Securitron MUNL-24 - Manual (Page 8)

PN# 500-18800
Page 8 Rev. E, 11/11
FIG. 8: ZENER DIODE PLACEMENT ( MUNL-24 , TRANSFORMER, BRIDGE RECTIFIER)
~
~ 120
VAC
IN
VAC
OUT
TP-24-2
PLUG IN
TRANSFORMER
~
~
BR-7
-
+ NC
C
NO
PB2E
C
TRIG
NC
NO
+
-
MUNL-24
1N5333B Zener diodes
RED
BLK
ORG
Note diode orientation:
BANDED SIDE
DENOTES CATHODE
CROSS BAR
DENOTES CATHODE
Note: The addition of the zener diodes is only required when using the MUNL-24 with 24VAC
transformer and bridge rectifier. When using the MUNL-12 with 12VAC transformer and bridge
rectifier the zener diodes are not necessary.
Power supply surge capacity, however, is ideally 4 Amps @ 12 VDC or 2 Amps @ 24 VDC for
best operation. This is because the Mortise UnLatch’s motor (like all motors) will momentarily
draw a lot more current if it has to “work harder”. This occurs for a very short time at motor
start. If power supply surge capacity is limited, the Mortise UnLatch will still operate, but more
slowly. The same condition of high current draw will occur for a longer period of time if the latch
is binding which could be a permanent condition of the latch or could be because the person
trying to enter is pulling on the door. Again, the Mortise UnLatch will function but more slowly
and weakly. If a full capacity power supply is not practical, a 1 Amp power supply which
includes battery backup such as Securitron’s model BPS-12-1 or BPS-24-1 (with battery) is an
equally effective device to power the Mortise UnLatch as the battery will provide extra surge
current as it is needed. Another point to note is that when one relatively large power supply
is serving a number of Mortise UnLatches, the extra current capability will always be there as
each unit will operate at different times so each can draw extra current from the power supply
when needed.
If you are using a smaller capacity power source, you may be satisfied with the operation
of the Mortise UnLatch since the reduction in torque is only noticeable when the door is pre-
loaded. There is, however, an important electronic effect. During the brief moment while the
motor is starting to move, the Mortise UnLatch will “try” to draw 4 or 2 Amps (depending on the
voltage). A power supply of lower capacity will react to this condition by sharply dropping its
voltage for the same period. While this will not harm the power supply, it may “crash”
microprocessor equipped devices like digital keypads (such as Securitron’s model DK-11 or DK-
26) that are being operated from the same power supply. If, therefore, your installation
includes a digital keypad, or other microprocessor equipped device, make sure you have full
power supply capacity (or a smaller supply with battery backup) or use a separate power supply
for the keypad or other device.
4.2 WIRE GAUGE SIZING
If the power supply is some distance from the lock, voltage will be lost (dropped) in the
connecting wires so that the Mortise UnLatch will not receive full voltage. The amount of voltage
that is dropped in the wires depends on the resistance of the “round trip” wire run compared to
the resistance of the Mortise UnLatch. The issue is complicated by the fact that the resistance of
the Mortise UnLatch is 20 Ohms @ 12 VDC or 80 Ohms @ 24 VDC when it is operating (the
motor is moving) but the resistance drops as low as 3 Ohms @ 12 VDC or 12 Ohms @ 24 VDC
for 100 milliseconds just as the motor starts. If the unit is denied full voltage during this motor
start period, it will operate but with less torque so it will not release the latch as crisply when the
latch has pre-load on it. Selecting thick wires to power the Mortise UnLatch will
maintain strong torque even during start up. The following table shows distances from the
Mortise UnLatch to the power source and calls out the wire gauge needed to fully maintain
torque. Note that if your installation requires the power source to be more distant or has other
complications, the factory can assist you.