Protection Parts

de:volt ships four two-terminal protection parts for guarding your circuit against transients, overcurrent, and high-frequency noise: the TVS diode, the PTC resettable fuse, the one-shot fuse, and the ferrite bead. All are passive and breadboard-friendly. For reverse-voltage clamping with a Zener, see also Diodes.

TVS Diode

A Transient Voltage Suppressor clamps fast voltage spikes by entering reverse breakdown. Below its breakdown voltage it is effectively invisible; once the line exceeds that threshold the diode conducts hard and holds the voltage down, absorbing the energy of ESD strikes and inductive kickback.

Pinout

Specs

The breakdown voltage is set by the vbr param (default 6.8 V) and can be overridden in the Inspector. While the line stays below vbr the reverse-overvoltage check is suppressed — that is normal standoff operation, not a fault.

Unidirectional vs. bidirectional

By default (bidirectional = 0) the TVS clamps only one polarity, like a Zener. For unidirectional use, point the cathode (K, the banded end) toward the positive rail. Set bidirectional to 1 for symmetric clamping on AC lines or bipolar signals; a bidirectional TVS has no functional polarity.

Example circuit

Signal in ─┬─── to MCU input
           │
          [TVS]   (K toward the line, A to GND)
           │
          GND

Place the TVS across the line you want to protect, as close as possible to the connector or the pin being defended.

PTC Resettable Fuse

A polymeric PTC (Positive Temperature Coefficient) fuse protects against overcurrent. When too much current flows it self-heats and jumps to a high resistance, throttling the current — it “trips.” Unlike a one-shot wire fuse it recovers on its own once the fault is removed and the part cools, so there is nothing to replace.

Pinout

Specs

How the trip is modelled

de:volt models the full trip-and-latch behaviour:

• Normal: the fuse is just rNormal (default 0.5 Ω) in series.
• Trip: when current rises to roughly 2× iHold the fuse latches into its tripped state, where resistance becomes 200 × rNormal (100 Ω at defaults). This collapses the current to a small trickle.
• Reset: the fuse recovers once the current falls below iHold / 2 and stays there for 2 seconds, simulating the cooldown of a real PTC.

Wire it in series with the supply line you want to protect — between a USB or battery feed and the rest of the board.

Example circuit

5V supply ─── [PTC fuse] ─── Load (+)
                              Load (−) ─── GND

If the load draws more than ~1 A (2× the 0.5 A default hold) the fuse trips and the load voltage drops. Fix the overload, wait for the cooldown, and the fuse resets itself.

Fuse (one-shot)

The plain fuse is the classic single-use overcurrent device: a thin element that carries current normally but melts open above its rated current, breaking the circuit. Unlike the PTC it does not recover — in real hardware you replace it. Reach for it when you want a hard cut-off rather than the PTC’s self-resetting throttle.

Pinout

Specs

In normal operation the fuse is just rNormal (default 0.1 Ω) in series. When the current exceeds iRating (default 1 A, editable in the Inspector) the fuse opens and stops conducting. Wire it in series with the supply line you want to protect.

Supply ─── [Fuse] ─── Load (+)
                      Load (−) ─── GND

Ferrite Bead

A ferrite bead suppresses high-frequency noise on power and signal lines. At DC it is nearly a short (a few hundred milliohms), but at RF it presents a large impedance — turning conducted switching noise into heat before it reaches a sensitive IC.

Pinout

Specs

DC-only model

The bead’s real job — high-frequency insertion loss — happens well above the simulator’s bandwidth (the engine steps at 50 µs). Modelling that impedance would add no accuracy, so de:volt stamps the bead as a plain series resistor equal to rDc. In practice this means:

• At DC and low frequency the part behaves like a small (~0 Ω) resistance.
• The HF noise suppression is not simulated — the part is here for BOM/layout realism and DC voltage-drop checks.

Example circuit

Noisy 5V ─── [Ferrite bead] ─── Clean VCC ─── Sensitive IC

Place the bead in series between a noisy supply output and the VCC pin of the chip you want to keep quiet, ideally right next to that pin’s decoupling capacitor.