TECHNICAL INFORMATION SHEET NUMBER 364
Snubbers
12/2/98
ISSUED BY THE APPLICATIONS ENGINEERING DEPARTMENT
EUROTHERM CONTROLS INC / BARBER-COLMAN CO. Page 1 of 2741-F
MILLER DRIVE, LEESBURG, VIRGINIA 20175-8993 USATECHNICAL SUPPORT:
703-669-1313 SALES: 703-669-1312 FAX: 703-669-1300 FAX-ON-DEMAND:
703-669-1310WWW:HTTP://WWW.EUROTHERM.COM
E-MAIL:[email protected]
[email protected]
Snubber is the term used to describe a device consisting of a
resistor and a capacitor connected in series.It is intended to be
connected in parallel with the contacts of a switch or a relay to
reduce arcing.
Although the original reason for using snubbers was to prolong
the life of contacts by reducing arcing in mechanicalswitches and
relays, snubbers now play several important roles in a variety of
control circuits.
Benefits of using snubbers with control relays
Note that the use of snubbers is particularly beneficial in
circuits controlling inductive loads such as the coils
ofelectro-mechanical contactors. The snubber may be connected
across the contacts of the control relay, or (better) inparallel
with the inductive load.
1. The snubber prolongs the life of the contacts by reducing
arcing.
2. The snubber helps to eliminate equipment malfunctions due to
interference by reducing electro-magneticnoise.
3. The snubber diverts the inductive surge current away from the
relay contacts (which may be close to sensitivecircuits).
4. The snubber reduces voltage stresses on insulation.
Important: Snubbers must never be connected across relay
contacts driving high impedance loads (such as solid-state relay
inputs) or on contacts used for isolation. A snubber will pass a
small alternating current, even though therelay contact is open.
This current is sufficient to turn on some solid-state relays, or
to cause electric shock topersonnel.In these cases it is
permissible to connect a snubber across the load.
Snubbers connected across SCRs, triacs and solid-state
relays
The use of snubbers is highly recommended in all circuits
controlled by solid-state devices, and is absolutelyessential in
circuits with solid-state devices controlling inductive loads. Most
solid-state relays and SCR powercontrollers have built-in
snubbers.
When used with solid-state devices, snubbers should be connected
across the SCRs or solid-state relays, not inparallel with the
load.
1. The snubber helps to prevent spurious firing of SCRs and
solid-state relays due to the rapid change of voltageproduced by an
inductive load when the SCR turns off.
2. The snubber helps to prevent spurious firing of SCRs due to
voltage spikes and notches on the supply lines.
3. The snubber enables pulse-fired SCRs to latch when feeding
inductive loads.
GWH 1998 December 1 Copyright Eurotherm Controls Inc 1998. All
rights reserved
