Power Indicator LED

Hardware version

CANBench Duo v1.1 — Fabricated prototype, sole built unit. V1.1 is electrically identical to the V1.2 schematic refresh but predates the InvenTree symbol-library migration; the schematic component metadata reflects legacy SCADYS naming. Testing and bench validation reference this V1.1 hardware.

Other versions: v1.2 — schematic refresh (next version)

Overview

A single XL-5050RGBC three-die RGB LED on the top extrusion encodes four supply-chain states. The encoding is entirely topological — there is no MCU, no firmware, no software. Each colour falls out of the rail relationships in the LISN supply-path protection chain.

This page covers a single sub-circuit — the Power Indicator LED — drawn on the power_indicator_led KiCad sheet. The LED is the fixture's front status indicator: it is in the operator's line of sight throughout a measurement, so the colour encoding is written to be readable at a glance during normal use.

Functional specification and design objectives

The power-indicator circuit must:

• give the operator an at-a-glance, always-on read of the bench-supply path state while a measurement is running;
• encode four distinct states — supply absent, normal operation, reverse polarity, and protection-chain fault — in a single front-facing RGB LED;
• derive every state from the LISN supply-path rail relationships alone, with no MCU, firmware, or active sensing;
• load the bench supply negligibly (target < 0.5 % of the LISN's 4 A continuous design current across the 12–48 V supply range); and
• keep each die comfortably below its 20 mA absolute-maximum across the full 12–48 V supply range.

State table

Indicator	Condition	What it means	What to do
Off	No bench-supply voltage	The bench supply is not connected, the supply is set to 0 V, or the supply cable to the SRC sockets is open.	Check the bench supply is on and wired to the SRC banana pair (front faceplate).
Green	Correct polarity, both protection FETs conducting, F1 intact	Normal operation. LISN delivers the filtered supply to the DUT bananas (J1 / J3) and the M12 N2K connector (J10).	Proceed with measurement.
Blue	Q2 protection FET not fully conducting — typically because F1 has blown	A DUT-side over-current event has usually blown the SRC+ rail's 5 A Nano2 Slo-Blo fuse, depriving the upstream P-FET (Q2) of its drain reference. The bench supply is still present at the SRC pair; the DUT pair is dead. A brief Blue flash at supply turn-on (see below) is a separate, benign variant of the same mechanism.	Power the bench supply down. Remove the DUT. Replace F1. If the LED stays Blue with no DUT attached after a fresh fuse, suspect Q2. Investigate the DUT for an over-current condition before reconnecting.
Red	Reverse polarity at the SRC pair	The bench supply cables are reversed at the SRC banana pair — V_BLACK > V_RED. The internal protection FETs prevent current flow downstream, so the DUT pair is not energised.	Power the bench supply down. Reverse the SRC cables to the correct RED-to-SUPPLY+, BLACK-to-SUPPLY− orientation.

A brief Blue flash for ~ ms at supply turn-on is normal and benign — Q2's gate-bias divider takes a few RC time constants to bring Q2 fully ON, and during that interval V(SUPPLY+) − V(VSS+) momentarily exceeds Q1's V_BE threshold. The LED settles to Green once Q2 reaches full conduction.

The LED is visible through the top extrusion of the YG-H10A enclosure (see render_1.PNG and render_2.PNG on the Connectors & Mechanical page), so the operator looking down at the bench can read the state at a glance.

Power Indicator LED

How it works

Five components implement the encoding:

Ref	Value	Role
D1	XL-5050RGBC	Three-die RGB LED — independent R / G / B dies with separate anode + cathode pairs
Q1	BC807-25 (PNP, SOT-23)	High-side switch driving the Blue die anode
R1	10 kΩ, 0805	Green-die current limiter (anode side)
R3	10 kΩ, 0805	Red + Blue shared current limiter (cathode side)
R2	100 kΩ, 0603	Q1 base bias — provides the DC path from VSS+ to SUPPLY− that lets Q1 sense the SUPPLY+ − VSS+ delta

The rail connections determine each state:

• Green — D1.GA (Green anode) sits on VSS+ through R1; D1.GK (Green cathode) sits on VSS−. When the protection FETs conduct, V(VSS+) ≫ V(VSS−), and Green is forward-biased.
• Red — D1.RA (Red anode) sits on the shared Red-anode / Blue-cathode node through R3 to SUPPLY−; D1.RK (Red cathode) sits on SUPPLY+. When the SRC pair is reverse-wired, V(SUPPLY−) > V(SUPPLY+) + V_F_red, and Red conducts.
• Blue — D1.BA (Blue anode) is driven by Q1's collector. Q1 is a PNP with E on SUPPLY+, B on VSS+. Q1 turns on when V(SUPPLY+) − V(VSS+) > V_BE_on ≈ 0.65 V. Under normal operation Q2 (the P-FET in the reverse-polarity protection chain) holds VSS+ within mV of SUPPLY+, so Q1 stays off and Blue is dark. When F1 blows, VSS+ no longer has a current path to SUPPLY+, R2 pulls VSS+ toward SUPPLY−, the V(SUPPLY+) − V(VSS+) gap opens up to the full supply voltage, Q1 saturates, and Blue lights via R3 to SUPPLY−.
• Off — no supply means no current path; all three dies are dark.

The cleverness is that the Blue-state mechanism is the gate-protection circuit's own behaviour under fault, not a separate fault-detection circuit.

Performance

LED current at the typical 12 V N2K supply and the maximum 48 V supply:

State	I_LED @ 12 V	I_LED @ 48 V	Notes
Green	0.98 mA	4.58 mA	Limited by R1 (10 kΩ); D1 die max is 20 mA — comfortable margin
Red	1.00 mA	4.60 mA	Limited by R3 (10 kΩ)
Blue	0.90 mA	4.50 mA	Limited by R3 (10 kΩ), reduced by Q1 V_CE_sat ≈ 0.2 V

Brightness ranges from comfortably-readable at 12 V to bright but not blinding at 48 V — appropriate for an always-on indicator.

Resistor power utilisation

Resistor	Footprint	Rated power	Worst-case dissipation @ 48 V	Utilisation
R1	0805, 0.25 W	250 mW	210 mW (Green ON)	84 %
R3	0805, 0.25 W	250 mW	210 mW (Red or Blue ON)	84 %
R2	0603, 0.1 W	100 mW	23 mW	23 %

At the 48 V supply ceiling, R1 and R3 sit at 84 % of their power rating when the corresponding die is on. V1.1 / V1.2 carry the 0805 footprint as a known steady-state limit at 48 V (see Gaps & next version).

Load on the supply

Total worst-case loading of the indicator circuit on the bench supply: ≈ 4.6 mA at 48 V (one die on). Against the LISN's 4 A continuous design current that's 0.1 % loading — entirely negligible.

PCB Layout

D1 is on B.Cu at (76.0, 90.0), facing up through the YG-H10A top extrusion. Q1, R1, R2, R3 are clustered on F.Cu beneath D1 inside a 5 × 3 mm island around (X = 75, Y = 90) — close to the SRC banana pair (J5 / J7 at X = 71.5) since that's the entry of the SUPPLY+ / SUPPLY− rails the indicator is sensing. The Q1.C → D1.BA trace stays under ~5 mm including one cross-layer via, keeping the high-impedance Blue-drive node short.

Why no firmware?

The indicator could equivalently be implemented as a single tri-colour LED driven by an MCU sampling rail voltages. The discrete topological version was chosen because:

1. No firmware to debug. The state encoding is wired into the rail relationships; there is no code path that can be wrong.
2. No power-up logic. Q1 + Q2 + Q3 all bias themselves up by the same rails they're protecting; there is no MCU initialisation sequence to fail.
3. Cost. Five small passives + one BJT in SOT-23 + one RGB LED beats a microcontroller + crystal + decoupling + firmware-flash effort for this single-function need.

This is consistent with the broader CANBench Duo philosophy — the instrument is fully passive, no MCU on the board, no firmware in the field. Everything that happens, happens because of physics.

Components

Ref	Value	Function	Datasheet
D1	XL-5050RGBC	Three-die RGB LED, SMD5050-6P — independent R / G / B dies, separate anode + cathode pairs	XINGLIGHT XL-5050RGBC
Q1	BC807-25	PNP BJT, SOT-23 — high-side switch driving the Blue die anode	Nexperia BC807
R1	10 kΩ	0805, 0.25 W — Green-die current limiter (anode side)	—
R2	100 kΩ	0603, 0.1 W — Q1 base bias (VSS+ to SUPPLY− DC path)	—
R3	10 kΩ	0805, 0.25 W — Red + Blue shared current limiter (cathode side)	—

Gaps & next version

Next version (V1.3)

• Upsize R1 and R3 to 1206 (0.4 W rating). At the 48 V supply ceiling, R1 and R3 sit at 84 % of their 0805 0.25 W rating when the corresponding die is on. V1.3 upsizes both to 1206 for better thermal margin. V1.1 / V1.2 carry the 0805 footprint as a known steady-state limit at 48 V.

Related pages

• LISN Supply Path — the fuse (F1), reverse-polarity protection FETs (Q2 P-FET on SUPPLY+, Q3 N-FET on SUPPLY−), and the VSS+ / VSS− rail definitions referenced by the state encoder
• Connectors & Mechanical — the YG-H10A top extrusion through which D1 is visible

References

• XINGLIGHT, XL-5050RGBC RGB LED, SMD5050-6P — D1
• Nexperia, BC807 PNP BJT — Q1