Problem 9
Fuse, 18 V TVS, and series Schottky so the rail survives +12 V, −12 V, and a +28 V pulse.
- Order: fuse, then the TVS to connector ground, then the series diode toward the load.
- Normal +12 V passes. −12 V must not reach the load. A short +28 V pulse is clamped.
Context
A connector is where the wrong supply shows up. A fuse is in series and opens a sustained overcurrent. A TVS is a shunt: it turns on above its voltage and steers the surge to ground. A series Schottky conducts only one way, so orientation decides whether a reversed supply is blocked.
Components on this challenge
- Fuse. Opens the series path when current stays too high.
- TVS. A diode that stays off at the normal voltage and clamps a spike to ground.
- Series diode. Conducts only one way, so a reversed supply does not reach the load.
Complete the required schematic checks to unlock PCB Layout.
Full brief
Order the fuse, the TVS shunt to ground, and the series diode so the intended polarity passes and a reverse supply or a surge meets the part that should handle it.
How PCBGrade measures this
Nominal input is 12 V (normal range 9–16 V) at up to 0.4 A. Build topology A: connector → 0.5 A resettable fuse → 18 V unidirectional TVS (cathode on the line, anode on GND) → series Schottky (anode toward the connector, cathode toward the rail). The 5 V and 36 V TVS parts are the wrong cards. This is a structured clamp model, not SPICE and not a transient: pulse energy is within every TVS rating, and the fuse does not trip on the surge. Fuse resistance is 0.5 Ω and the Schottky drop is 0.45 V. The 5 V card clamps the TVS node to 9.2 V, the 18 V card to 24.0 V, and the 36 V card does not clamp (the node stays 28 V). Topology A then subtracts the diode drop; a TVS on the rail side of the diode does not. Normal must be ≥ 11.0 V, reverse ≥ −0.30 V, and surge ≤ 24.0 V, compared in millivolts and inclusive. Failing the fuse-in-series check, the normal test, the reverse test, or the surge test caps the score at 69. Without that cap a 5 V TVS would score 76 and a 36 V TVS would score 77, and both would pass. The connector-to-TVS length is the copper from the connector pin to the fuse pin on that net, plus the fuse pad-centre span, plus the copper from the other fuse pin to the TVS cathode, and it must be ≤ 10.0 mm. TVS anode to the connector GND pin is ≤ 5.0 mm of GND copper. A via barrel on this 2-layer board is 1.565 mm. An unfused TVS (ahead of the fuse) fails reverse. Points: fuse between LINE and CLAMP 15, Schottky between CLAMP and RAIL 15, 18 V TVS 15, normal 9, reverse 8, surge 8, each placement path 10, DRC 10.
- circuit · 45 pts
- stress tests · 25 pts
- protection placement · 20 pts
- DRC · 10 pts
- Cap 69: A missing series fuse, or a normal, reverse, or surge result outside its limit, caps the score at 69.
- Cap 69: Copper outside the outline cannot pass, and neither can a short between nets.
Reference
0.5 A resettable fuse
Must sit in series on every path from the connector to the protected rail.
- 1. 1. Either terminal
- 2. 2. Either terminal
1 A / 40 V Schottky
Anode toward the connector, cathode toward the protected rail. Forward drop is 0.45 V.
5 V TVS
Pin 1 is the cathode (line). Pin 2 is the anode (GND). Clamps a +28 V pulse to 9.2 V. Conducts at the +12 V normal test and fails it.
- 1. K. Cathode, toward the line
- 2. A. Anode, toward GND
18 V TVS
Pin 1 is the cathode (line). Pin 2 is the anode (GND). Clamps a +28 V pulse to 24.0 V. Does not conduct at +12 V.
- 1. K. Cathode, toward the line
- 2. A. Anode, toward GND
36 V TVS
Pin 1 is the cathode (line). Pin 2 is the anode (GND). Does not clamp the +28 V pulse. The surge stays 28 V and the rail fails.
- 1. K. Cathode, toward the line
- 2. A. Anode, toward GND