In many industrial pumping applications, two identical pumps are used for the same job. A standby unit is available in case the first pump fails. However, a completely idle pump might deteriorate and provide no safety margin. Alternating relays prevent this by assuring that both pumps get equal run time.
A CMOS seven-stage ripple counter (CD4024) can be used to build a simple alternating relay (Fig. 1). In this circuit, the relay will change state each time the control switch opens. Using the counter chip is cost-effective and offers several advantages:
- Exceedingly slow input transitions may be applied to the Clock input. There's no need for special signal-conditioning circuitry.
- The counter is reset to "zero" by a high level on the RESET input. When power is applied to the unit, the relay will remain in its de-energized position. The first closure of the control switch will therefore always turn on the number 1 pump.
- Output transitions occur on the falling edge of clock pulse. The relay changes position only when the control switch opens and the contacts carry no current. This prevents arcing and wear of the relay contacts.
Resistor R6 is placed across the power supply when the relay de-energizes. This resistance is selected to be equal to the resistance of the relay coil, in order to minimize supply-voltage variations.
The circuit in Figure 2 shows a typical pump-down application incorporating a normally open float switch. The switch should have a wide differential between its closing and opening points, otherwise a rapid continuous switching of the pumps can occur.
The alternating relay approach isn't limited to pumping applications. The control switches could be thermostats or pressure switches, and the loads could be fans and compressors.
Further refinements are possible but beyond the scope of this article. For example, by using three float switches, the alternating relay could be wired to energize both pumps under peak load or pump failures. A fourth float switch could actuate an alarm if the level can't be successfully controlled.
NOTE: This low-voltage logic circuit isn't isolated from the 120-V ac line, and dangerous voltages may be present at all points in the circuit.