Source: Thermal Control Circle
Part I
If the instrument person engaged in automatic control work is not clear about passive signals, active signals, passive contacts (dry contacts), and active contacts (wet contacts), they will inevitably be confused. This article is a dry article that introduces passive signals, active signals, passive contacts, and active contacts, and I hope it will be helpful to you.
(1) Passive signals and active signals (analog)
(1) Definition of passive signal and active signal
For current signals, if the device has an independent working power line, the signal output (e.g., 4-20mA) provided by the device is an active signal, and if the device itself does not have an independent working power supply, the signal it provides is a passive signal. The output signal of the three-wire instrument and the four-wire instrument is an active signal, and the output of the two-wire instrument is a passive signal.
(2) How to collect passive signals and active signals
(1) For the acquisition of active signals, it is very simple, first of all, it is necessary to connect the power supply of the signal source equipment (usually there are two kinds of three-wire system and four-wire system) to make the equipment work normally, and secondly, the positive and negative ends of the input interface of the acquisition equipment are respectively connected to the positive end and negative end of the signal source equipment.
(2) For the acquisition of passive signals, a DC 24V power supply is first needed (because the two-wire instrument that provides passive signals does not have an independent working power supply, when it needs to output 4-20mA signals, its working voltage needs to be provided by external equipment.) This DC24V power supply can be an independent power supply, or it can be provided by the power distribution function of passive signal acquisition equipment). A correct circuit should be formed between the DC24V power supply and the passive signal acquisition equipment: the positive pole of the DC24V power supply is connected to the positive pole of the 4-20mA output of the two-wire instrument, the negative pole of the 4-20mA output of the two-wire instrument is connected to the positive pole of the 4-20mA signal input terminal of the acquisition device, and the negative pole of the 4-20mA signal input terminal of the acquisition equipment is connected to the negative pole of the DC24V power supply, so that a complete loop is formed.
(2) Active contact and passive contact (switch)
(1) Definition of passive contact
Passive contact is commonly known as dry contact or passive switch, dry contact is a kind of electrical switch, with two states of closed and open, there is no polarity between the two contacts, and it can be interchanged.
(2) Common passive contact signals
(1) All kinds of switches, such as: limit switches, travel switches, foot switches, rotary switches, temperature switches, liquid level switches, etc.;
(2) Various buttons;
(3) The output of various sensors, such as: sensors in environmental dynamic monitoring: water immersion sensors, fire alarm sensors, glass breakage, vibration, smoke and condensation sensors, relay and reed switch output.
(3) Definition of active contact
The active contact is commonly known as the wet contact or the active switch, which has two states: electric and non-electric, and there is polarity between the two contacts, which cannot be reversed.
(4) The most common active contact signal
(1) Connect the above dry contact signal to the power supply, and then use the other pole of the power supply as the output, which is the active node signal. The voltage range of wet connection commonly used in industrial control is DC0-30V, and the standard is DC24V, and the voltage range of wet connection in the secondary circuit of electrical equipment is AC110-380V, and AC220V or AC380V wet contact is commonly used.
(2) It is not impossible to use the TTL level output as a wet contact; under normal circumstances, the TTL level needs to be buffered output, such as: 7407, 245, 244, etc., to form a loop with VCC, etc., 244, 245 can also form a loop with GND, in order to drive the distant optocoupler.
(3) The collector output and VCC of the NPN transistor.
(4) Collector output and VCC of the Darlington tube.
(5) The output of the infrared reflex sensor and the through-beam sensor.
(5) Advantages of passive contact
In the field of industrial control, the number of passive contacts (dry contacts) is much more than that of active contacts due to the lack of polarity limitations, and passive contacts have the following advantages:
(1) Casual access, reduce project costs and engineering personnel requirements, and improve project speed.
(2) The number of dry contact switches is large.
(3) The wire connecting the dry contact will not damage the local control equipment or the distant equipment even if it is short-circuited for a long time.
(4) Easy access and easy unification of interfaces.
Part II
1. Definition of passive node: passive switch, with two states of closed and disconnected; there is no polarity between the two nodes, which can be interchanged; common passive node signals are: (1) various switches such as: limit switches, travel switches, foot switches, rotary switches, temperature switches, liquid level switches, etc.; (2) various buttons; (3) the output of various sensors, such as: sensors in environmental dynamic monitoring: water immersion sensors, fire alarm sensors, glass breakage, vibration, smoke and condensation sensors; (4) relays, the output of the reed switch;
2. The definition of active node is: active switch, with electric and non-electric two states; there is polarity between the two contacts, can not be reversed; the most common active node signal is: (1) the above dry node signal, connected to the power supply, and then with the other pole of the power supply, as the output, is the active node signal; in industrial control, the voltage range of the commonly used wet node is DC0~30V, the standard is DC24V;AC110~220V output can also be an active node, although this is relatively rare.
Now the field instrument and the secondary instrument are connected if the two-wire signal is adopted, which is generally 4-20mA, and the signal is divided into active and passive two, active: 4-20mA has a field instrument to provide a signal with voltage
Passive: The secondary instrument needs to provide power distribution (DC24V) and the field instrument in series to form a two-wire 4-20mA circuit in order to work normally.
The maximum current of the two-wire instrument loop is less than 25mA, and the voltage is fixed DC24V, so the power is also within this range, if the power consumption of the field instrument is greater than this, the two-wire system can not provide this energy, so only the power supply is used separately, and the signal is output separately, that is, the four-wire signal mode.
Active and passive are relative to transmitters, with transmitters providing 24V active (four-wire) and transmitters not providing 24V being passive (two-wire).
Active means that all kinds of transmitters or signal sources and other equipment have a set of power supply loops to supply power to it. Directly output a set of 4~20mA positive and negative signal loops for direct measurement to the acquisition equipment.
Passive means that all kinds of transmitters or signal sources and other equipment do not have a power supply loop to supply it with power. There must be a collection device to supply it with working power before it can output a set of 4~20mA positive and negative signal loops for direct measurement by the acquisition equipment.