Design Decisions: Choosing the Right Resistor for Energy Metering

Energy meter design has changed radically in recent years as application-specific standard ICs from several vendors have enabled digital designs to replace electromechanical ones at costs compatible with high-volume applications. Many designs go further down the digital road with flexible multi-rate billing and remote reading. However, even with significant advances in digital design, all energy meters depend on analog front-end components for reliability and accuracy.

Line input
A typical transformerless power supply section for an energy meter is shown in Figure 1, below. Prior to voltage regulation the high-voltage supply is stepped down by a capacitive divider and rectified. The remaining components provide protection against supply-bourne EMC disturbances. These include radio frequency interference (RFI), filtered by the choke and X2 capacitor; electrical fast transient (EFT) pulses, shunted mainly by the X2 capacitor; and lighting strike transients, clamped by the MOV.

The input protection resistor R1 serves a number of functions here. The first relates to circuit function, namely limiting the zener peak current at switch-on to a safe level. The remainder relate to protection functions. Regarding RFI, a resistor can assist not only by contributing to series inductance, but also by reducing the Q factor of the input network, thereby minimizing the effect of any resonances. Critically, it serves to limit the peak MOV current during a lightning strike transient, reducing the stress on the MOV by dissipating a share of the pulse energy. And finally, it can offer fail-safe flameproof fusing in the event of a short circuit failure.

The pulse used to test immunity to lightning strike transients is defined in IEC61000-4-5 as having a rise time of 1.2mus and then exponential decay, with a 50 percent amplitude pulse width of 50mus. It is important to realize that a MOV has a finite lifetime and that permanent and progressive changes occur at each pulse event. If a safe number of pulses is exceeded during the product lifetime then the MOV voltage will begin to rise then drop rapidly until reaching short circuit failure. The use of an input resistor placed before the MOV, as shown, can greatly extend the lifetime of the MOV, and also permits lower cost parts to be selected.

Wirewound technology combined with flameproof cement coating is often used, with 3W to 5W sizes generally being chosen. Care should be taken to ensure that the pulse capability is guaranteed by the manufacturer. Successfully testing a sample may not be sufficient as there is generally some flexibility in winding design causing batch-to-batch variation in the energy capacity of normal wirewound resistors.