Master Catalog — Gems Sensors & Controls - X-3

REED SWITCH PROTECTION

Reed Switch Protection
APPENDIX X

The hermetically-sealed reed switch used in GEMS level switches are
extremely rugged and designed to operate reliably for many years -
2 million cycles under ideal conditions. To achieve the maximum
service life, reed switches benefit from protected electrical supply.

The dependable reed
switch is at the heart of
most level switch shown
in this catalog.

IMPORTANT:
* Don't be misled by the resistive ratings of the switches. Most applications involve
inductive loads.
* Don't be misled by the wattage ratings of loads. Low wattage loads are often high
inductive devices, making contact protection very important.

Contact Protection Requirements
When switching inductive loads such as relays, solenoids and transformers, reed switch
contacts require protection in order to ensure long, dependable life. When current
is interrupted, the inductance or electrical inertia of the load generates a large high
frequency voltage, which appears 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 shorting this voltage through an
alternate path.

Recommended Protection
D.C.
A 1N4004 diode (or equivalent) connected cathode-to-positive, as shown in Figure 1,
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.
A.C.
A resistor and capacitor, connected in parallel with the switch, as shown in Figure 2,
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.

Notes:
1. Don't be misled by low voltage ≤10V, low current ≤1MA type of loads. These loads
may require special gold plating on contact surfaces to operate reliably at these
low voltage/low current levels. For long term reliable low current switching action,
Gems 20VA switches should be operated at a minimum of 12V to assure contact
make; e.g., break through an oxide film which may form over long periods of time.

Figure 1
D.C. Contact
Protection
(Drain the Load)

(DRAIN THE LOAD)
REED
SWITCH

INDUCTIVE
LOAD

1N4004
H
Figure 2
A.C. Contact
Protection
(Protect the Switch)

2. Incandescent lamp loads are very destructive to reed switch applications. These
type of loads have a 6-10 time the normal operating current (inrush current) when
first energized. This high current level is a prime factor which decreases the life of
the switch.

N
(PROTECT THE SWITCH)

REED SWITCH

INDUCTIVE
LOAD

100 Ω, 1/4 WATT .1 µf, 600 VOLT

3. The following rating may be used for selection.
VRMS = 130 volts Energy = 30-50 joules Peak Amps = 4000-6000

X-3

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Appendix / p3of13 / 10-JUN-14
http://www.GemsSensors.com
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