| Caution for On-Off control circuits with SCR. |
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- When SCR's are used to operate relays with half-wave rectified power
source to make release easy, almost all operations and releases could
occur at the same point in the wave form.
- In the case of heavy loads such as heater loads of temperature
controllers, the opening and closing of contacts can occur at any point
in the wave form of the control.
Thus, contact life can vary
greatly. A thorough check of the controlled equipment should be made to
determine true operation condition.
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| Connect one side of coil terminals in common. |
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Bad
example has the possibility of making a short-circuit in the direction
shown by a black arrow.
As shown in the good example, connect one side
of the coil terminals in common. |
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| Avoid these two circuits. |
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| In the event of contact
arcing, short-circuit can occur in these reversing circuits as shown by
the arrows. |
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| Circuits operating with gradual current change. |
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| In
circuits in which the operating current is not turned ON or OFF completely
but increases or decreases gradually, the operating time will be unstable
or the snap action of the contacts will be weak. Contact life may be
shortened due to arcing and the specified electrical life may be thus
shortened. |
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| It is dangerous to switch DC loads ON and OFF. |
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When DC load
is turned ON and OFF by contacts:
- DC ON-OFF capacity is less than AC ON-OFF capacity. For example, the
AC rating of 5 x 105 operations for a specific set of
contacts is 10 A at 125 V but the equivalent life for DC is 0.5 A at 125
V.
- Transposition may cause contact corrosion and pitting, reducing
service life.
- Metallic parts may corrode abnormally, reducing service life.
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| Ground line should not be switched ON and OFF. |
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| A
ground fault could cause the relay to operate improperly. |
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| Arc short-circuit that shortens life of contacts. |
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(1) Forward and reverse rotation of motor.
If the rotation of a motor
is reversed while it is running, arc is generated across contacts of the
relay, which ionizes air around the contacts, resulting in arc
short-circuit of power source circuit and its damage. Therefore, it is
necessary to extinguish arc completely, and then change over. |
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When the switch S is closed, the relay operates, and
when (3)-(4) is opened, arc is generated. At the same time (1)-(2) is
closed, so that, if arc still continues across (3)-(4), a circuit which
short circuits the power source circuit is formed between
(1)-(2)-(3)-(4), damaging the contacts to a great degree.
(2)Change over of different power sources.
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When changing over power source by the contacts of the
relay, bridge accident due to heterogeneous power sources because of
short change-over time as well as damage of contacts due to arc may
occur.
It is desirable to form an interlock circuit in the control
circuit comprising a push-button and a relay, and after turning on one
power source, turn on the other power source. |
16475
Canyon Hills Road #3