Datasheet
STPM10 Theory of operation
Doc ID 17728 Rev 4 27/53
7.14 Energy to frequency conversion
The STPM10 provides energy to frequency conversion both for calibration and energy
readout purposes. In fact, one convenient way to verify the meter calibration is to provide a
pulse train signal with 50% duty cycle whose frequency signal is proportional to the active
energy under steady load conditions. In this case, the user chooses a certain number of
pulses on the LED pin that correspond to 1 kWh. This value is called P.
Let us consider the case in which the LED pin is configured to be driven from internal signal
AW (active energy) whose frequency is proportional to the active energy. The signal AW is
taken from the 11
th
bit of the active energy register, and consequently a relationship
between the LSB value of the active energy register and the number of pulses provided per
each kWh (P) can be defined as:
Equation 2
Due to the innovative and proprietary power calculation algorithm, the frequency signal is
not affected by any ripple at twice the line frequency. This feature strongly reduces the
calibration time of the meter.
In a practical example where the desired P is 64000 pulses/kWh (=17.7 Hz*kW), we have:
Equation 3
k
AW
=7.63*10-6 Wh
This means that the reading of 0x00001 in the active energy register represents 7.63 µWh,
while 0xFFFFF represents 8 Wh.
The LED pin can be driven from AW (active energy wide band), AF (active energy limited at
fundamental), RW (reactive energy) or SW (apparent energy) according to the value of the
KMOT bit.
In this case, since the LED pin is driven by signals different from that of AW, some other
relationship between the LSB of the register and the number of pulses per kWh provided by
the meter (P) must be defined:
Equation 4
k
AF
= 4*k
AW
[Wh]
k
RW
= 2*k
AW
[VARh]
k
SW
= k
AW
[VAh]
Table 9. LED pin configuration
KMOT (2 bits) Signal available on LED pin # of pulses
0 AW Type0* P [kWh]
1 AF Type1* P [kWh]
2 RW P [kVARh]
3 SW P [kVAh]
K
AW
1000
2
11
P⋅
--------------------
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