SENTROL Application notes - Alarms BC

......................................................................................................................................................................... Introduction
PROTECTING AGAINST VOLTAGE SPIKES
The Sticking Reed Switch Problem
When magnetic reed switches are wired in line with relay devices with coils, the
reverse voltage spike generated when current is removed from the relay can fuse
the switch contacts together.
Figure 1 shows that when the contact is closed, current flows through the relay coil
and magnetizes the iron core. When the contact is opened, current from the power
supply stops and the magnetism of the relay iron drops to zero instantly. The
collapsing magnetic field draws voltage into the relay coil. Since the coil is open
(open reed switch) and there is no circuit loading to limit the voltage, spikes from
the open relay can reach 500 volts or more, causing arcing across the reed contacts.
(Most of Sentrol’s reeds arc between 150 and 200 volts, well within this reverse
voltage kick.)
Repeated arcing roughens and pits the reed switch blades until they eventually
stick together mechanically. Installers often find that tapping on the switch will
cause the contacts to release (open). The “fix” is only temporary, however.
Correcting the Problem for DC Circuits
Voltage kicks or transients from DC circuits are easily controlled by adding a diode
across the relay coil, as in Figure 2. The diode has no affect on the circuit when
the current flows in the right direction. However, when current from the power
supply stops suddenly and the relay coil generates a reverse voltage, it shorts
through the diode. Note that the stripe on the diode points toward the + voltage.
The diode used should be a #1N4002, 1N4003, 1N4004 or equivalent.
... and for AC Circuits
In AC circuits, current flows through the relay coil in both directions. Using a
diode would simply short out half of every cycle. However, a transient protection
diode, called a TransZorb ® , can be used to clip off voltage spikes in either
direction as shown in Figure 2. TransZorb must be selected to fit the voltage used
in the circuit. To find the correct TransZorb voltage rating, multiply the circuit
voltage times 1.414. For example, in a 24 volt circuit a TransZorb rated at 34 volts
or more may be used (24 x 1.414 = 34).
+
Relay
+
Relay
MAGNETIC CONTACTS
D.C.
Power
Supply
-
Switch
D.C.
Power
Supply
-
Diode
TransZorb®
Switch
Magnet
Magnet
Figure 1
Figure 2
3.9