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MX4428 MXP Engineering /Technical Manual Document: LT0273
MXP Technical Description
Page 7-6 24 March 2006 Issue 1.5
7.2.3.4 +5V ISO & +5V Batt
A second L1 secondary winding is used to produce an 8V supply, This 8V supply is poorly
regulated and may vary from 7.5V to 10V depending on the 40V ISO load. The 8V supply is
regulated by U11 to 5.2V.
This supply is then passed through D32 to produce the 5V supply for the CPU and logic
circuitry. The 5.2V supply also passes through D31, to produce +5V Batt, and this supply
contains a supercap (C68) which is used to keep this supply up after the power supply has
shutdown. R66 is used to limit the charge current to the supercap. This supply is used to
power the RAM on the MXP, and retain its contents during short (up to a few hours) power
downs.
7.2.4 MX4428 LOOP INTERFACE
7.2.4.1 Loop Disconnect Circuitry
The MXP, like all MX4428 compatible Responders, includes a DISCONNECT relay (RL3)
which is used to isolate shorts on the power supply loop.
Normally 24V power passes from one Responder to the next via the 24VIN terminal /
DISCONNECT relay / 24 VOUT terminal path, supplying power to the Responder on the way
through. D35, D37, R90 form a diode gate such that a loop short on either 24VIN or
24VOUT applies a low voltage to comparator U7:D. This generates a DISCON IN- signal to
the microprocessor which then opens the DISCONNECT relay to isolate the fault.
Depending on the time taken for the shorted section to be isolated, the power output of the
MX4428 panel may collapse completely, removing power from all responders. Therefore it is
necessary for them to respond rapidly to the DISCON IN signal and open the DISCONNECT
relay in their last dying gasps before their power supplies collapse to zero.
Generally, all Responders on the loop respond in like fashion and break the loop supply.
(Depending on the position of the short, and loop resistances, some may not open their
DISCONNECT relays.) Starting from the Responder nearest the MX4428 Master, each
Responder then makes a decision, based on the value of “MIN V” (refer to
Figure 7.1)
whether to re-connect the relay or not. If MIN V is less than +V/2, the loop fault is on one of
its 24V terminals, so it will not re-close its DISCONNECT relay. If, however, MIN V is
greater than +V/2 the fault lies beyond the next Responder and it can therefore apply power
to that Responder.
The newly powered up Responder then makes a similar decision, followed by each
successive Responder up to the Responder with the loop fault on its far side which will not
close its DISCONNECT relay.
Similarly Responders on the other end of the Responder loop will close their DISCONNECT
relays, up to the Responder connecting to the section of the loop with the short circuit.
With a single short circuit, all responders will eventually be powered up (receiving power
from one end of the loop or the other), however the two responders on either side of the
short will have their DISCONNECT relays open.
7.2.4.2 Disconnect Relay Driver
U8A and U8B with Q10 and Q11 form a bridge circuit to drive DISCONNECT relay RL3.
This is a magnetically latched relay to save power consumption. Its position can be switched
by providing a short pulse of voltage, with the polarity of the voltage controlling the position.