Thermocouple Symbol
Definition: The Thermocouple symbol represents a temperature sensor consisting of two dissimilar metal wires joined at a measurement junction, drawn in circuit diagrams as two diagonal lines meeting at a point (the hot junction) with the positive leg labelled '+' and the negative leg labelled '−', generating a small EMF (millivolts) proportional to the temperature difference between the hot and cold junctions, as standardised in IEC 60617 and IEC 60584.
Also known as: TC, thermoelectric sensor, Type K thermocouple, Type J thermocouple, thermoelectric couple, Seebeck sensor.
What the Thermocouple symbol means
The Thermocouple symbol denotes a self-generating temperature transducer that exploits the Seebeck effect: when two dissimilar metals are joined at one end (the hot or measurement junction) and maintained at a different temperature at the other end (the cold or reference junction), a voltage proportional to the temperature difference is produced. In schematics the symbol flags the point where thermocouple leads are connected to the measurement circuit.
Thermocouples are the most widely-used industrial temperature sensors, capable of measuring from −200 °C to over +2300 °C depending on the wire alloy type (Type K, J, T, E, N, S, R, B). The schematic symbol uses '+' for the positive lead (higher Seebeck voltage metal) and '−' for the negative lead, matching IEC 60584 colour and polarity conventions.
How to identify the Thermocouple symbol
The thermocouple symbol is drawn as two diagonal lines converging to a point (the measurement junction), forming a V or chevron shape. The left line is typically labelled '+' (positive/hot leg) at pin pos, and the right line is labelled '−' (negative/cold leg) at pin neg. Some schematics enclose the junction point in a small circle. The symbol may be labelled TC, TC1, or include the type identifier (e.g. K, J). In IEC 60617, the thermocouple may also be represented by a dedicated symbol showing two unlike metals joined at a point with the qualifying letter for the alloy type.
Function in a circuit
A thermocouple generates a small thermoelectric EMF (typically 1–80 µV/°C, depending on type) when its measurement junction is at a different temperature to its reference (cold) junction. This voltage is measured by a thermocouple interface circuit — typically a dedicated IC such as the MAX31855 or AD8495 — that applies cold-junction compensation (CJC) to convert the differential voltage into an absolute temperature reading. Thermocouples are passive (no supply voltage required at the sensing junction), rugged, fast-responding, and usable at extreme temperatures where semiconductor sensors fail.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 defines the graphical symbol for a thermocouple in schematics. IEC 60584-1 specifies thermoelectric EMF characteristics and tolerances for standard thermocouple types (K, J, T, E, N, S, R, B). IEC 60584-2 defines tolerances. IEC 60584-3 defines extension and compensating cable requirements. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 defines the thermocouple symbol for North American schematics. ANSI MC96.1 (now superseded and aligned with IEC 60584) defined thermocouple calibrations for the US market. The designator TC is standard in both conventions. |
| Key difference | The IEC and ANSI thermocouple symbols are essentially identical in form — two converging lines meeting at a junction point. The main difference is in labelling conventions for the lead polarity and in the colour codes for thermocouple extension wire: IEC 60584-3 uses a yellow overall sheath for Type K, while ANSI/ANSI MC96.1 uses a yellow-positive / red-negative convention. |
Terminals / pins
| Pin | Name |
|---|---|
| pos | + |
| neg | - |
Typical values
Output EMF: Type K (Chromel-Alumel): ~41 µV/°C, range −200 °C to +1372 °C. Type J (Iron-Constantan): ~52 µV/°C, range −210 °C to +1200 °C. Type T (Copper-Constantan): ~43 µV/°C, range −270 °C to +400 °C. Type E (Chromel-Constantan): ~68 µV/°C, range −270 °C to +1000 °C (highest sensitivity of standard types). Type S/R (Pt-PtRh): ~10 µV/°C, range 0 to +1768 °C. Type B: range +200 to +1820 °C.
Where the Thermocouple symbol is used
- Industrial furnace and kiln temperature control, where temperatures exceed the range of semiconductor sensors
- Gas turbine and jet engine exhaust temperature monitoring using Type K or Type S thermocouples
- Automotive exhaust gas temperature (EGT) sensing in turbo and diesel engine management
- Laboratory and scientific instruments requiring traceable temperature measurement per IEC 60584
- Plastic injection moulding machines and extruders for barrel and mould temperature control
- Household gas appliance flame sensors (thermocouple as safety cut-off: no flame = no EMF = valve closes)
Example
In an industrial oven controller, a Type K thermocouple is inserted into the oven chamber; its '+' lead (yellow, Chromel) and '−' lead (red, Alumel) connect to the thermocouple input terminals of a MAX31855 SPI interface IC. The IC performs cold-junction compensation using its internal temperature sensor and outputs the oven temperature over SPI to a microcontroller that drives the heater relay.
Key facts
- The thermocouple symbol is two converging lines meeting at a measurement-junction point, with the positive terminal labelled '+' (pin pos) and the negative terminal labelled '−' (pin neg); the standard designator is TC.
- Thermocouples generate a Seebeck voltage (EMF) proportional to the temperature difference between the hot junction and the cold (reference) junction — they require cold-junction compensation for absolute temperature measurement.
- Type K (Chromel-Alumel) is the most widely used industrial thermocouple, producing approximately 41 µV/°C over a range of −200 °C to +1372 °C.
- IEC 60584-1 defines the EMF-temperature relationships and tolerances for all standard thermocouple types (K, J, T, E, N, S, R, B); IEC 60617 governs the schematic symbol.
- Thermocouples are passive sensors — they generate their own voltage at the measurement junction and require no supply voltage, unlike resistance-based sensors (RTDs, thermistors).
- The polarity of thermocouple leads is critical: reversing the '+' and '−' terminals causes the measured temperature to read below (not above) the reference junction temperature.
- Extension cables for thermocouples must be made of the same alloy pair (or a specified compensating alloy) as the thermocouple itself to avoid introducing additional parasitic junctions that corrupt the reading.
- Thermocouple output voltages are very small (millivolts), requiring high-input-impedance, low-noise amplifier circuits or dedicated thermocouple interface ICs for accurate measurement.
Diagrams that use this symbol
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Frequently asked questions
What does the thermocouple symbol look like in a circuit diagram?
The thermocouple symbol is drawn as two diagonal lines converging to a point, forming a V or chevron shape representing the measurement junction where the two dissimilar metal wires are joined. The positive wire ('+', pin pos) and negative wire ('−', pin neg) form the two legs of the V. The symbol may be labelled TC, TC1, or annotated with the alloy type (K, J, T).
What does the thermocouple symbol mean in a schematic?
The thermocouple symbol indicates that the circuit includes a thermoelectric temperature sensor. It marks the point where the two dissimilar-metal leads connect to the measurement circuit. The symbol tells the reader that a small voltage proportional to the junction temperature (relative to the cold reference junction) will appear between the '+' and '−' terminals.
What is the difference between a Type K and Type J thermocouple?
Type K (Chromel-Alumel) is the most common industrial thermocouple, covering −200 °C to +1372 °C with a sensitivity of ~41 µV/°C. Type J (Iron-Constantan) is slightly more sensitive (~52 µV/°C) but has a narrower range (−210 °C to +1200 °C) and is more prone to oxidation above 700 °C. Both types use the same schematic symbol; the type is noted as text on the symbol or in the parts list.
Do thermocouples need a power supply?
No. Thermocouples are passive sensors — they generate their own thermoelectric EMF at the measurement junction without any external power supply. However, the measurement circuit (amplifier, interface IC, or data acquisition system) that reads and converts the millivolt signal does require power.
What is cold-junction compensation in thermocouple circuits?
Cold-junction compensation (CJC) corrects for the fact that a thermocouple measures the temperature difference between its hot junction and its cold (reference) junction, which is at the measurement instrument's terminals. If the cold junction is not at 0 °C, the raw EMF reading is offset. Modern thermocouple interface ICs (such as the MAX31855) measure the cold-junction temperature using an integrated sensor and add it to the thermocouple reading to give the absolute hot-junction temperature.
What is the designator letter for a thermocouple?
The standard designator for a thermocouple in circuit diagrams is TC (thermocouple). Multiple thermocouples in the same schematic are numbered TC1, TC2, TC3, etc. This convention is consistent in both IEC and ANSI/IEEE 315 schematics.
What standard governs thermocouple specifications?
IEC 60584-1 defines the EMF-versus-temperature relationships and standard tolerances for all common thermocouple types (K, J, T, E, N, S, R, B). The schematic symbol is defined in IEC 60617. In North America, ANSI MC96.1 historically governed thermocouple calibrations, but US practice has largely aligned with IEC 60584 since the 1990s.
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