Megger DLRO - Section E Circuit Description

AVTM24-1J REV A April 2006
15
Section E
CIRCUIT DESCRIPTION
Ra and RA’ set the test current to the values listed in Section 0,
Specifications; there is a pair of these resistors for each range. On the
6 mΩ 10A range, test current is also affected by the resistance of
the test leads connected to the C1and C2 terminals. The
instrument is designed to operate with current leads having a
resistance of O.02Ω each. Using leads that have a resistance
other than 0.02Ω will not affect the accuracy of the instrument,
but will cause the test current to be different.
A digital meter, powered by four size D nickel cadmium cells,
displays the ratio:
REF
IN
E
E
6×
EIN is the voltage across Rx where the potential leads are
connected.
(There are no length restrictions on the potential leads).
E
REF
. scaled down by divider RD-RE, is set by the range
network and represents the current in Rx.
The calibration network on each range is set to produce, at
the specified test current, nominally 60 mV at the E
REF
input. Consider a test specimen, Rx of 1.5 milliohms and a
test current of 10 amperes.