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Technical Note Using the 770MAX Relays
to Drive Inductive Loads
THORNTON
Leading Pure Water Analytics
Technical Note
Technical Note TN-0117
When the 770MAX Relay outputs are used to drive some types of inductive loads, protection should be
provided for the relay contacts.
Opening the relay contact to an inductive load can cause the stored energy in the inductor to create a
counter EMF (electro-magnetic force). This voltage across the open relay contacts can cause arcing
which can cause contact wear as well as possible process upsets within the 770MAX meter. The use of
an RC Snubber will provide the protection needed to the relay circuit.
Mettler-Toledo Thornton Snubber Kit, part number 58 079 014 provides the components needed for pro-
tecting four relay circuits. Each kit includes four RC Snubber networks(Reference RC Snubber network
part number is Quencharc 104MO6QC100 from CDE Cornell Dublier or ITW Paktron). One RC Snubbers
network is required for each 770MAX relay circuit utilized.The kit also includes wire insulation used to
cover the leads from the Snubber to eliminate the risk of electric shock.
RC Snubber Installation:
1.
Place supplied insulation tubing over lead wires on RC Snubber network
2.
Snubber leads are to be connected in parallel with the inductive load contacts as the wiring diagram
below demonstrates.
INDUCTIVE
LOAD
RC SNUBBER
AC
POWER
C
NC
NO
770MAX RELAY
CONTACTS Technical Note
Mettler-Toledo Thornton, Inc.
36 Middlesex Turnpike, Bedford, MA 01730 USA
Phone: +1-781-301-8600 Fax: +1-781-301-8701
Toll-Free: 1-800-510-PURE
info@thorntoninc.com
Customer/Technical Service
Phone: +1-781-301-8690
Toll-Free: 1-800-642-4418
Customer Service Fax: +1-781-271-0214
Technical Service Fax: +1-781-271-0675
Subject to Technical Changes
© Mettler-Toledo Thornton, Inc.
www.thorntoninc.com
Visit for more information
For further assistance or more information regarding the use and installation of these components, please
contact Thornton Technical Service at +1-781-301-8690 or 800-642-4418.