through workpiece in a precise, MCU-controlled pulse (1–50ms). **Requirements:** - Handle peak current: at 40V into ~10mΩ joint resistance → up to 400A peak (real-world much less due to tip/wire resistance, but design for headroom) - Very low Rds(on) to minimize losses in the switch - Fast switching — MCU GPIO drives gate directly or via gate driver **Approach:** - Single large N-channel MOSFET or 2–3 in parallel - Gate driver IC (e.g. TC4420) for clean switching — avoids MCU GPIO current limits - Flyback diode across output to protect FET from inductive kickback - Fuse on output (fast-blow, 50–100A) for catastrophic fault protection **Parts:** - IRFP4668 or similar (200V, 130A, 3.7mΩ Rds) — beefy, easy to heatsink - TC4420 gate driver - 50A fast-blow fuse - Schottky flyback diode (e.g. MBR20200) --- ### 4. MCU & Control Logic **Platform:** STM32F103 (Blue Pill) — cheap, fast, good ADC, easy to program via Arduino IDE or STM32CubeIDE. Arduino Nano is also viable if you want simpler toolchain. **Responsibilities:** - Monitor cap bank voltage via ADC - Control charge circuit (enable/disable, set voltage via PWM) - Trigger discharge FET via GPIO → gate driver - Measure and enforce pulse duration (microsecond timer precision) - Read UI inputs (encoder, trigger button) - Drive OLED display - Safety interlocks (see below) **Pulse control:** ``` Pulse width: 1ms – 50ms (adjustable) Pulse energy: set by charge voltage (20–40V) × cap value Presets: SOLDER | FOIL-LIGHT | FOIL-HEAVY | CUSTOM ``` --- ### 5. UI Panel - **128×64 OLED** (SSD1306, I2C) — shows mode, charge voltage, pulse width, ready state - **Rotary encoder with push button** — navigate menus, adjust values - **RGB LED or discrete LEDs** — CHARGING (red), READY (green), FAULT (yellow) - **3.5mm jack** — trigger input from pen handpiece button (keeps handpiece wiring simple) --- ### 6. Pen Handpiece **Design goal:** Light, pencil-grip, interchangeable tips, single trigger button. **Construction:** - Aluminum or 3D-printed handle (PETG minimum — PLA will deform from heat) - 2× output wires from base unit (silicone insulated, flexible, 10–14 AWG) - Copper-tungsten or pure tungsten tips (press-fit or set-screw retention) - Momentary tactile button wired back to base via thin signal wire (can run alongside power cables) **Tip geometries (interchangeable):** - **Twin-point** (2.54mm spacing) — standard header pin pitch - **Wide twin-point** (5mm) — larger pads - **Single point + ground clip** — for tricky geometry --- ### 7. Safety Systems **Critical — capacitor discharge at these voltages/currents is dangerous.** - **Charge cutoff:** MCU stops charging if bank reaches max voltage (hardware comparator backup independent of MCU) - **Bleed resistor:** 10kΩ resistor permanently across cap bank — bleeds charge when unplugged (caps do NOT self-discharge, this is mandatory) - **Interlock:** Discharge FET cannot trigger while charging circuit is active (mutual exclusion in firmware + hardware OR gate) - **Tip detect:** Sense tip resistance before allowing pulse — prevent firing into open air - **Pulse watchdog:** Hardware timer limits max pulse to 100ms regardless of firmware state — MCU crash can't hold FET on indefinitely - **Enclosure:** Base unit must be fully enclosed. Capacitor terminals should not be user-accessible. --- ## Bill of Materials (Draft) | Subsystem | Key Parts | Est. Cost | |---|---|---| | Power input | HUSB238 PD module, LTC4412, Schottkys | $8 | | Charge circuit | LM3478 boost IC + passives, or XL6009 module | $6 | | Cap bank | 3× 4700µF 50V Nichicon | $12 | | Discharge FET | IRFP4668 + TC4420 + heatsink | $8 | | MCU | STM32 Blue Pill | $4 | | UI | SSD1306 OLED + encoder + LEDs | $8 | | Safety | Bleed resistor, fuse, comparator | $4 | | Handpiece | Hardware + tips + wire + 3D print | $15 | | Enclosure + misc | PCB, connectors, case | $20 | | **Total** | | **~$85** | --- ## Next Steps (Build Order) 1. Breadboard the charge circuit + cap bank — verify charge/hold/bleed behavior 2. Test discharge through a resistive load with a manual switch — measure pulse energy 3. Bring in MCU — controlled pulse timing, ADC monitoring 4. Build UI loop — display + encoder + presets 5. Add safety interlocks 6. Design and build handpiece 7. First weld test on solder paste + copper pad 8. Iterate tip geometry --- *Version 0.1 — architecture draft*