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FAQ02804 of General Purpose Relays FAQ

FAQ No. FAQ02804

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Choosing from among CR elements, diodes, varistors, and other kinds of surge suppressor elements, which is the most effective for protecting contacts?


For a DC load, a diode is generally the most effective, and the next most effective is CR elements. For an AC load, a varistor or CR elements are the most effective.

Examples of Surge Suppressors:

ItemCircuit exampleApplicabilityFeatures and
Element selection guidelines
OK*Load impedance
must be much
smaller than the CR
circuit impedance
when using the
Relay for an AC
When the contacts
are open, current
flows to the
inductive load via CR.
Use the following as guides for C and R
C: 0.5 to 1 μF per 1 A of contact current (A)
R: 0.5 to 1 Ω per 1 V of contact voltage (V)
These values depend on various factors,
including the load characteristics and
variations in characteristics. Capacitor C
suppresses the discharge when the
contacts are opened, while the resistor R
limits the current applied when the contacts
are closed the next time. Confirm optimum
values experimentally.
Generally, use a capacitor with a dielectric
strength of 200 to 300 V. For applications in
an AC circuit, use an AC capacitor (with no
polarity). If there is any question about the
ability to cut off arcing of the contacts in
applications with high DC voltages, it may
be more effective to connect the capacitor
and resistor across the contacts, rather than
across the load. Perform testing with the
actual equipment to determine this.
OKOKThe release time of
the contacts will be
increased if the load
is a Relay or
NGOKThe electromagnetic
energy stored in the
inductive load
reaches the inductive
load as current via
the diode connected
in parallel, and is
dissipated as Joule
heat by the
resistance of the
inductive load.
This type of circuit
increases the
release time more
than the CR type.
Use a diode having a reverse breakdown
voltage of more than 10 times the circuit
voltage, and a forward current rating greater
than the load current. A diode having a
reverse breakdown voltage two or three
times that of the supply voltage can be used
in an electronic circuit where the circuit
voltage is not particularly high.
Diode +
NGOKThis circuit effectively
shortens the release
time in applications
where the release
time of a diode
circuit is too slow.
The breakdown voltage of the Zener diode
should be about the same as the supply
OKOKThis circuit prevents
a high voltage from
being applied across
the contacts by
using the constant-
voltage characteristic
of a varistor. This
circuit also
somewhat increases
the release time.
Connecting the
varistor across the
load is effective
when the supply
voltage is 24 to 48
V, and across the
contacts when the
supply voltage is 100
to 240 V.
The cutoff voltage Vc must satisfy the
following conditions. For AC, it must be
multiplied by √2.
Vc > (Supply voltage × 1.5)
If Vc is set too high, its effectiveness will be
reduced because it will fail to cut off high

Do not use a surge suppressor in the manners shown below.

This circuit arrangement is very effective for diminishing arcing at the contacts when breaking the circuit.
However, since electrical energy is stored in C (capacitor) when the contacts are open, the current from C flows into the contacts when they close. This may lead to contact welding.
This circuit arrangement is very useful for diminishing arcing at the contacts when breaking the circuit.
However, since the charging current to C flows into the contacts when they are closed, contact welding may occur.

Note:Although it is thought that switching a DC inductive load is more difficult than a resistive load, an appropriate contact protection circuit can achieve almost the same characteristics.

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