Design Ideas: February 1, 1996

Figure 1 shows a low-parts-count circuit that can drive one- or two-coil latching relays. Latching relays are useful in sensitive analog designs. The 74ACT174 hex flip-flop has ample capability to drive the Aromat TQ2E series of latching relays. Both windings for the TQ2E-L2-5V two-coil relay are on the same magnetic element, so the windings' fields add. You simply place the coils of a two-coil relay in series and drive the relay as if it were a one-coil device. The two 125 Ohm coils add to 250 Ohms, the same resistance as the coil in a one-coil relay.

Pulses as short as 2 msec set and reset the relays. Selecting the relay to drive requires loading a special data code into the register. You drive all relays not intended for switching to the same level as the Set/Reset signal. For example, suppose you wish to set relay K₁. The correct code is 111110, which provides a voltage to the coil of relay K₁ only. The correct code to reset K₂, is 000010. After the relay operates, you deactivate it by setting the register to all zeros.

Driving all coils to ground when the relays are not operating provides an advantage in sensitive analog designs. Consider the traditional alternative scheme in Figure 2. When the relay is not energized, its coil pins have 5V potential at a distance of only 0.1 in. from the sensitive summing junction. When the coils have 0V potential (except during switching), as in Figure 1, no leakage can occur into the summing junction. You may even want to connect the ground on the register to analog ground instead of to traditional logic ground. You can do so, because the CMOS part draws almost no current when it's not switching. To avoid coupling of switching transients, you should take care to prevent any switching on D0 to D5 and CLK during a measurement.

This circuit does not require clamp diodes. Unlike in the traditional approach, the relay coil is never open-circuited. The CMOS output has either low impedance to ground or to 5V, except for the few nanoseconds during transitions (during which time, stray capacitance easily handles the 20-mA coil current). You should energize only one coil at a time. Therefore, the Clear signal should be in the low state at power-up. (DI #1821)