Brochure

20
Technical Information – Relays
Dark Current in OFF Time
A circuit that produces a control output as soon as the relay
operates must be carefully designed. In the example on the left,
electrode dark current flows as shown when the relay operates.
When dark current flows into the relay coil, the relay’s resistivity to
shock and vibration may degrade.
Overcoming Beat in DC Relays
When using AC power to generate power for operating a DC relay,
the use of half-wave rectification causes the formation of a
pulsating current. Therefore, when the capacitance of the
smoothing capacitor C is low, the relay generates a beat.
However, when a bridge rectification circuit is used, the frequency
of the pulsating current doubles, generating no beat even when a
smoothing capacitor C is not provided. The bridge rectification
circuit can provide a higher rectification efficiency to increase the
contact attraction, which is desirable in terms of prolonging the
service life of the contact.
Voltage Considerations for AC Relays
For stable relay operation, a voltage +10% to -20% of the rated
voltage should be applied to the relay. The voltage applied to the
relay must be a sine wave. When a commercial power source is
used, there should be no problem. However, if an AC stabilized
power source is used, either beat or abnormal heating may occur,
depending on the wave distortion of the power source. A shading
coil is used to suppress beat in an AC current coil, but wave
distortion defeats this function.
When a motor, solenoid, transformer, or other device is connected
to the same power line source as the relay controller, and any of
these devices causes a drop in the line voltage, the relay may
vibrate, damaging the contact. This commonly occurs when a
small transformer is added to the line, when the transformer is too
small, when long wiring is used, or when thin wiring is used in the
customer’s premises. Be aware of this phenomenon, as well as
normal voltage fluctuations. Should this problem occur, check the
change in voltage with a synchroscope or the like, and take
appropriate countermeasures. Effective countermeasures include
replacing the relay with a special relay suited to the
circumstances, or use of a DC circuit and inclusion of a capacitor
to compensate for the voltage change, as shown in the following
circuit diagram.
Voltage change compensation circuit
incorporating a capacitor
Incorrect
Correct
TE
Io
Incorrect
Correct
100 VAC
50/60 Hz
100 VAC
50/60 Hz
C
C
5µF
SW
SW
Switch
24 VDC
100 VAC
C
Omron A5 Catalogue 2007 1-282 11/9/06 10:16 am Page 20