The AMPTEC 620UK Explosive Safety Ohmmeter is “4-Wire” Kelvin version ohmmeter.
See diagram below.
6
A-5.1 Principle of Operation
The AMPTEC 620UK Safety Ohmmeter
fundamentally uses the familiar principle of "Ohm's Law"
. It outputs
a constant DC current through a unknown resistance. The DC voltage drop across it is measured and it displays it
(voltage drop) as resistance. The unit’s 2.0 Ohm range outputs an
intrinsically safe
5mA level of test current. When
the AMPTEC 620UK outputs 5mA through a 1.0 Ohm resister you should measure a 5mV drop. When you put that
voltage drop into the x10 (times ten) amplifier (only on the 2.0 Ohm range) it becomes 50mV, which inputs into the
DVM PCB and ratiometrically displays the measurement as 1.0000 Ohm. The unit’s 20 Ohm range outputs the
same constant 5mA level (without the x10 amp). When this constant 5.0 mA level is put through a 10.0 ohm
resister, you get a 50mV drop, which inputs into the DVM PCB and is displayed as 10.000 Ohms. The 200 Ohm
range outputs a constant 500uA DC current. When this current level (500uA ) is routed across a 100 Ohm resistor
you get a 50mV drop, which routes into the DVM PCB and is displayed as 100.00 Ohms. As the 620UK selected
range resistance goes up by a factor of 10 the constant DC current level goes down by a factor of 10 keeping the
voltage at 50mV.
4-Wire Kelvin
Connections
- The
V4
version
should
no
t use the front panel zero pot (only used with the 2 wire
version) - It is best to keep it (zero pot) turned in the
full clockwise position
. Connections are made to the front
panel terminals using a 4-wire (
V
high,
I
high,
V
low and
I
low) configuration. Proper 4 Wire Kelvin connections
automatically eliminate test lead wire (length) resistance and test lead connection contact resistance offsets (which
is what the zero pot does for 2 wire users). The notched connector end plugs directly into the single access notched
main connector. Turn the connector clockwise until you feel the notch or click of the mating connector of the
620UK. They have the 4 wire
current
high,
current
low as well as the
voltage
sense high and
voltage
sense low routed to the end of the leads. The 4 wires
(
Kelvin config.)
then terminate depending on the style with
either a 4
banana jacks or a set of double tipped handheld probes
. The 620UK keyed single access connector
also make it extremely difficult if not virtually impossible during normal operation to mis-connect measurement
leads to the 620UK
v4 version
.
* contact resistance
RESUST
ANCE
UNDER
TEST
LEAD RESISTANCE
*
*
LEAD RESISTANCE
DC CURRENT
LIMITED
CONSTANT
CURRENT
SOURCE
FAIL SAFE
OHMMETER
VOLTAGE
LIMITING
RESISTOR
4.8 VDC BA
TTER
Y
T
N
L
E
O
R
O
R
P
U
C
DVM
(
V4 Users
only)
Disregard the unit's front panel “ZERO” knob
. It was only intended for the 2 Wire ohm
version of the AMPTEC 620UK Ohmmeter.
It should not be used with the 4 Wire Kelvin ( V4) Version.
The
front panel "
zero" knob should always be turned all the way to the far clockwise position
. For the 2 Wire
NATO connector 620UK users the zero pot allows them to "zero out" a certain amount of "in-series" the test
lead resistance. The
4 Wire Kelvin ( V4) version automatically compensates for this effect -That is p
roper
4 Wire Kelvin connections automatically eliminate test lead wire (length) resistance and test lead connection
contact resistance offsets (which is what the zero pot does for 2 wire users)
