2-, 3-, or 4-wire connection for resistors
When measuring resistance, particularly for Pt-100 sensors, one uses, according to accuracy requirements and economic constraints 2-, 3- or 4 wire technology. The advantages and disadvantages of the technologies are discussed below.
The obvious advantage of 2-wire connections is their simple wiring. However, this technology can easily lead to large errors: an additional resistance of one Ohm (lead resistance, junction resistance at solder joints, connectors, etc.) can cause an error of 2.5 degrees in a Pt-100 transducer. The error acts as an offset (zero point shift) and can, in principle, be trimmed. However, it must be noted that these resistances are not constant (due to temperature, aging), particularly in the case of junction resistances at points of contact (connectors, screw terminals) and even the lead resistance (copper) can alter by approx. 0.4%/K. These variations can lead to significant errors.
In practice, 2-wire technology is only recommended in cases where great accuracy is not required or where the connections are short with very low junction resistance (e.g., good solder joints). When mounting a module in the Type B DIN43729 head of a sensor, these requirements are usually well fulfilled.
In a 3-wire connection, one wire (the sensor wire) measures the lead and junction resistance and with the help of a suitable electronic circuit, the influence of the additional resistances can be largely eliminated. However, this only applies under a certain condition which is often not observed: the resistances of the three wires and of the junction points must be matched exactly. A difference of only 0.39 Ohms results in an error of 1 degree (Pt-100). Even in the case of exact matching (which is uneconomical) the problem of variation caused by temperature and aging exists. In this respect, the 3-wire technology is hardly better than the two wire variation. Compensation of lead resistance is usually achieved by unequal amplification (+2, -1) of the positive and negative signal inputs. Due to this asymmetry, the three wire technology is also more sensitive to interference voltages (thermo-voltages, electromagnetic interference) than the 2- or 4-wire versions.
In practice, the 3-wire connection is usually more unstable than either the 2- or 4-wire measurement. The measured offset drift in a 3-wire system can often be attributed to the thermo-voltage at the connection.
With this type of connection, 2 wires are used to deliver and return the constant measurement current. Two additional wires (sensor wires) make a high ohmic measurement of the voltage drop across the resistance sensor. The influence of the lead and junction resistance is all but eliminated (approx. error 0.002-0.004%/Ohm). These resistances may also have completely different values and may alter with time without affecting the output signal. Although this connection system requires more wiring, it is generally still the most economical since it requires no adjustment.
Connection Possibilities for RTM Modules
All Soclair Electronic Pt-100 (or resistance) modules are available for 3- or 2-/4-wire connection. Generally, the 2-wire version is realised in a 4-wire module which must be short circuited as close to the module as possible (see diagram).
In the programmable modules (e.g., RTM 90), the connection type (2-/4-wire, 3-wire) is selected via the mini switches.
In modules with fixed measurement ranges, the type of connection cannot be changed (except for 2-wire connection on a 4-wire module).
|Point of connection for the sensor wire (F) as close as possible to the resistor.|