Lesson · Problem 9
Protect the Connector
Series and shunt protection
You should be able to
- Place a fuse, a TVS shunt, and a series diode in the order the fault requires.
- Orient the series diode so the intended polarity conducts and the reverse does not.
Datasheets & Components · Schematic Reasoning · Power
What it is
A fuse in series opens a sustained overcurrent. A series Schottky blocks a reversed supply when it is oriented to conduct only for the intended polarity. A TVS is a shunt: it turns on above its voltage and steers the surge into ground. This challenge grades a specific order. A clamp that is not on the node the stress hits is not the protection that case is checking.
Why this matters
A connector is where the outside world attaches the wrong supply. The protection parts have to be in the path the fault actually takes.
What you are building
A connector, a series fuse, a TVS to ground, and a series diode toward the load.
Prerequisites
This lesson uses net, device card, closed loop, anode, cathode. It introduces fuse, TVS, shunt, series diode, polarity.
Component guide
What the parts are. The requirements panel is still the list that is graded.
Fuse
- What it does
- Opens the series path when current stays too high.
- Why it is here
- It is the first series element after the connector in this protection chain.
- Symbol
- A rectangle in series with the line.
- Beginner mistake
- Putting it after the load, where it no longer sees the fault current.
- What an engineer checks
- The current at which it opens, and that it is in series.
TVS
- What it does
- A diode that stays off at the normal voltage and clamps a spike to ground.
- Why it is here
- It takes the surge that the fuse is too slow to stop.
- Symbol
- A diode from the line to ground. The bar, the cathode, is on the line for a positive rail.
- Polarity
- Cathode on the protected line, anode at ground, for the positive rail in this course.
- Beginner mistake
- Turning it around so it conducts during normal operation.
- What an engineer checks
- Standoff voltage and which way the cathode faces.
Series diode
- What it does
- Conducts only one way, so a reversed supply does not reach the load.
- Why it is here
- It sits after the TVS, toward the load.
- Symbol
- A diode in series. The triangle points toward the load.
- Polarity
- Anode toward the source, cathode toward the load.
- Beginner mistake
- Pointing it back at the connector.
- What an engineer checks
- Forward direction and the current it must carry.
Interview lens
Draw the TVS cathode on the line and the series diode pointing at the load. Say which fault each part stops.
What PCBGrade measures
Polarity and order. A part that is present but reversed fails the stress model. This is not a SPICE transient.
Where this shows up
Reverse battery and a cable transient are why this chain exists on a barrel jack or a field connector.
Terms
- TVS
- A diode that conducts heavily above a trigger voltage and dumps that current into its other pin, usually ground.
- Shunt
- A part wired across the line to ground, not in series with the load.
What is happening electrically
A connector is where a person can attach the wrong supply. A series fuse opens if current stays too high. A TVS is a shunt: above its voltage it conducts heavily and steers the surge into ground. A series diode conducts one way, so a reversed supply stops at it if the cathode and anode face the right direction.
Why the geometry matters
Order is the layout. The fuse has to be in series with the energy. The TVS has to sit on the node the surge actually reaches, with its ground pin on connector ground. The series diode has to be between that protected node and the load, pointing so the intended polarity passes.
How an engineer reasons
Take each stress alone. Normal +12 V should reach the load. −12 V should not. A short high pulse should be clamped at the TVS rather than applied in full downstream. If a part is not on the path that stress takes, it does not answer that case.
Worked example
Fuse first, then the TVS from the fused node to connector ground, then the series Schottky toward the load. The three stress cases are graded separately. The requirements name the nets.
Good and bad
Fuse, then TVS to ground, then the series diode toward the load, handles the three cases in the order this challenge grades. A TVS after the load never sees the connector surge. A series diode turned around blocks the supply you meant to pass.
Common mistake
Putting the TVS on the connector side of the fuse, or reversing the series diode so the normal supply is blocked. The graded order is fuse, then the TVS to ground, then the series diode.
Where this rule stops
The stresses are a structured model, not a laboratory surge generator and not an IEC waveform. TVS clamp voltage, fuse time, and diode drop on a real part need the datasheet. The principle that survives is: each part has to sit on the path its fault actually takes.
Before the challenge
The challenge applies three stresses. Order the fuse, the shunt, and the series diode so each stress meets the part that is supposed to handle it.
Ready for the challenge
You can place the three parts in order and state the cathode direction of each diode.
Question 1 of 2