Induction Heater Layout

flux

A 5 A fuse can be used temporarily, but 4 A is safer and preferred for this ~3 A / 72 W design.
A 5 A fuse does not limit normal current to 5 A; depending on its time-current curve, it may carry substantially more than 5 A for seconds or minutes. That reduces protection for the perfboard wiring, chokes, MOSFETs, connectors, and supply.
If using the 5 A fuse:
  • Use it only if the supply itself has a reliable 3 A current limit.
  • Ensure the fuse is rated for at least 32 VDC; an automotive blade fuse is suitable for 24 V DC.
  • Use 14–16 AWG input wiring and heavy copper buses.
  • Do not increase the supply current above 3 A merely because the fuse is 5 A.
  • Fit the fuse in the +24 V input, close to the connector.
  • Replace it with a 4 A time-delay fuse when available.
For initial tests, set the bench supply to 0.5–1 A, verify stable oscillation, and then increase gradually toward 3 A. If your power supply cannot current-limit, I would not recommend powering this build with the 5 A fuse alone.
J

jmwilson2019

E102 20Kv is this ceramic cap to large to substitute C3???

flux

No. 102 means 1,000 pF = 1 nF = 0.001 µF. C3 requires 100 nF = 0.1 µF, normally marked 104.
Therefore, the E102 is 100× too small in capacitance. Its 20 kV rating is harmless electrically but unnecessary, and the part may be physically large with long leads—poor for high-frequency bypassing.
For C3, look for:
  • Marking: 104
  • Capacitance: 100 nF / 0.1 µF
  • Voltage: 50 V minimum; 100 V preferred
  • Type: ceramic disc or small film
  • Mount with the leads as short as possible
The required part may say 104, 104K, 104Z, 0.1uF, or 100nF.
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Files

Schematic

Layout

3D View

Project Specification
Project Overview
  • Status: Draft
  • 24 V induction-heater controller for an Original Prusa MK3S-class printer.
  • Replaces the resistive hotend heater load while retaining the printer thermistor and firmware temperature control.
  • Target input power: approximately 60–80 W, nominally 24 V at up to 3 A.
Intended Use
  • Prototype/validation board installed near the printer electronics and induction work coil.
  • Powered directly from the printer PSU, not through the Einsy heater power path.
What the Device Should Do
  • Detect the Einsy hotend-heater PWM command.
  • Enable and disable a 24 V ZVS/Royer induction stage in response.
  • Fail off when the control cable is disconnected or the heatsink overheats.
  • Protect the 24 V input against overcurrent and switching transients.
Main Features
  • Fused 24 V power input.
  • Mazzilli/Royer-style resonant induction power stage.
  • Printer heater-command input with electrical buffering; no direct connection from HEATER+ to a power MOSFET gate.
  • Input TVS protection and bulk decoupling.
  • Normally-closed thermal cutoff input.
  • Screw-terminal connections for VIN, printer heater command, thermal switch, and work coil.
System Architecture
Diagram



  
    
  
  
    
  


















24 V printer PSU
Fuse + TVS + bulk capacitance
Input enable switch
ZVS resonant power stage
External work coil
Einsy HEAT0 output
Buffered heater-command detector
NC heatsink thermal switch
Existing hotend thermistor
Hardware Subsystems
  • Power entry: 24 V screw terminal, 4 A slow-blow fuse target, 33–36 V TVS, local ceramic and electrolytic bulk capacitance.
  • Control sensing: Detect voltage across the Einsy HEAT0 connector. The Einsy output is low-side switched: one terminal is continuously tied to the heater supply and the other is MOSFET-switched to ground. The control input must therefore sense the differential heater command rather than treating HEATER+ as a PWM logic output.
  • Enable stage: Power MOSFET or equivalent switch sized above the 3 A input target with gate clamp, defined off-state, and thermal-switch override.
  • Induction stage: Two power MOSFETs, dual feed chokes, resonant polypropylene capacitor bank, and external work coil.
Interfaces and Connections

Table

InterfaceFunctionExpected rating
VIN+ / VIN-Printer PSU input24 V nominal, 3 A target
HEATER+ / HEATER-Einsy HEAT0 command sense24 V PWM differential input, low current
THERMAL SWNormally-closed bimetal cutoffOpens at approximately 90–100 C
COIL A / COIL BExternal work coilHigh-frequency, high circulating current
Power and Runtime Expectations
  • Continuous operation is controlled by printer PWM and thermal feedback.
  • Design point: 70 W input; worst-case design current target 3 A at 24 V.
  • PSU capacity and printer total power headroom must be checked before installation.
Power Tree and Power Budget

Table

PathVoltageTypical/target currentDesign margin
PSU to board24 V2.5–3.0 AConnectors, fuse, switch and copper sized for at least 4.5 A continuous margin
ZVS tankResonant ACCirculating current exceeds input currentMOSFETs, capacitors and coil connections require substantial pulse/ripple margin
Control input24 V PWMLow mAMust not materially load the Einsy output
Manufacturing and Assembly Expectations
  • Through-hole power MOSFETs suitable for bolted heatsinking.
  • Through-hole or chassis-rated screw terminals for power and coil wiring.
  • Polypropylene resonant capacitors with verified RMS ripple-current capability.
  • Heavy copper preferred; otherwise use large pours and provision for soldered copper reinforcement.
Firmware-Relevant Hardware Requirements
  • No printer firmware changes assumed.
  • Existing hotend thermistor remains connected to the Einsy and provides closed-loop temperature feedback.
Physical Design Expectations
  • Bare controller board; enclosure and exact mounting envelope remain open.
  • Power stage and coil connector placed for minimum high-current loop area.
  • MOSFET tabs aligned along a board edge for a common external heatsink while maintaining electrical isolation where required.
Important Design Decisions
  • The attached direct HEATER+ to MOSFET-gate proposal is rejected because HEATER+ is the constant supply terminal on the low-side-switched Einsy output.
  • The board will use a buffered differential heater-command detector and default-off enable stage.
  • Initial implementation targets a conservative 24 V / 3 A current-limited prototype, not an unrestricted high-power heater.
Assumptions
  • External work-coil geometry and exact resonant capacitor value will be treated as a prototype tuning set unless the user supplies measured coil inductance.
  • Normally-closed 90–100 C thermal switch mounts directly to the power MOSFET heatsink.
  • Standard rectangular PCB with no enclosure-driven size limit yet.
Change Notes
  • Initial specification created from the supplied design description.
  • Corrected the Prusa heater-output polarity/topology before schematic capture.

  • Project Overview

  • Intended Use

  • What the Device Should Do

  • Main Features

  • System Architecture

  • Hardware Subsystems

  • Interfaces and Connections

  • Power and Runtime Expectations

  • Power Tree and Power Budget

  • Manufacturing and Assembly Expectations

  • Firmware-Relevant Hardware Requirements

  • Physical Design Expectations

  • Important Design Decisions

  • Assumptions

  • Change Notes