PM9904BPD
where:
Vnom is the mains voltage and correspond to 14µA in the
voltage inputs of the SA9904B.
Imax is the maximum mains current to be measured and
correspond to 16µA on the current inputs of the SA9904B.
The pulse rate required for a meter is usually expressed in
pulses/kWh. A single pulse on the LED is mostly a fraction of a
kWh and is converted to energy in Ws/pulse
Energy per LED pulse is (Ws/pulse):
Epp energy = 1000 x 3600 / Mpr
where:
Epp is energy per LED pulse
Mpr is the meter pulse rate or meter constant in pulses/kWh
The threshold is calculated by dividing the energy represented
by a LED pulse by the energy per register count.
Active energy threshold = Epp / Epc
The threshold is thus the amount of energy to be measured
(accumulated / integrated) by the meter before a LED pulse is
generated.
sames
The simplest way to implement the creep threshold is to relate
it to the time between meter pulses. If the time between pulses
is more than the limit, the energy accumulator is cleared.
Pulse rate of meter at rated conditions (Hz):
Rf = ( Vnom x Imax / 1000 ) x ( Mpr / 3600 )
where:
Vnom is the mains voltage and correspond to 14µA in the
voltage inputs.
Imax is the maximum mains current to be measured and
correspond to 16µA on the current inputs of the device.
Mpr is the meter pulse rate in pulses/kWh.
Creep threshold time (s):
Ct = 1/(Cc / Imax) x Rf
where:
Cc is the specified creep current; energy below this value is
discarded.
Imax is the maximum mains current to be measured and
correspond to 16µA on the current inputs of the device.
Rf is the rated current frequency.
The flow diagram (figure 6) for the timer interrupt shows how
the time between pulses is measured, if the time since the last
pulse is more than the time measured, the integrator is reset
and a new count down is started.
Start Timer Interrupt
Pulse threshold
Integrator Amplitude
Threshold value subtracted
from integrator
Integrator zero
No
If LED On
Pulse LED
Yes
Decrement LED ON timer
Pulse Generated
Reg 8 add to Integrator
Reg 4 add to Integrator
Reg 0 add to Integrator
Yes
If LED ON Timer = 0
Switch LED off
Figure 5: Implementation of an overflow integrator
No
If creep timer > 0
Yes
Decrement creep timer
Meter creep current
For the SA9904B meter creep must be taken care of in
software. From the explanation above on how to generate
pulses, the meter must also be prevented from pulsing in
cases where the energy measured is less than the creep
threshold as per the meter specification. The creep current is
defined as the limit for measured energy, any energy less than
the creep threshold is discarded, and energy above the creep
threshold is measured.
Yes
If creep timer = 0
No
Exit Interrupt
Reset integrator
Load creep timer
Figure 6: Interrupt flow diagram
5/22
SAMES [ SAMES ]
