[CIRCUIT IDEAS]
DEEPAK GUPTA
Latchable Relay
Frontline Electronics, Pvt. Ltd., Salem
Armature relays are made of coils and contacts.
When the coil is energized, the induced magnetic
field moves the armature, which opens or closes the
contacts.
There are 2 types of electromechanical relays:
•
latching
•
non-latching. A non-latching relay has an
initial position of normally closed (NC) maintained
by the force of a spring or permanent magnet while
no current flows. The normally open (NO) contact
is maintained by the force of a magnetic field while
current flows through the coil. When the current
stops, the relay reverts back to its initial NC
position. A normal relay is "on" when and only when
power is applies to the coil. It is "off" then no power
is applied.
Non-latching electromechanical relays are useful
in control applications when the switch must return
to a known state if power is lost. A latching relay can
have 1 or 2 coils. Latching relays have no default
position and remain in their last position when the
drive current stops flowing. While the relays
themselves may be latching, their reset position in a
module is based on the control circuitry Latching
relays are useful in applications where power
consumption and dissipation must be limited
because, once actuated, they require no current
flow to maintain their position. A latching relay has
two coils. Whey you first get the relay it may be ON
6 4 8
9
RLY1
11
R1
2.2k
13
+12V
1
2
RESET
SET
15
C1
22uF
25V
+
-
16
A
A
K
K
D2
D1
+
-
S1
D1,D2: 1N4004
A
K
or OFF we don't know. Apply power to the on-coil
for more than 25mS will cause the relay to be ON. It
will stay ON for years with out power.
To turn off the relay apply power to the off-coil for a
short period of time and now the relay is OFF.
Sending power to the off-coil again will not change
anything.
This circuit allows an SPST momentary pushbutton
to act as a push-on push-off switch, using a DPDT
latching (bi-stable) relay. It was originally intended
to allow a single pushbutton switch on the dash of a
vintage car to provide a latched function. The relay
only draws current when it is being switched. At
other times, the only current drain on the 12V
supply is the leakage current of one 22µF
capacitor, which is very low. It works as follows.
Assume that initially the latching relay is in the reset
state, with pins 4 and 6 connected together. In this
state, C2 charges up to +12V via 2.2kO resistor
R2 while capacitor C1 remains discharged as it is
not connected to the 12V supply. If S1 is pressed,
C2 discharges via the relay’s “set” coil, diode D2
and S1. This switches the relay into its set position,
connecting pins 4 and 8. C1 then begins to charge
via R1. While S1 is being held down, the relay does
not return to the reset position because the current
supplied via R1 is insufficient for the coil to latch the
armature. As soon as S1 is released, current no
longer flows though the coil so C1 can
finish charging, ready for the next button
CON1
press.
Once the relay has switched and C1 has
finished charging, pressing S1 again
causes the relay to switch back to the reset
R2
state via the same process. The unused set
2.2k
of relay contacts can be used as an SPST or
SPDT switch. The circuit as shown has been
tested with the Jaycar SY4060 relay. It will
C2
work with other DPDT twin-coil latching
22uF
25V
relays but the resistor and capacitor values
may need to be adjusted to suit. Relays with
lower resistance coils will need larger
value capacitors and smaller value
resistors.