FAQ No. FAQ02182
Question
What causes contact welding and what can be
done about it?
Answer
Causes:
Overload that does not match the contact
switching capacity
Inrush current larger than rating
Breaking current larger than rating
Switching frequency in excess of allowable
operating frequency
Usage in a location subject to continuous
vibration
Countermeasures:
A protective circuit is required so that
loads such as relays, motors, incandescent bulbs, and solenoids can be
used properly.
Cause:
Countermeasures:
Typical Examples of
Contact Protective Circuits (Surge Killers)
|
Circuit example |
Applicable
current |
Feature |
Element selection |
|
AC |
DC | |
CR
circuit |
 |
*
Conditional |
Applicable |
* When AC is switched, the load impedance must be lower than the C and R impedance. |
The following is a general guide for C and R:
C: 0.5 to 1 μF per switching current (1 A)
R: 0.5 to 1 Ω per switching voltage (1 V) The values may change according to the characteristics of the load.
The capacitor suppresses the spark discharge of current when the contacts are open. The resistor limits the inrush current when the contacts are closed again.
Consider these roles of the capacitor and resistor and determine the ideal capacitance and resistance values from
testing.
Generally, use a capacitor with a dielectric
strength between 200 and 300 V. Use an AC capacitor for an AC circuit i.e.,
a capacitor that has no polarity. | |
 |
Applicable |
Applicable |
The operating time will increase if the load is a relay or solenoid.
It is effective to connect the CR circuit in parallel to the load when the power supply voltage is 24 or 48 V and in parallel to the contacts when the power supply voltage is 100 to 200 V. | |
Diode method |
 |
Not
applicable |
Applicable |
Energy stored in the coil is changed into current by the diode connected in parallel to the load. Then the current flowing to the coil is consumed and Joule heat is generated by the resistance of the inductive load. The reset time delay in this method is longer than that of the CR method. |
The diode must withstand a peak inverse voltage 10 times higher than the circuit voltage and a forward current as high as or higher than the load current. | |
Diode and Zener diode method |
 |
Not
applicable |
Applicable |
This method will be effective if the reset time delay caused by the diode method is too long. |
If
a suitable Zener voltage is not used, the load may fail to operate depending
on the environment. Use a Zener diode with a Zener voltage that is about 1.2
times the power supply voltage. | |
Varistor method |
 |
Applicable |
Applicable |
This method makes use of constant-voltage characteristic of the varistor so that no high-voltage is imposed on the contacts. This method causes a reset time delay more or less. It is effective to connect varistor in parallel to the load when the supply voltage is 24 to 48 V and in parallel to the contacts when the supply voltage is 100 to 200 V. |
--- |
Do not apply contact protective circuit as shown below.
 |
This circuit
effectively suppresses arcs when the contacts are OFF. The capacitance
will be charged, however, when the contacts are OFF. Consequently,
when the contacts are ON again, short-circuited current from the
capacitance may cause contact weld. |
| |
This circuit
effectively suppresses arcs when the contacts are OFF. When the
contacts are ON again, however, charge current flows to the capacitor,
which may result in contact weld. |
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|
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