Dual-coil relay driver uses only two MOSFETs
Latching relays change their states when you apply a short voltage pulse to their coils. Because these relays require no continuous coil currents to keep their states, you can save considerable power in the driver circuit. In one type of latching relays, you can alternately energize dual coils to change the relay state. Simply apply voltage to one coil for the set state and to the other coil for the reset state. Applying a 25- to 50-msec-wide voltage pulse to the coils is sufficient for operating the relay. Many relays can operate with a continuous coil current, and some dual-coil relays have internal contacts that interrupt the coil current after it completes a state change. Continuous coil voltages can drive such relays if energy efficiency is not a big concern.
The need to differentially drive the coils results in crowded drive circuits for dual-coil relays. Drivers usually include logic elements to make sure to energize only one coil at a time. The design in Figure 1 uses only two MOSFETs to drive a dual-coil RF relay. The Agilent Technologies N1810UL RF switch has dual 24V coils and internal current-interrupting contacts.
When logic input is high, Q1 conducts and changes the relay state by activating L1. The states of the current-interrupting contacts also change. Meanwhile, Q2 is off because Q1 pulls down its gate, which avoids fighting between the coils. If you then apply a low signal to the logic input, Q1 turns off and keeps the L1 coil inactive. Because R1 pulls up Q2's gate, Q2 turns on and energizes L2. The 1N4007 diodes prevent inductive kickback. The idea is applicable to dual-coil relays with continuously rated coils or with current-interrupting contacts. In the absence of current-interruption contacts, L1 can serve as a pullup, and R1 therefore becomes redundant.