AXICOM Signal and High Frequency Relays (RF Switches)

APPLICATION NOTE #5

Latching Relays
1. Latching relays in general
Latching relays are commonly used in low power consumption or high temperature applications where applying
coil power for a long time cannot be afforded due to power consumption or self heating of the coil. Instead of acontinuous voltage applied to the coil, they are operated with short voltage pulses instead. Latching relays change
contact position when a coil voltage is applied and remain in that position even if the voltage is disconnected. (it iscommon to use the term SET for operating a latching relay). To reset a latching relay another voltage pulse needs tobe applied. There are two basic designs for latching relays on the market:

1.1 Mechanical latching relays
They have features like mechanical locks or gears inside keeping the contacts in the switching position. Mechanicallatching relays are not very common in recent telecom and signal relay designs and can be disregarded.

1.2 Magnetical latching relays
This design is being used for telecommunication and signal
relays. The relays are polarized and use an internal magnet

Non latching

instead of mechanical springs to apply mechanical forces tothe relay armature when no voltage is applied to the relay
coil. For non-latching relays the magnets are magnetizedin a way that the flux, generating the drop-out force ismuch stronger then the flux on the pull-in side. To operate
the non-latching relay a voltage is needed to be appliedlatching relay 1on the coil and the current through the coil in generating
an additional flux on the pull-in side and overpowers themagnetic force on the drop-out side of the relay.
For latching relays the magnet needs to be charged
in a different way. Both sides, pull-in and dropout aremagnetized equally. To move the armature from one

latching relay 2

position to the other the magnet’s force needs to be
overpowered by the coil. Once the armature changed
positions (SET) a flux in the opposite direction is necessary
to switch the relay back (RESET). This can be done bychanging polarity on the coil (single coil latching relays) or
operating separate SET and RESET coils (dual coil latchingrelays) (Figure 1)

AXICOM /// APPLICATION NOTE

Figure 1: Coil and contact signals for latching relays

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Latching Relays

2 How to operate a latching relay
2.1 Single coil latching relays
2.1.1 Reverse voltage
This is the simplest way to operate a latching relay. Applying
positive voltage by a push button set the relay. Applying the reverse
polarity voltage by a push button reset the relay (Figure 2)

Figure 2: Simple latching relay circuit

2.1.2 “C”-circuit
A more advanced way to operate an one-coil latching relay is the“C”-circuit. A capacitor is used in the path of the relay coil.
The capacitor has to be discharged. If the voltage is applied tothe coil, only the circuit’s resistance limits the current (capacitor
works like a short circuit). When the capacitor starts to be charged,
the current decreases to almost zero (there might be a leakage
current through the capacitor). The coil voltage can be applied alltime, it is not necessary to pulse the relay. The almost zero current
is responsible for the very low power consumption of the circuit(Figure 3).

Figure 3: „C“- circuit – SET conditions

Resetting is very simple. The coil circuit needs to be opened andshortened to the coils ground potential. The capacitor will bedischarged through the coil and drives a current pulse with opposite
polarity through the coil, that resets the relay (Figure 4).

Figure 4: „C“-circuit – RESET conditions

AXICOM /// APPLICATION NOTE

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Header
TitleRelays
Latching

Relay drivers can be used in those circuits instead of a switch.
The circuit performance will be the same (Figure 5).
The circuit capacity and timing is very important to drive the relay
properly. Every relay has a certain operation time. A minimum
magnetic force is necessary at this time to move the armature. Thisforce is generated by the coil current. The capacitor has to allowthat value of current within the operation time. Otherwise the relay

Figure 5: Circuits with relay drivers

might not pull through the armature.
The capacitor needs to be fully charged, too, to allow the maximum
RESET current. During resetting, the discharge circuit should beclosed until the capacitor is fully discharged. If not, the capacitor
already has a certain resistance and there will be a lower current

Equation 1: Minimum capacitanceduring the next relay set. The relay might not set.
An example for dimensioning the capacitor is shown in (Figure6: simulation). The circuit has been simulated with a P2 relay(V23079C1111B301). The minimum value of the capacitor should be(Equation 1):
Cmin: minimum capacitance
L: coil inductance
Rcoil: coil resistance
Figure 6: Capacitor calculation

2.1.3 IC-Circuit

AXICOM /// APPLICATION NOTE

PAGE 3HeaderTitleRelaysLatching

2.2 Dual coil latching relays
Dual coil latching relays are being set and reset by two separated coils (Figure 7: Dual coil latching relay). Two
independent circuits can drive the relay.

Figure 7: Dual coil latching relay

Figure 8: Variations of relay control

The unique feature of the Axicom P2 relay is, that no specific
set or reset coil needs to be defined. In fact, both coils could
operate the relay as single coil latching relays (Figure 8).

3 Remarks
Magnetic latching relays do not have the same armature forces than the nonlatching counterparts during operation.
Therefore, most of them are more sensitive against shock and vibration and probably have lower contact ratings.
Some relays may change the armature position during transportation.

te.com© 2017 TE Connectivity. All Rights Reserved. AXICOM, TE, TE Connectivity, and the TE connectivity (logo) are trademarks of the TE Connectivity Ltd.
family of companies. Other logos, product and Company names mentioned herein may be trademarks of their respective owners.
While TE has made every reasonable effort to ensure the accuracy of the information in this brochure, TE does not guarantee that it is error-free, nor does TE make anyother representation, warranty or guarantee that the information is accurate, correct, reliable or current. TE reserves the right to make any adjustments to the information
contained herein at any time without notice. TE expressly disclaims all implied warranties regarding the information contained herein, including, but not limited to, any
implied warranties of merchantability or fitness for a particular purpose. The dimensions in this catalog are for reference purposes only and are subject to change withoutnotice. Specifications are subject to change without notice. Consult TE for the latest dimensions and design specifications.

1-1773931-5

07/2017JN

AXICOM /// APPLICATION NOTE

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