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| Contact
Protection Requirements |
| When switching inductive loads such as relays,
solenoids and transformers, reed switch contacts require protection
in order to insure long dependable life. When current is interrupted,
the inductance or electrical inertia of the load generates
a large high frequency voltage, which spreads across the switch
contacts. If the voltage is large enough, it can break down
the medium in the gap between them, making a conductive path.
This phenomenon, called "arcing" is the spark you
see. Arcing can cause the contacts to burn, weld together
or stick; thus, giving unreliable performance. The purpose
of protection circuits is to prevent arcing, by shortening
this voltage through an alternate path. |
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Recommended Protection - D.C.
|
| A 1N4004 diode (or equivalent) connected cathode-to-positive,
as shown in Figure 1 (see above), is recommended. The diode
does not conduct when the load is energized, but conducts
and shorts out the generated voltage when the switch opens.
The generated voltage always acts in series with the applied
voltage |
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Recommended Protection - A.C.
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| A resistor and capacitor, connected in parallel
with the switch, as shown in Figure 2 (see above), is recommended.
The capacitor is a high impedance to 60 hertz, but is essentially
a short circuit to high frequencies of generated voltages.
Transient suppressors or varistors may also be used to
dissipate the transient and protect the switch contacts. |
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