Eload 4ch

eload-60v — 4-channel electronic load, 60 V · 20 A · 300 W

An open-hardware DC electronic load with four independent constant-current channels, 20-bit measurement on every channel, a hardware fail-safe that does not depend on firmware, and WiFi control through a plug-in ESP8266 module.

Status: rev 0.3 — design complete, not yet built or bench-tested. ERC and DRC (with schematic parity) are clean, and the control loop and thermal budget have been analysed numerically. Treat it as a reviewed design, not a proven product. Bench reports, issues and pull requests are very welcome.

Specifications

How it works

• Power path. DUT+ → 25 A ATO fuse → VLOAD bus. A D²PAK Schottky (VS-30CTQ100S) acts as a reverse-polarity crowbar that blows the fuse, and an SMCJ64CA TVS clamps transients.
• Channels. Each channel is an OPA2196 integrator driving an IXYS IXTH75N10L2 Linear L2 MOSFET (extended FBSOA, designed for linear operation) with a 20 mΩ 4-terminal shunt (Vishay WSK2512). The setpoint is the DAC output divided by 25, referenced to the shunt's cold Kelvin pad: I = V_DAC × 2 A/V.
• Measurement. One INA228 per channel reads the shunt (Kelvin) and the remote-sense voltage. Its ALERT pin is wired to the common ARM line, so a programmed over-current or over-power limit shuts the load down in hardware.
• Fail-safe. ARM is active-high with a pull-down. It can be pulled low by the MCU, by the LM393 OVP/OTP comparators or by any INA228 ALERT. ARM low drives CLAMP high, which shorts all four setpoints to ground. No MCU, a reset or a boot glitch all leave the load off.
• Thermal. The four TO-247 parts are mounted under the PCB on an 80 × 70 × 6 mm copper spreader. The spreader sits at VLOAD (drain) potential, so it is isolated from an SP3/TR4-class CPU cooler by a thermal pad. Two NTC thermistors sit on the spreader.
• Front panel (rev 0.3). A 10-pin IDC connector carries the I²C bus to a 128 × 64 OLED (0x3C) and the encoder, its push switch, the OUTPUT button and its LED to a PCF8574 expander (0x20) on the main board. The expander's INT goes to GPIO0. The encoder works as a vernier: push toggles coarse/fine, long-press selects the channel. OUTPUT ON is the only way to arm the load from the panel.
• Supplies. An isolated 12 V adaptor feeds the op-amps and the fan. A 78L05 supplies the analog 5 V (DAC, comparators). A separate AP63205 buck feeds only the WiFi module, and its 3.3 V regulator powers the logic.

The full walkthrough, including SOA, thermal budget and loop-stability analysis, is in docs/Electronic_Load_Circuit_Walkthrough.pdf (20 slides).

Analysis highlights

SOA	Loop gain

• SOA. The worst steady point is 60 V × 1.25 A = 75 W per MOSFET. The guaranteed FBSOA (240 W at 80 V, T_C 75 °C), derated to T_C ≈ 96 °C, gives ≈ 173 W, a 2.3× margin.
• Thermal. T_J ≈ 119 °C at 35 °C ambient with a 0.12 K/W cooler. At the OTP trip point (spreader ~100 °C) T_J ≈ 139 °C. To run 300 W continuously the cooler plus pad must be ≤ 0.22 K/W.
• Loop. Rev 0.1 used 10 nF compensation: stable, but only 0.6 – 1.5 kHz bandwidth and a turn-on delay of about 2 ms. Rev 0.2 uses C6 = 1 nF and C5 = 10 nF: PM ≈ 80°, 6 – 15 kHz, about 28 µs for small steps. A 10 mA keep-alive in firmware cuts the turn-on from the clamp to about 39 µs.

The model and scripts are in analysis/. Op-amp second pole and output resistance, the cooler's thermal resistance and the pad are assumptions, listed in the slides.

Repository layout

hardware/
  kicad/          KiCad 10 project (eload_4ch.kicad_pro), custom symbols, DRC rules
  fabrication/    Gerbers + drill (zip) and pick-and-place positions, rev 0.3
  bom/            BOM from KiCad (CSV) and priced BOM with sources (XLSX)
docs/
  Electronic_Load_Circuit_Walkthrough.pdf   slide deck (English)
  assembly/       interactive HTML BOM, top/bottom assembly drawings
  eload_4ch_schematic.pdf, eload_4ch_pcb.pdf
  images/, reports/ (ERC / DRC)
firmware/         ESP8266 firmware (Arduino core), web page, unit tests, mock server, prebuilt bin
analysis/         loop model (sympy/scipy), large-signal sim, SOA/thermal plots
tools/generator/  Python scripts that generated the schematic and placed/routed the PCB

Schematic, PCB and the KiCad BOM are in English. The priced BOM workbook (eload_4ch_bom_priced.xlsx) and the code comments in tools/ are still in Italian.

Building one

1. PCB. Use hardware/fabrication/eload_4ch_gerbers_rev0.3.zip: 2 layers, 1.6 mm, 2 oz copper. Many fabs require ≥ 0.2 mm spacing with 2 oz copper, while the INA228 MSOP-10 pads are 0.15 mm apart (the project has a local DRC exception for them). Confirm with your fab, or order 1 oz outer layers.
2. Parts. See hardware/bom/eload_4ch_bom_priced.xlsx. The electronics cost is about €126 per board at single quantities (October 2026). The four IXTH75N10L2 are about half of that and can have long factory lead times.
3. Mechanics and front panel. Copper spreader, insulating thermal pad (≥ 100 V), CPU cooler with a 4-wire PWM fan, M3 nylon standoffs, M5 bolts for the DUT studs. Front panel: 2.42″ SSD1309 or 1.3″ SH1106 I²C OLED, EC11 encoder with push, OUTPUT button with LED, 10-wire ribbon (≤ 20 cm). J6 pinout: 1 3V3 · 2 GND · 3 SDA · 4 GND · 5 SCL · 6 ENC_A · 7 ENC_B · 8 ENC_SW · 9 OUTPUT · 10 LED (cathode, 470 Ω on board).
4. Assembly. Open docs/assembly/eload_4ch_ibom.html in a browser: an interactive BOM that highlights where each part goes, with checkboxes to track sourced and placed parts. docs/assembly/assembly_top.pdf and assembly_bottom.pdf (the four MOSFETs) show every reference inside its part outline. The silkscreen has no overlaps: each reference designator is next to its component.
5. Firmware. See firmware/README.md. A prebuilt image is in firmware/bin/. On first start the load opens a WiFi access point for setup; afterwards the page is at http://eload.local.

Firmware

v0.2.0 for the D1 mini Pro. It compiles and its control math is unit-tested, but it has not yet run on hardware.

• Control: CC / CP / CR with the setpoint shared by the channels, hardware and firmware protections with the trip cause reported, battery cutoff, auto-trim.
• Pulsed load: the sink switches between the setpoint and 10 mA at 0.1 Hz – 1 kHz with an adjustable duty, set from the panel menu or the web page.
• Front panel: OLED with the logo at boot, encoder used as a vernier and to drive the menu, OUTPUT button.
• Logging: CSV on the 14 MB flash.
• Web: live chart, controls, OTA update.

⚠️ Safety

• The whole controller, including the Wemos USB port, is referenced to DUT−. USB is not isolated: only program the module with the DUT disconnected, then update over the air. The 12 V adaptor must be isolated.
• The copper spreader and the MOSFET tabs are live at the DUT voltage.
• 300 W of heat is real. Do not run full power without the cooler and fan, and verify the OTP before trusting it.

To verify on the bench

• Loop step response (1.0 ↔ 1.2 A) into a stiff supply, then with 1 m of DUT cable.
• D1 crowbar surviving the fuse I²t on reversed connection.
• Thermal resistance of the actual cooler and pad; OTP trip point.
• WiFi range with the antenna overhanging the board edge inside the enclosure.
• I²C signal integrity with the OLED on a 20 cm ribbon (2.2 kΩ pull-ups).

Changelog

• firmware 0.2.0 — pulsed load (frequency 0.1 Hz – 1 kHz, duty 1 – 99 %, phases ≥ 100 µs) in the panel menu, the web page, the API and the CSV.

• firmware 0.1.0 — first firmware: control, protections, OLED + encoder panel, CSV logging, web interface, OTA.

• rev 0.3 — front panel: PCF8574 expander, 10-pin IDC connector, debounce, I²C pull-ups 2.2 kΩ. Schematic and PCB translated to English (sheets: input, channels, control, power). Silkscreen cleaned up: no overlaps, references next to their parts; interactive BOM and assembly drawings.

• rev 0.2 — loop compensation C6 = 1 nF, setpoint filter C5 = 10 nF (10× bandwidth, PM ≈ 80°); WiFi control with D1 mini Pro replaces STM32 + isolated USB.

Credits

Inspired by shufps/eload2. Designed by IV3HGA.

License

• Hardware (everything under hardware/ and docs/): CERN-OHL-S-2.0 — see LICENSE.
• Software (firmware/, analysis/, tools/): MIT — see LICENSE-MIT.