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MNL107-900-000D
3.6
OPERATIONAL CONSIDERATION FOR DC PREFERRED OPTION
The standard model AC / DC 1200 is designed to operate primarily from the AC line. If the AC
line fails, operation from the DC supply (Batteries) is automatic. When the AC is recovered, the
unit will again automatically resume operation from the line. These units are considered to be “AC
preferred”. The D1 option provides the opposite function. When both the AC and DC sources are
available, the D1 unit will operate primarily from the DC power. When the DC is removed the unit
will automatically switch to AC line operation. In both cases, if the inverter were to fail, the AC line
will be “by-passed” directly to the output terminals.
Operating Considerations:
Because this operates primarily from the DC source (most likely batteries) it should be noted that
current (and power) will always be drawn from the source even when no load is connected to the
unit. There are certain fixed minimum losses associated with DC operation that determine this
current. For a 48Vdc unit this quiescent current is about 2 Amps. The 125Vdc input requires less
than 1 Amp.
Switching from DC to AC power is accomplished by a relay with a transition time of between 2
and 10mSec. If the unit is heavily loaded, a dip in the output voltage during switch over may be
encountered. When selecting components to design a back-up power system, use devices with
“hold up” times greater than 10mSec whenever possible. This is not an issue with AC preferred
units.
To determine which version is being used, look at the rating label affixed to the right side panel of
the unit. DC preferred units will be designated D1.
The internal impedance of the DC source will affect the transient capability of these units.
