SLVAF85 September 2026 TPS2596 , TPS25961

2 Conventional RC Circuit-Based Design

Figure 2-1 Block Diagram of Conventional RC Circuit-Based Design

In this conventional RC-based implementation, the latching relay peak power requirement is supplied by both hold-up capacitor ‘C_{hold_up}’ and power supply . ‘C_{hold_up}’ capacitor is connected to power supply with a series resistor ‘Rlimit’ (22Ω to approximately 100Ω), as shown in Figure 2-1, to limit the inrush current.

In a typical single phase smart e-meter:

Nominal supply voltage = 12V
Relay coil resistance = 96/2 = 48Ω
Pulse duration = 50ms
Relay operating voltage range = 12V to 8.4V
Rlimit = 100Ω to limit inrush current to 120mA

During the latching period of 50ms, while the capacitor supplies the energy required to relay ,the voltage across the capacitor decreases . The voltage across relay (which is equal to voltage across capacitor) must be in the operating voltage range for the latching to be successful. The hold-up capacitor needs to support the peak energy consumed by relay, as well as maintain the relay voltage in the operating range for the latching to be successful. The minimum hold-up capacitor for successful latch is calculated using the Equation 2:

Equation 1.

C_{hold_up} = \frac{t_{pulse}}{(\frac{R_{c} \times Rlimit}{R_{c} + Rlimit}) \times \ln (\frac{V_{in} \times Rlimit}{[V_{\min} \times Rlimit]) - [V_{in} - V_{\min}]) \times R_{c}})}

In the typical scenario described above, we need to install a capacitor of 3.3mF calculated using Equation 2 for the latching to be successful. A 3.3mF capacitor, typically 16mm in diameter and 25mm in height, is required to supply the relay's energy during latching, as the power supplied by the power supply is limited by the RLIM resistor in the RC configuration. This large capacitor occupies significant space in space-constrained smart meters. The test result of conventional RC circuit with 3.3mF of hold-up capacitor is shown in Figure 2-2.

Figure 2-2 Test Result of Conventional RC Circuit With 3.3mF Of Hold-Up Capacitor

So, Customers require a design that:

Reduces the capacitor size as it becomes a bottle neck in limited size of the energy meter while meeting the relay energy requirements
Is capable of utilizing the available energy from the SMPS
Provides overload and short-circuit protection on the 12V rail powering these relays
Must bring e-meter fully functional within five seconds once mains power is applied. So, hold-up capacitor charging time must be <5sec