The AMPTEC 620 display should indicate a
stable reading when the test leads are securely
attached to the device under test. If the display
appears to be erroneous when connected to the
resistance under test, recheck the test leads for
integrity and cleanliness. If all external items
appear to be functioning properly, the next step
in problem isolation and diagnosis envolves
general trouble-shooting principles. If a
measurement problem appears on the 2.0 Ohm
range of the meter, test for a zero offset problem
first. Plug the 620 with Kelvin test leads into a
0.10 test resistor (i.e. AMPTEC # FTB-620ES).
If the meter doesn’t display a value close to 0.1
Ohms adjust the zero (see calibration procedure
chapter). The zero adjustment trimpot only has
enough span to zero out the 620 series test leads.
The meter’s zero adjustment pot wasn’t
designed to zero out a 100 feet of 2 wire
harness.
D-5.
Failsafe Operation
The AMPTEC 620 Series of Igniter Testers or
failsafe ohmmeters incorporate a constant
current source design that renders them
incapable of delivering excessive voltage or
current to the device under test. The typical
fail-safe current for each range is indicated with
most AMPTEC models under the corresponding
range switch on the 620 series meter front panel.
Please refer to section E-6 for a technical
description of the failsafe circuitry specifics.
As a further precaution the 620 Series Igniter
Tester is isolated from the AC line whenever the
POWER switch is in the ON position.
The 620 series igniter tester receives its power
from an internal rechargeable battery pack (4
“D” Cell Ni-Cad batteries). The 620 series
igniter tester “main power” switch (see item 12
of the Front Panel Diagram) must be in the
OFF/CHARGING position in order to charge
the batteries. Of course, the battery charger must
also be plugged into the unit’s rear panel jack. .
The “main power switch” must be switched
“OFF” to charge the batteries. In the "OFF"
position, the main power switch disables all
ohmmeter measurement circuitry and connects
the internal battery pack to the charging jack
(Safety Feature).
Constant Current Circuit Operation
Assume that terminals
I
and
I
of Figure E-3 are
hi
lo
shorted, and 0.5 volt is applied to
E
so that
I
is
in
hi
positive. To equalize the 0.5 volt applied to
E
in ,
the inputs of IC202, IC201 must be driven to
zero. This condition occurs only when the
voltage drops across R212 and R222 are equal to
the drops across R213 and R221. For these
voltage drops to be equal, the output of IC202
must be at +1.0 volt. Since the output of IC201
must be zero, the drop across R213 is 0.5 volts,
making the inverting input 0.5 volts. The drops
across R212, R221 and R222 will also be 0.5
volts. Since the inputs to IC201 are essentially
equal, its output is zero (offset by the few
microvolts required to drive IC202 to +1.0 volt).
Under these conditions the sum of the voltages
across R212, R213 , R221 and R222 equals the
sum of
E
plus the output of IC202.
in
Consider now that the short is removed from the
I
and
I
terminals and a 100-ohm resistor (R ) is
hi
lo
L
connected in its place. The current through R
L
increases the voltage at the input to IC201. A
balanced condition will be reached when the
output of IC201 is equal to the non-inverting
input of IC202. Again, this condition occurs
when the voltage drops across R212 and R222
are equal to the voltage drops across R213 and
R221. At this time the output of IC202 is 1.0 V.
The 620ES Explosive Safety Igniter Tester
measurement circuitry is failsafe current limited,
even under worst case component failure.
For the 620ES Explosive Safety Tester the normal
or typical operating current level is less than 10
mA, and <15 mA on 20 Ohm range as a Failsafe
Level
.
The voltage drop across the range resistor is 1.0
V, just as it was when the output terminals were
shorted. The current through R is 10 mA, just as
L
it was through the jumper when the output
terminals were shorted.
