Charging Jack
The battery charging jack is a barrel type and is
located on the 620A-4 rear panel. The center
pin of the connector is positive. The charging
requirements of the internal battery pack are
6VDC @300mA. The correct charging voltage
is supplied by the adapter included with the
instrument. Additional AC/DC Battery
Chargers are available as Option "620-DC".
with the AMPTEC 620A-5, the
charging jack is mounted on the unit’s front
panel.
D-4. 4-Wire Resistance Measurement
The four-terminal configuration of the 620A-
4 eliminates errors normally caused by in
series test lead resistance and contact
resistances. In many applications the contact
resistance can exceed the value of the load by
several orders of magnitude.
The 620A-4 avoids this potential error source
by providing two terminals of constant current
and an additional two terminals for high
impedance voltage measurement. The constant
current can be thought of as a
current loop
that
overcomes lead length resistance and contact
resistance encountered (all series resistance)
along its loop path. The result is a fast, accurate
resistance measurement of the load,
independent of the resistance of the current
carrying leads.
Figure D-1 (above) illustrates the 4-wire
principle and how it is used to eliminate lead,
wire, and contact resistances as potential error
sources. The internal current source inherently
overcomes all series resistance (within
compliance voltage limits) and delivers a
precise constant current.
The internal high-impedance DVM senses the
voltage drop across the load. There is
negligible contact and lead resistance error
created by the voltage measurement because the
high input impedance of the DVM limits
current flow in the voltage leads.
LEAD RESISTANCE
*
IGNITER
*
VOLTAGE
LEAD RESISTANCE
*
contact resistance
Figure D-1. Error Sources in Resistance
Measurements eliminated with Four Wire
Kelvin Connections
D-5.
Connections
Connections are made to the front panel
terminals using a 4-wire configuration as
described in section 4-4. When using AMPTEC
test leads, the tabbed or
negative (-) side of the
banana jack is plugged into the current
terminals of the 620A-4
. This ensures that the
current is carried in the largest conductor and
that the voltage input is shielded. The 620A-4
features five-way gold plated input jacks. The
gold plating on the input jacks helps minimize
thermal EMFs. Wire connections often generate
a DC offset voltage with the contact between
two dissimilar metals (also called the thermo-
couple effect).
In addition, the 620A-4 five-way input jacks
allow for clean, simple connections to bare
wires, cables terminated with spade lugs, and
banana jacks.
V high
I high
V low
I low
620A-4
CLOSEUP - INPUT JACKS (4 wire connection)
LIMITING
RESISTOR
LIMITED
CONSTANT
CURRENT
SOURCE
FAIL-SAFE
OHMMETER
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