VBA Curve Tracer — Vol 3: Building It

Figure 1 — Inside the finished unit: the mains inlet, line filter and DUT-supply transformer at the rear left, the heatsinked pass devices and the 250 WVDC reservoir cap centre-rear, the Main and Front boards… — Figure 1 — Inside the finished unit: the mains inlet, line filter and DUT-supply transformer at the rear left, the heatsinked pass devices and the 250 WVDC reservoir cap centre-rear, the Main and Front boards mid-chassis, and the fully labelled front panel. Source: Paul Versteeg / paulvee, VBA-Curve-Tracer GitHub repo.

3.1 What “building it” means here

The VBA is a from-published-files build, not a purchasable kit. Everything you need lives in the public paulvee/VBA-Curve-Tracer GitHub repository: schematics, 2D and 3D board renders, bills of materials, connector/wiring lists, build instructions, verification-and-calibration procedures, and the Gerber sets for all four PCBs. Mark Allie captured the schematics and laid out the boards in Altium. You fabricate the boards (Versteeg shares them via PCBWay as a four-part project), source the parts from the BOMs, and assemble. This volume is an orientation to those files and the practical bring-up — not a substitute for reading the repository’s own build and calibration PDFs, which are authoritative.

3.2 The four boards

Figure 2 — The four PCBs and roughly how they sit in the chassis. Front Panel carries the controls and DUT sockets; the Front Board holds the step generator and isolated supply; the Main Board holds the trian… — Figure 2 — The four PCBs and roughly how they sit in the chassis. Front Panel carries the controls and DUT sockets; the Front Board holds the step generator and isolated supply; the Main Board holds the triangle generator, DUT supply and X/Y amplifiers; the Back Panel carries mains, transformer and the BNC outputs. Source: hand-authored SVG.

The design is partitioned across four PCBs, each with its own folder and Gerber set in the repo:

Table 1 — The design is partitioned across four PCBs, each with its own folder and Gerber set in the repo

Board	What it carries
Main Board	Triangle generator, regulated DUT supply (transconductance amp + MOSFET pass devices), current-limit reference, current-sense shunt, X/Y amplifiers, and the local power rails
Front Board	The isolated step generator (counter + resistor-ladder DAC), its floating ±15 V supply, step buffering, offset, and the DUT-protection / fault-detect circuitry
Front Panel	The user interface — rotary switches, potentiometers, toggle switches, indicator LEDs, and the DUT connections (banana sockets and the Left/Right device sockets for matching)
Back Panel	Mains inlet and filter, the reconfigurable DUT-supply transformer, and the X / Y / Z BNC output connectors

One explicit fabrication note from the README: when you order the boards, select the black solder-mask colour for the Front Panel and Back Panel, since those double as the visible faces of the instrument. The 3D render files in each board folder are worth studying before assembly to understand component orientation and clearances, especially around the heatsinked pass devices.

3.3 The design files and BOMs

The repository’s key documents, by role:

Schematics — one set per board, with the Main Board’s sheets split by function (TriangleGenerator, DUTSupply, CurrentSource, TriangleSupply, IsolatedSupply, ACSupply) and the Front Board’s by function (D2A, StepGenBuffer, CurrentAndVoltageSet, FaultDetect, XYAmp). Read these alongside Vol 2’s block descriptions.
Bills of materials — on-board parts are listed per board; off-board parts (switches, pots, connectors, hardware) are collected separately. The repo includes files such as an off-PCB parts BOM (OffPCBPartsBOMRev2c.xlsx) and a connector/wiring list (ConnectorWiring.xlsx). Community builders have corrected BOM errors over successive revisions, so use the latest revision and read the repo’s recent-changes notes before ordering.
Gerbers — one zip/folder per board, ready for a fab house.
Build instructions — separate install notes for the Front Board and Main Board.
Specifications and Verification and Calibration documents — the performance envelope and the step-by-step alignment procedure. These are the load-bearing documents for bring-up and are covered only in outline here.

The transformer is the one component that needs thought: the three DUT-supply envelopes (35 V/2 A, 70 V/1 A, 200 V/100 mA) are produced by reconfiguring transformer windings — paralleled, series, and with an auxiliary winding for the 200 V range — so follow the repo’s transformer and wiring documentation exactly. Getting the winding configuration and phasing right is a prerequisite for the supply behaving.

3.4 Assembly notes

A sensible build order, consistent with the repository’s structure:

Populate the Main Board and Front Board from their per-board BOMs. Fit the passives, then semiconductors, then the pass MOSFETs and their heatsinks. The pass devices dissipate real power on the low-voltage/high-current range — mount and heatsink them per the layout.
Wire the transformer and power rails. Bring up the raw supplies first, before any active bring-up, and confirm each rail (the DUT supply reservoir, the isolated ±15 V step rails, and the X/Y and control rails) is present and at the right polarity and voltage. Respect the isolation barrier around the step generator — do not accidentally bond its floating ground to chassis.
Mount the Front Panel controls and wire them to the Front Board per the connector/wiring list. The panel carries: Step Delay, Offset (Pos/Off/Neg with Min–Max), Polarity (P/N), Steps (0–7), a BJT/FET device-type switch, the Step Output range knob, DUT Select (Left/Right) with the E/S, B/G, C/D banana posts and the two device sockets, Current and Voltage level controls with the CL indicator, Current Range (×1…×0.02), Sweep, the X-Amp ×1/×10 switch, and the Voltage Ranges selector.
Fit the Back Panel with the mains inlet/filter and the X/Y/Z BNCs.

Two safety points carry over from the shared bench-discipline notes and apply squarely here: the instrument contains mains wiring and a 200 V DUT supply with a large reservoir capacitor that stays charged after power-off. Treat it like any mains-connected, high-voltage bench tool — discharge the reservoir before working inside, and bring it up for the first time on a current-limited variac if you have one.

3.5 Bring-up and calibration

Once the rails check out, follow the repo’s Verification and Calibration procedure rather than improvising. In outline, bring-up confirms:

The triangle generator runs (nominal ~160 Hz, ~7.5 Vpp) and its sync trim locks the sweep to the step transitions.
The DUT supply regulates across all three ranges and its current limit tracks the front-panel Current Range multipliers — verify the 1 V reference and the ×1…×0.02 scaling.
The step generator produces a clean 0–7-step staircase in the selected 1-2-5 magnitude, with Polarity and Offset behaving, and the fault-detect / protection trips as specified.
The X and Y amplifiers are calibrated: X to a 1:1 device-voltage scale, Y for correct current scaling on ×1 and ×10. A known part (a resistor gives a straight line whose slope equals 1/R; a reference diode gives a known knee) is the quickest sanity check that the axes read true.

A useful first smoke test after calibration is to trace a plain resistor: in X-Y it should draw a straight line through the origin, its slope set by the resistance and the current range — a fast confirmation that both axes and the sweep are wired and scaled correctly before you risk a real semiconductor. A reference diode is the natural second test: a known forward knee confirms the X-axis voltage scale, and stepping into its reverse breakdown on the 200 V range exercises the high-voltage path under a safe current limit.

3.6 Sourcing and cost notes

Because the VBA is a published-files design rather than a stocked kit, expect to gather parts from the usual distributors against the BOMs. The reconfigurable transformer, the enclosure, the rotary switches and the front-panel hardware tend to dominate both cost and lead time — order those early. The community-maintained BOM revisions in the repo matter here: they exist precisely because builders hit part-availability and errata issues, so cross-check the latest revision and the repo’s issue history before committing an order. None of the active silicon is exotic, but the pass MOSFETs and their heatsinking, plus the high-voltage reservoir capacitor, are the parts worth buying to spec rather than substituting casually.

With the boards populated, the transformer and rails verified, the protection confirmed to trip, and the X/Y axes calibrated against known parts, the instrument is ready for the measurements in Vol 4.