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MX4428 MXP Engineering / Technical Manual Document: LT0273
MXP Current Consumption
Page 5-2 24 March 2006 Issue 1.5
5.1 THEORY
The MXP current consumption is considerably higher than that of the other responders (even
higher than the MPR, in fact it can be considerably higher than the MPR depending on the
sounder load). It must be carefully considered when engineering the MX4428 responder
loop.
Use of the F4000CAL PC program is strongly recommended as it performs the following
calculations automatically.
A formula for predicting the MXP current is.....
I(mA) = (ITOT(mA) * (40V / VIN) * ( 1 / PCE) ) + (IQ(mA) * (24V / VIN))
Where .....
PCE = Power converter efficiency = 0.80
IQ = MXP quiescent current at 24V = 50mA.
ITOT = Total current sourced into the AL and AR terminals, which can
be calculated as shown in Section
3.2.2
VIN = MX4428 Responder Loop voltage
5.1.1 ALARM CURRENT
The alarm current calculated for all responders can be calculated and used to ensure that
1) The total current to be sourced from the MX4428 does not exceed 2.0A
2) No responder will see a supply voltage of less than 17V, allowing for the minimum
battery voltage at the MX4428 and voltage drops in the responder loop wiring.
Once MXP currents are calculated, the voltage drops around the responder loop can be
calculated. This will give a more accurate figure for the operating voltage of each MXP which
will result in a slightly different current consumption. The full calculation is an iterative
process when performed manually, and it is recommended that the PC program F4000CAL
is used.
Taking the example from section
3.2.5, where the alarm current was calculated at 221mA,
the MXP supply current at (22V) will be
I(mA) = (ITOT(mA) * (40V / VIN) * ( 1 / PCE) ) + (IQ(mA) * (24V / VIN))
= (221 * (40 / 22) * (1 / 0.8)) + (50 * (24 / 22))
= 502mA + 55mA
= 557mA.
This current, together with the load of other responders on the MX4428 responder loop, can
be used to calculate the voltage drops on the responder loop power wiring, and possibly
refine the value used for the MXP supply voltage (22V above). The calculations can be
performed iteratively until little change is evident.