Use coil impedance to detect position of latching relay

Latching relays, which are switched back and forth by short current pulses applied to either of two relay coils, retain their last contact position after the coil is de-energized. Detecting the actual contact position typically requires an auxiliary contact. Many relays, such as high-voltage (HV) types, don’t provide additional contacts for determining the position.

The circuit shown detects the actual contact position of a latching Kilovac HV-relay from the small difference between the inductances of the two relay coils (see the figure), which stems from the different magnetic paths inside the relay mechanism. The circuit produces a continuous output signal, valid shortly after the relay coils are de-energized, even after power (V_CC = +5 V) was interrupted.

Op amp OP1A generates a square wave (f @ 1 kHz), which pulses the two relay coils through diode D1 and resistors R1 and R2. The induced voltage peaks charge capacitors C1 and C2 through Schottky diodes D4 and D5. The voltage difference ΔV, mainly dc, is amplified by OP1B, which works as comparator. Finally, the LED signals the contact position. The output of OP1B, VOUT, also can drive CMOS logic inputs.

D1, R3, and R4 protect the circuit against the nominal relay drive pulses (amplitude V_DD = 26 V) through transistors Q1 and Q2. Capacitors C1 and C2 are slowly discharged through resistors R7 and R8. Resistors R5 and R6, along with potentiometer P1, are used for offset adjustment, which corrects for the asymmetries of the relay coils and diodes D4 and D5. The setting of P1 is correct when the voltage DV is approximately symmetric around 0 V for the two contact positions (e.g., with the prototype after driving coil X, DV = +17.0 ΔV was measured, and DV = −17.1 ΔV after driving coil Y).

The bodies of diodes D4 and D5 should be in thermal contact to compensate for their thermal drifts. The circuit draws approximately 2.5 mA/8.8 mA (LED off/on). Since other relays might use different latching principles, experimenting and modified dimensioning of the circuit might be required.