Johdota device guide

Glossary

Short explanations of control circuit terms. Each link takes you to the page where the topic is explained in more detail.

01Relay control

Normally open contact (NO): A contact that is open at rest and closes when the device operates. On contactors and relays its terminal numbers end in 3 and 4, for example 13–14. Contactor →
Normally closed contact (NC): A contact that is closed at rest and opens when the device operates. Its terminal numbers end in 1 and 2, for example 21–22. Contactor →
Changeover contact: A contact whose common terminal switches over from the NC side to the NO side, for example 15–16–18 on a timer relay. Auxiliary relay →
Coil A1–A2: The control terminals of a relay or contactor. When the coil is energised, the relay pulls in. Contactor →
Self-holding (latching): The contactor's own NO contact in parallel with the start button keeps the contactor on after the button is released. Self-holding circuit →
Interlocking: NC contacts of two contactors, each in the coil circuit of the other, so that they cannot energise at the same time. Reversing and interlocking →
On-delay: The contact of a timer relay changes state only after the coil has been energised for the set time. Timer relay →
Off-delay: The contact of a timer relay returns only when the set time has elapsed after the control signal ended. Timer relay →
Positive-opening contact: An NC contact that the actuator forces apart mechanically. Even a contact that has welded shut is torn open. IEC 60947-5-1 calls this direct opening action. Emergency stop button →
Reset: A separate action that acknowledges a fault or emergency stop state before the next start. Emergency stop circuit →
Control circuit and power circuit: The control circuit controls relays and contactors. The power circuit carries the current of the load, for example a motor. Contactor →
Utilisation category AC-3: Contactor rating for starting squirrel-cage induction motors and switching them off while running. Contactor →
Overload protection: Protection that trips when the current is too high for too long. For example a thermal overload relay or the thermal trip of a motor protection circuit breaker. Thermal overload relay →
Short-circuit protection: Protection that breaks a short-circuit current at once. For example fuses or the magnetic trip of a motor protection circuit breaker. Switch-fuse →
Trip class: Tells how fast overload protection trips. A class 10 device trips from cold in 4–10 seconds at 7.2 times the set current. Thermal overload relay →
Isolation: Reliably disconnecting a circuit from the mains for maintenance work, for example with a local isolator. Local isolator →
Star and delta: The two ways of connecting the windings of a motor. In star the voltage across each winding is lower than in delta. Induction motor →
Feedback loop: An NC contact of a contactor in the reset circuit of a safety relay. A contactor that has welded shut prevents the reset. Safety relay →
PNP sensor: A three-wire sensor whose output switches the positive supply to the load: brown +, blue 0 V, black output. Sensors →

02Automation

PLC (programmable logic controller): A device with push-buttons and sensors wired to its inputs, and contactors, indicator lights and valves to its outputs. The conditions that switch the outputs are written in the program. Small PLC →
Input and output (I, Q): The connections of a PLC. Inputs I1, I2 … read push-buttons and sensors, outputs Q1, Q2 … control loads. Small PLC →
Scan cycle: The PLC reads all inputs, runs the program and writes the results to the outputs. The scan repeats continuously. Small PLC →
Ladder diagram (LAD): A program drawn like a relay diagram turned on its side: contacts on the left, coil on the right. Small PLC →
Memory bit (M): An internal auxiliary relay of the program, with no terminal. Small PLC →
Relay output: A PLC output that is the NO contact of a relay with its own terminals. It can switch a different voltage from the PLC supply. Small PLC →
Transistor output: A PLC output that gives the PLC's own 24 V to the load. The output current is small. Small PLC →
RUN and STOP: The operating modes of a PLC. In RUN mode the program is running. In STOP mode the inputs are not read and the relay outputs are open. Small PLC →
4–20 mA: An analogue signal where the start of the measuring range is 4 mA and the end is 20 mA. Transmitter 4–20 mA →
Two-wire (loop-powered) transmitter: A transmitter that takes its supply from the same current loop it signals on. Transmitter to an analogue input →
Live zero: The measuring range starts at 4 mA and not at zero, so 0 mA means a fault, for example a broken wire. Transmitter 4–20 mA →
Scaling: Converting an analogue signal into a measured value. For example, on a 0–4 m range 12 mA is 2 m. Transmitter to an analogue input →
NAMUR NE 43: A recommendation that a current of 3.6 mA or less, or 21 mA or more, means a fault. Transmitter 4–20 mA →
Ramp: The drive raises or lowers the frequency steadily over a set time. Variable speed drive →
Speed reference: A voltage or current to the analogue input of the drive that sets the required speed. For example, 0–10 V is 0–50 Hz. Drive control with a PLC →
5/2 valve: A solenoid valve with five ports and two positions. It is used to control a double-acting cylinder. Solenoid valve →
Monostable and bistable: A monostable valve returns to its rest position by spring when the coil is de-energised. A bistable valve stays in its position until the other coil is energised. Solenoid valve →
Double-acting cylinder: A pneumatic cylinder that compressed air moves in both directions. Pneumatic cylinder →
Cylinder position sensor: A sensor fixed to the side of a cylinder that detects the piston magnet and tells the PLC that the cylinder is at its end position. Cylinder control →
Reed sensor: A position sensor with a contact inside that the piston magnet closes. Pneumatic cylinder →