Summary
A prototype only has to survive you. A shipped product has to survive thousands of strangers plugging in the wrong adapter, dragging their feet across carpet first, and yanking the power mid-write. This is the fast pass over the seven protections that stand between "works on my bench" and a field failure — or worse, smoke.
#7 — Reverse polarity
- Risk: wrong barrel-jack adapter or backward battery kills the board instantly.
- Fix: P-channel MOSFET on the high side. Its body diode conducts on correct polarity, then the MOSFET turns fully on — near-zero drop instead of the 0.3-0.7V a series diode costs you. For higher current, use an ideal-diode controller instead.
Keying isn't protection
A keyed connector is a convenience, not a circuit. Users defeat it constantly — put real reverse-polarity protection behind it regardless.
#6 — ESD
- Risk: carpet-to-doorknob static hits 8-15kV. Every exposed pin — USB, jack, SD slot, screw terminal — is an entry point. IEC ESD immunity testing is mandatory for EU sales.
- Fix: ESD protection diode on every externally accessible line, right at the connector, with a short ground return. Use low-capacitance parts on high-speed data lines so you don't degrade signal integrity.
Protect the power pins too
A grounded USB shield isn't enough — one unit died in the field because the power pin had no ESD diode while the data lines did. Cover every pin, not just data.
#5 — Overcurrent
- Risk: a shorted component or a damaged cable turns into melted traces and a liability problem with your name on it.
- Fix:
PTC resettable fuse— cheap, self-resetting, but slow — won't catch fast transients.eFuse IC— faster, precise, often bundles overvoltage lockout.One-time fuse— last line of defense on safety-critical rails; blows permanently on purpose.
#4 — Overvoltage / transient
- Risk: a 12V adapter fits the same jack as your 5V one. USB-PD can negotiate up to 48V on the same cable — a negotiation bug pushes that straight into a 5V rail.
- Fix:
TVS diodeon the input for fast spikes (standoff above max normal voltage, clamp below your parts' damage threshold). Pair with aneFuse/OVP IC for sustained overvoltage — a TVS only absorbs short bursts.
#3 — Inductive kickback
- Risk: relays, solenoids, motors store energy in a magnetic field. Cut the current and the collapsing field fires a high-voltage spike back into your driver/MCU pins. Survives bench testing, kills units one at a time in the field.
- Fix:
flyback diodeacross every DC coil, placed as close to the coil as physically possible — shorter path, less chance to do damage on the way.
#2 — Battery (lithium)
- Risk: over-discharge silently kills a cell; overcharge or a short causes swelling, venting, fire.
- Fix: start from
protected cells(OV/UV/OC/short-circuit built in at the cell). Add adedicated charger ICon-board for correct termination — never charge below freezing or above rated limit.
Compliance
UN38.3 testing is required to legally ship lithium cells, on top of whatever your market/retailer stacks on top of that.
#1 — Watchdog / brownout
- Risk: the only one on this list that's free — and the only one that protects the board from its own firmware instead of the outside world. A lockup means a support ticket; a power dip mid-flash-write can corrupt firmware permanently.
- Fix: enable the MCU's
independent watchdogfor production builds, fed from the main loop only after critical tasks check out — never from an always-fires interrupt. Set thebrownout detectorabove the minimum voltage needed for a reliable flash write.
Pre-ship checklist
Source
Condensed from Predictable Designs' Production-Grade Protection Circuits Checklist.