PM9904BPD datasheet(5/22 Pages) SAMES

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PM9904BPD

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Figure 6: Interrupt flow diagram

Reg 0 add to Integrator

Reg 4 add to Integrator

Reg 8 add to Integrator

Pulse threshold

Integrator zero

Pulse LED

Threshold value subtracted

Pulse Generated

from integrator

Figure 5: Implementation of an overflow integrator

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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.

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.

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.

If LED On

If LED ON Timer = 0

Switch LED off

Decrement creep timer

If creep timer = 0

Decrement LED ON timer

If creep timer > 0

Load creep timer

Reset integrator

Yes

Exit Interrupt

Start Timer Interrupt

If LED On

If LED ON Timer = 0

Switch LED off

Decrement creep timer

If creep timer = 0

Decrement LED ON timer

If creep timer > 0

Load creep timer

Reset integrator

Yes

Exit Interrupt

Start Timer Interrupt

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