VI Curve Tracing Explained: How FADOS Finds PCB…

What Is VI Curve Tracing?

VI curve tracing (also called signature analysis) is a power-off test method that identifies faults on populated circuit boards by comparing the voltage-current (V-I) characteristic of each component or node against a known-good reference. Unlike traditional troubleshooting methods that require schematics, powered test fixtures, or functional knowledge of the circuit, VI curve tracing works on any board, powered off, no schematic needed.

The principle is simple: apply a controlled, current-limited AC test signal to a point on the board and measure the resulting V-I curve. Every component, resistor, capacitor, diode, transistor, IC, produces a distinctive V-I signature. Compare the signature from a working board against a suspect board, and deviations immediately reveal the fault location and type.

How FADOS Implements VI Curve Tracing

FADOS (Fault Detector and Oscilloscope) from CBT Electronic implements VI curve tracing with a dual-channel architecture. One channel connects to a known-good reference board, the other to the board under test. The FADOS software overlays both V-I curves in real time, any difference between the two curves indicates a fault.

The key advantages over manual multimeter probing:

Speed: probe a point and instantly see if it matches the reference. No need to look up expected values.
No schematic required: you only need one working board as a reference. FADOS stores reference signatures in a database for future comparisons even without the reference board present.
Component-level resolution: the V-I curve shape reveals the component type and condition. A shorted capacitor, a leaky transistor, a wrong-value resistor, each produces a distinctly different curve deviation.
Safe: testing is done with the board powered off. No risk of further damage to already-faulty boards, no risk to the operator.

What Faults Can VI Curve Tracing Detect?

Fault Type	V-I Curve Indicator
Short circuit	Curve collapses to a straight line (pure resistive)
Open circuit	Curve flattens to horizontal (no current flow)
Wrong-value resistor	Curve slope changes (steeper = lower resistance)
Leaky capacitor	Elliptical curve shifts or distorts
Failed semiconductor	Curve loses characteristic diode knee
Degraded component	Subtle curve shape change vs. reference
Solder joint failure	Intermittent or shifted curve

FADOS 9F1: Beyond Basic VI Tracing

The FADOS 9F1 adds two capabilities that no other VI tester offers:

1. Built-in programmable power supply (0–40V, 0–2A): apply DC power to the board and observe current draw. Faulty components that draw excessive current under power are immediately visible in the V-I characteristics, complementing the power-off signature analysis.

2. Non-contact IR temperature sensor: after briefly powering the board, use the IR sensor to detect components that are overheating due to fault-related excessive current draw. Combined with the power supply, this technique can reduce troubleshooting time by 5–10x for certain fault types.

Industries Using FADOS in India

GSAS provides FADOS systems to repair centres and production facilities across multiple industries:

Automotive: ECU repair, powertrain control module diagnostics
Aerospace & Defence: avionics board repair, radar system maintenance
Industrial Automation: PLC and drive repair, motor controller diagnostics
Medical Devices: service depot repair of diagnostic equipment
Consumer Electronics: high-volume board repair operations
Telecom: base station and networking equipment maintenance

Getting Started with FADOS

GSAS Micro Systems is India’s authorized CBT Electronic partner, providing FADOS sales, training, and support from 7 offices across India. All FADOS units are assembled and tested at our Bangalore facility.