| Episodes | 56 |
| Mentions | 71 |
| Cited here | 25 |
| First — last | #3 — #696 |
| Top guests | Luke Beno, Art Kay, Rick Altherr |
| Related | internet of things · microcontroller · sensor · accelerometer · analog to digital converter |
A temperature sensor is a transducer that converts thermal conditions into an electrical signal readable by an electronic system. An ordinary integrated temperature sensor measures the behaviour of a silicon junction on its own die, so it reports the temperature of the chip itself, whereas non-contact instruments such as thermopile parts collect incident infrared radiation in a package cavity to measure remote objects.[53] Temperature measurement is among the most thoroughly commoditised functions in electronics, which shifts the engineering problem from how to measure temperature to where to place the sensor and what to do with the resulting number.[153]
Operating principles
The most common integrated temperature sensor exploits the temperature-dependent behaviour of a silicon junction on the sensor’s own die; consequently, such a device measures the temperature of the chip on which it is fabricated rather than that of any external object.[53] Non-contact measurement requires a different instrument entirely: a thermopile part incorporates a cavity in the package that collects incident infrared radiation, which is what makes measurement without physical contact possible.[53]
Some voltage regulators expose their own die temperature as an analogue output whose voltage range is set by a single external resistor. Because the range can be matched directly to the converter that will read it, the op-amps and resistor dividers that signal scaling would otherwise require are eliminated.[154]
In modern soldering irons, the heating element and the temperature sensor are integrated into the tip itself, so changing the tip changes both. The feedback path is far more direct than sensing through a mechanical contact or an air gap; older iron construction is recognisable because the tip is bare metal with no electrical contacts on it.[384]
Placement and thermal coupling
A temperature sensor reports conditions at its own location and nothing else, which makes placement the central design question. In a reflow oven with an infrared heating element and a control sensor mounted in a corner, the board being soldered can run one hundred to two hundred degrees hotter than the controller believes, because nothing in the control loop measures the object that matters.[170] A convection oven is more forgiving in this respect: the sensor sits in circulated air that closely matches what the board experiences, which suppresses both hot spots and the divergence between the reading and the reality.[170]
Relocating the sensing point to where temperature actually matters can transform control performance. Replacing an air conditioner’s built-in control with a sensor placed at the workbench rather than at the unit brought the hysteresis band down to two or three tenths of a degree, a band that becomes harder to hold as the controlled space grows larger.[643]
Characterising a circuit board thermally calls for several channels of accurate measurement with probes small enough to sit directly on individual integrated circuits. Small RTDs placed right on ICs are what turn a vague overheating complaint into an identified source, sink and thermal path.[425]
Failure modes and mitigation
Self-heating and co-located heat sources
A device that transmits also heats itself, and in one precision thermostat the radio’s own dissipation was sufficient to corrupt the temperature reading. The remedy was scheduling rather than filtering: the wireless stack and the measurement were synchronised so that the system knew exactly when the radio had been active.[526]
Where a sensor is exposed to a heat source that is not the quantity of interest, the reading can be weighted down rather than discarded. In one installation, several thermostats were averaged while the weight of the unit mounted near a boiler was progressively reduced as that room warmed, falling to zero above about eighty-five degrees Fahrenheit; this removed the disturbance without losing the sensor entirely.[657]
Threshold chattering
A single switching threshold applied to a temperature reading chatters on the sensor’s own noise, toggling repeatedly as the value crosses back and forth. Two thresholds — one to switch on and a separate, lower one to switch off — stop the output cycling on noise alone.[696]
Security
Firmware in a device as unremarkable as a temperature sensor constitutes an attack surface. Faking an overheat is a plausible way to make a system shut itself down, and the security attention applied to application software has largely not reached this layer.[590]
Selection and identification
Temperature sensing is a commodity right up to the moment it is not. Filtering a distributor catalogue for a generic part returns thousands of results, so choosing something familiar is the pragmatic answer — but a requirement as ordinary-sounding as a 200 °C operating limit narrows the field to almost nothing.[645] The market moves quickly: a smaller, cheaper part can appear within months of a previous selection.[153]
Identification of salvaged parts presents its own difficulty. A three-pin temperature sensor and a small-signal diode in the same TO-92-style package are indistinguishable by inspection once the markings are gone, so identifying such parts is a curve-tracing problem rather than a visual one.[87]
System integration
Battery systems rely heavily on distributed temperature sensing. One battery management design instrumented every group of cells for temperature alongside per-cell voltage measured to a few millivolts, with the shunt current returning on its own separate path specifically so that balancing current could not corrupt the voltage measurement.[112] Battery pack connectors likewise carry more than power: a thermistor for temperature and an identification pin sit alongside the cell connections, so the charger can establish both what the pack is and whether it is safe to charge before delivering any current.[662]
Systems that cannot be serviced after deployment carry dense instrumentation. A small satellite carried on the order of sixty temperature sensors — including sensors already integrated inside devices — alongside a converter on every voltage rail and current measurement on every subsystem, because nothing aboard can be probed after launch.[220] Sensor counts are not unlimited, however: the number of channels a system can carry is a link-budget calculation done on paper from packet size, protocol headers and bytes per sample. Doing that arithmetic early is what lets an engineer state, with a reason attached, that a requested complement of two hundred temperature sensors is not going to happen.[584]
A separate management processor whose only job is the health of the hardware can read temperature sensors, collect serial numbers and control power while the main processor is off — which is exactly when a thermal problem most needs to be visible.[357] A limitation of the long-standing management standard in this space is that it reports that temperature sensors exist and what they read, but not which components each one is measuring; at scale, that gap is what stops readings from being actionable, because a number without a subject cannot drive a decision.[357]
Temperature sensing also participates in sensor fusion. A gas volume corrector combines flow, pressure and temperature through the gas law into a standard volume, and it is that computed figure rather than any single measurement on which the customer is billed.[635]
Communications and data handling
A temperature reading without context is nearly useless: whatever transports the number must also carry where it was taken, because the value alone cannot be acted on by anyone who was not present when the sensor was installed.[205]
Because temperature changes slowly, transmission strategies can be frugal. Sending a periodic keep-alive and otherwise transmitting only when the value changes means a flat line indicates both that the device is alive and that the temperature is steady — the same information at a fraction of the traffic and energy.[376] Equivalently, a sensor repeating the same value is spending its energy budget saying nothing; computing the difference on the device and sending only the change lets the receiver reconstruct the series from a much smaller stream.[370] Temperature and humidity tolerate lost samples, which makes them a genuinely forgiving class of system and permits architectural choices such as fire-and-forget transmission that a system with real-time stakes could never accept.[272]
A lightweight publish-and-subscribe protocol fits sensing naturally: the sensor announces its reading without knowing who is listening, and displays or loggers subscribe independently, decoupling the addition of a consumer from any change to the device.[203] Industrial transmitters costing hundreds to thousands of dollars demonstrate a related property worth copying into cheap embedded sensors: interrogated with one or two commands, such a device states what it is, the units it reports in and the current value, making it self-describing.[458]