Nivotemp NT-EL
4.3 Information on the correct operation of reed contacts in Bühler level switches
Based on their construction, reed contacts are very long lasting and reliable components. Yet the following should be considered
when using them:
Life of reed switches
The life of reed switches can be up to 10
9
cycles. This is reduces by high stress and / or incorrect or the absence of protective cir-
cuits when switching inductive, capacitive or lamp loads.
It’s therefore important to ensure NEVER to exceed one or several of the maximum approved limits, even temporarily, and to
install a contact protective circuit for loads which are not purely ohmic. Using test lamps when installing the devices is also pro-
hibited, as these can temporarily allow too much current to flow, which can damage the reed contacts. In this case non-volatile
testing equipment should always be used.
Contact protective circuits for reed switches
For direct current voltage a recovery diode per figure A must be connected parallel to the contact.
For alternating current voltage an RC circuit per Figure B and Table 1 must be connected parallel to the contact.
A
B
Load
Contact
Recovery diode
Load
Contact
RC circuit
Load in VA
10
25
50
Voltage at contact V
R/Ohm
C/µF
R/Ohm
C/µF
R/Ohm
C/µF
24
22
0.022
1
0.1
1
0.47
60
120
0.0047
22
0.022
1
0.1
110
470
0.001
120
0.0047
22
0.022
230
470
0.001
470
0.001
120
0.0047
Please note the max. voltage/load ratings of the respective level contacts!
Voltages and currents
All Bühler level contacts with reed switch can switch minimal Switching voltages of 10 µV and minimal switching currents of
1 µA.
The maximum values specified for the respective contact types apply.
Level contact with reed switches can therefore be used for SPS applications as well as for high loads (within the maximum lim-
its) without hesitation.
Contact material
All reed switches in Bühler level contacts use rhodium as the contact material for the actual contact areas.
Magnetic fields
Avoid external magnetic fields, including from electric motors. These can interfere with the function of the reed switches.
Mechanical loads
Do not expose the level switch to strong blows or bending.
8
Bühler Technologies GmbH
BE100025 ◦ 08/2020
Summary of Contents for Nivotemp NT-EL
Page 18: ......
