OHMLENS // EDA

Lesson 5.2 · Module 5: Power Regulation & Circuit Protection

Input Protection: Reverse Polarity, TVS & Fuses

The protection parts at the input of a gadget that save it from being plugged in backwards, from static zaps and from short circuits.

The live, interactive simulation for this lesson opens here when JavaScript is on.

Key ideas

  • A P-channel MOSFET blocks reverse voltage while wasting almost nothing.
  • A TVS diode clamps short spikes such as a static electricity zap.
  • A PTC fuse heats up and chokes the current during a short, then resets when it cools.

Try this in the simulation

  • With no protection, set the input negative and watch the board die. Press Replace.
  • Turn on each protection part one at a time and repeat each abuse test.
  • Fire the ESD strike with and without the TVS diode.

How do circuits protect against reverse polarity, static and short circuits?

Gadgets are protected at their power input by a few cheap parts. A series diode or a P-channel MOSFET blocks a battery connected backwards, with the MOSFET wasting almost no voltage. A TVS diode clamps very short spikes such as a static zap, and a resettable fuse heats up and chokes the current during a short circuit, then recovers when it cools. Try each abuse test with and without the protection and watch what survives.

What you’ll learn

  • Three ways to survive a battery connected backwards.
  • How a TVS diode clamps a fast voltage spike.
  • How a fuse or a resettable PTC fuse limits a short-circuit current.
  • How to compare the voltage cost of each protection part.

The formula, letter by letter

Uboard=11.9 V  ∈[−0.3,16] VTVS: U≈UBR+I Rdyn=13.3+0.00 A×0.33PTC: R=0.00 Ω  \begin{aligned}\htmlClass{mv mv-v_prot}{\htmlStyle{font-size:1.00em}{\htmlData{sym=U_board}{U_{\mathrm{board}}}}} &= \htmlData{funit=V,v=11.89099917423617}{\text{11.9 V}}\;\in [-0.3, 16]\,\htmlData{funit=V}{\text{V}}\\ \text{TVS: } \htmlData{sym=U}{U} &\approx \htmlData{sym=U_BR}{U_{\mathrm{BR}}} + \htmlData{sym=I}{I}\,\htmlData{sym=R_dyn}{R_{\mathrm{dyn}}} = 13.3 + \htmlData{funit=A,v=0}{\text{0.00 A}}\times0.33\\ \text{PTC: } \htmlData{sym=R}{R} &= \htmlData{funit=Ω,v=0}{\text{0.00 Ω}}\;\end{aligned}

Shown with this lesson’s starting values. In the simulation the numbers change as you move the sliders.

UboardU_{\mathrm{board}}
Voltage reaching the board (V · volt)
UU
Voltage (V · volt) : The electrical push between two points.
UBRU_{\mathrm{BR}}
Breakdown voltage (V · volt) : The voltage where a TVS diode starts to conduct and clamp. Datasheets usually write it V_BR.
II
Current (A · ampere) : How much charge flows each second.
RdynR_{\mathrm{dyn}}
Dynamic resistance (Ω · ohm) : How much a clamp’s voltage rises for each extra amp.
RR
Resistance (Ω · ohm) : How strongly something opposes current.

Common questions

How do I protect a circuit from reverse polarity?

Put a diode in series with the supply, which is simple but loses about 0.6 V, or use a P-channel MOSFET that conducts only when the polarity is right and loses far less.

What does a TVS diode do?

A transient voltage suppressor sits across the supply or a data line and does nothing in normal use. When a spike pushes the voltage past its rating, it conducts and clamps the voltage to a safe level.

What is a resettable fuse?

A PTC (positive temperature coefficient) fuse. Its resistance jumps when too much current heats it, so the current falls to a safe trickle. After the fault is removed and it cools, it returns to normal.

Keep going

Words used in this lesson

Parts in this lesson

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