17
OHMMETER
DISPLAY
LCD DISPLAY &
PIC CPU SMART
CONTROL
INTRINSICALLY
SAFE CURRENT
LIMITED SOURCE
PCB
HIGH INPUT
IMPEDANCE
VOLTMETER
PCB
ISOLATED “AA”
BATTERY SUPPLY
A-5 BLOCK DIAGRAM AND CIRCUIT DESCRIPTION
The AMPTEC 620UK Explosive Safety Igniter Tester is a portable digital milli-ohmmeter specifically
designed for ultra-safe resistance testing on bonds in a highly flammable fuel air environment. With 4 1/2-
digit resolution and a low Ohm range of 2.0 Ohms full-scale, the AMPTEC 620UK Ohmmeter provides
ordnance safe resistance measurements and viewing even in direct sunlight.
The AMPTEC 620UK Ohmmeter is constructed in a metal EMI enclosure and internally contains 5 VDCt
HC-MOSS clock operating at 100 KHz for the display, and an HC-MOSS clock operating at 32.7KHz for
the PIC computer chip.
OUTPUT
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
