8.3
Length of cable and supply voltage
For sensors with current output, the required supply voltage increases by the voltage
dip on the connected cable. The most significant voltage dip on the cable occurs when
the maximum current of 20 mA is flowing, caused by the resistance of the cable (R
L
).
The partial resistances of the go and return line must also be taken into account. The
sensors have been qualified up to a maximum length of cable of 30 meters according
to standard EN 61326-1. Longer lengths of cable are technically possible, but were not
evaluated taking into account EMC aspects.
Conductor resistance R
L
can be calculated using the following formula:
R=p*l/A
Cross-sectional area of TMS/TMM22 cable: A = 0.34 mm²
Specific conductor resistance of copper: ρ = 0.0171 ohm*mm²/m
In general, this also applies: Conductor resistance R = 0.0503 ohm/m
All specified values refer to the recommended cable cross-section of 0.34 mm² / AWG
22.
This results in the following exemplary conductor resistances:
1 m length of cable: R = 0.05 ohm conductor resistance
10 m length of cable: R = 0.50 ohm conductor resistance
30 m length of cable: R = 1.51 ohm conductor resistance
The conductor resistance must be considered when designing the supply voltage.
8.4
Load resistance at current output
The set load resistance essentially determines the power loss in the sensor, based on
the supply voltage. To keep the power loss low and to avoid the sensor overheating, a
load resistor appropriate for the supply voltage should be used. The following table and
corresponding diagram illustrate the relationship between supply voltage and permissi‐
ble load resistance.
The minimum and maximum load resistances specified below should always be under‐
stood as the total resistance at the output. This total resistance comprises the load
resistance and the resistance of the cable.
U
S
[V]
Minimum load resistance R
[Ω]
Maximum load resistance R
[Ω]
12
0
350
18
200
600
24
200
900
30
200
1100
CONNECTION
8
8026576/1EF6/2021-12-13 | SICK
O P E R A T I N G I N S T R U C T I O N S | TMS/TMM22
13
Subject to change without notice
