Nivotemp NT-M
4.2 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 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.
8 Bühler Technologies GmbH BE100021 ◦ 03/2017
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