D-3. 4-Wire Resistance Measurement
The four-terminal configuration of the 620LM
eliminates measurement errors normally caused
by "
in series
" test lead resistance and "
contact
"
resistance.
In many resistance measurement applications the
contact resistance and can exceed the value of
the test resistance by several orders of magni-
tude. The AMPTEC 620LM overcomes this
potential error source by providing two terminals
of constant current (
I
high and
I
low) and an
additional two terminals for high impedance
voltage measurement. The constant current
source uses a variable compliance voltage circuit
to overcome lead and contact resistance until the
current loop is a constant level. The result is a
fast, accurate resistance measurement of the test
resistance, independent of the resistance of the
current carrying leads.
Figure D-1 above illustrates the 4-wire principle
eliminates lead, wire and contact resistances as
potential error sources. The internal constant
current source inherently overcomes all series
resistance (within compliance voltage limits) and
delivers a precise constant current.
Separate DC
to DC isolation circuitry provides independent
circuit voltage supplies for both polarities of
the constant current source circuit
. The internal
high-impedance Digital Voltmeter (DVM)
senses the voltage drop across the test resistance
(i.e. squib or detonator). 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 (V
hi
and V
low
) leads.
D-4.
Connections
Connections are made to the front panel termi-
nals using a 4-wire configuration as described in
section D-3.
Next connect the 4 wire Kelvin wires or test
leads into the AMPTEC 620LM Igniter Tester
rear panel.
For 620 series test leads other than those termi-
nated with banana plugs, RG-58 Shielded Coax
Cabling is recommended. Customer built test
wiring should maintaining the four wire Kelvin
measurement if possible. Make the current con-
ductor the largest conductor and keep the voltage
sense input shielded or inside the current shield.
The AMPTEC 620 series Igniter Tester's five
way input jacks allow for customized wire con-
nections, extended kelvin wiring (beyond 100
feet depending upon conductor gauge), cables
terminated with spade lugs, and special banana
jacks can all be used with most AMPTEC 620
series Igniter Testers.
All AMPTEC ohmmeters use a high impedance
voltmeter as part of the resistance measurement
process. This voltmeter is a highly accurate and
stable 4½ digit analog-to-digital converter (A to
D). The high impedance DVM must receive a
voltage drop in order to display the proper value.
When the DVM is not receiving a definite input
signal, the output reading from the Analog to
Digital Convertor can appear random and wan-
dering. The display may indicate a randomly
wandering number or it may indicate an
overrange (flashing) condition. This unpredict-
able display (No input to the DVM) condition is
not a malfunction, in fact, just a characteristic of
the high impedance voltmeter circuit and
should
not be mistaken for a fault in the instrument
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IGNITER TESTER
Figure D1 - Igniter Tester Kelvin Block Diagram
* contact resistance
*
*
IGNITER
LEAD RESISTANCE
LEAD RESISTANCE
P
O
O
L
T
N
E
R
R
U
C
DC to DC ISOLA
TOR
HIGH IMPEDANCE
VOL
TMETER
4.8 VDC BA
TTER
Y
DC to DC ISOLA
T
OR
COMPLIANC
VOLTAGE
LIMITED
CONSTANT
CURRENT
SOURCE
E
