Other devices used in electronics
A 10 kΩ NTC thermistor in a divider with another 10 kΩ resistor departs almost 8 °C from a straight line between 0 and 70 °C. With 5.1 kΩ the departure drops to 2.2 °C: the fixed resistor is also part of the sensor.
01Thermistors
A thermistor is a semiconductor resistor, made of sintered metal oxides, whose resistance changes strongly with temperature. NTCs lower their resistance when heated, by about 4 % per degree at room temperature, ten times more than a metal; they are used to measure temperature and to limit the inrush current of power supplies. PTCs raise their resistance abruptly above a transition temperature, and serve as protection and as self-regulating heaters. Their resistance follows, over a limited range, the β equation:
The curve is highly nonlinear. Connected in a divider with a fixed resistor \( R_s \), the output voltage is an S-shaped curve, with an inflection point where it is almost straight. Placing the inflection at the center of the measurement range \( T_m \) gives the resistor:
A 10 kΩ NTC at 25 °C with β = 3950 K, with a fixed resistor Rs below it to ground, supplied with 3.3 V. The blue curve is the voltage across Rs; the dashed line, the straight line between 0 and 70 °C.
02Protection devices
| Device | How it acts | Use |
|---|---|---|
| Fuse | Melts by \( I^2 t \); fast or time-delay | Overcurrent and short circuit, one time only |
| Resettable PTC (polymeric) | Goes to high resistance when heated and recovers on cooling | USB ports, batteries, moderate overloads |
| Varistor (MOV) | Zinc oxide that conducts above a voltage; degrades with each surge | Mains overvoltages |
| TVS suppressor | Large-area avalanche diode, very fast | Electrostatic discharges and transients on data and power lines |
| Gas discharge tube (GDT) | Arc in a gas; large energy, but slow and with a high breakdown voltage | Telephone and antenna lines, first stage of protection |
Protections are combined in stages: a discharge tube for the large energy, a separating resistance or inductance, and a TVS close to the circuit to clamp the voltage precisely.
03Relays and optocouplers
An electromechanical relay galvanically isolates the control circuit from the power circuit and has a very low contact resistance, but it is slow, has bounce and a limited mechanical life. Its contacts are rated for current and voltage, and the DC breaking capacity is much lower than the AC one, because the arc does not extinguish by itself. The coil needs a freewheeling diode so as not to destroy the transistor that drives it. A solid-state relay, with a triac or MOS transistors and an optocoupler, has no wear or bounce, but it dissipates and has leakage current. An optocoupler transmits a signal across an isolation barrier with an LED and a phototransistor, and is specified by its current transfer ratio (CTR), which falls as the LED ages.
04In the lab
Measure the resistance of an NTC at three temperatures with a bath and a reference thermometer, and compute the Steinhart–Hart coefficients.
Measure the voltage across an NTC with different currents and detect the self-heating error.
Observe on the oscilloscope the overvoltage when the coil of a relay is switched off, with and without a diode.
05Common mistakes
- Using the β equation over the whole range. Outside the interval where it was fitted, the error reaches several degrees.
- Forgetting the self-heating of the thermistor. A high measurement current puts it above the temperature to be measured.
- Trusting a varistor forever. Each surge degrades it and it can end up short-circuited.
- Switching DC with a relay rated for AC. The arc does not extinguish and burns the contacts.
06Self-assessment
What resistance does a 10 kΩ NTC with β = 3950 K have at 70 °C?
About 1.76 kΩ.
What fixed resistor linearizes the divider best for measuring between 0 and 70 °C?
One close to 4.75 kΩ, the one that places the inflection at 35 °C; in the E24 series, 4.7 or 5.1 kΩ.
Why is a gas discharge tube combined with a TVS?
The tube absorbs the large energy but lets a high spike through while it ignites; the TVS clamps the voltage quickly and precisely.
What is the diode in parallel with a relay coil for?
To give a path to the coil current when it is switched off and avoid the overvoltage.
07Further reading
- Ramón Pallás Areny. Sensores y acondicionadores de señal. 4th ed., Marcombo, 2003 (in Spanish). Thermistors, their linearization and their conditioning.
- Jacob Fraden. Handbook of Modern Sensors. 5th ed., Springer, 2016. Temperature sensors and the physics of each one.
- Charles A. Harper (ed.). Passive Electronic Component Handbook. 2nd ed., McGraw-Hill, 1997. Thermistors, varistors, relays and protection devices.