The 640 Tester measurement circuitry is also
failsafe current limited, even under worst case
component failure.
For the 640 Tester the normal or typical
operating current level is less than 5 mA, and
< 8mA on most 20 ohm range versions as a
Failsafe Level
A simple startup test
procedure which also has the 640 Tester user
perform a functional check using the
milliammeter would also detect any current
level even getting close to the Failsafe level.
.
E-7 . Ultrasafe Power Supply Scheme
The +5 volt power supply is provided directly
by the batteries (for driving the LED displays
and digital logic). The ±5VD is used for driving
IC8, the low battery detection circuit. The ±15V
power supply is generated by IC7 for the digital
voltmeter (DVM) chip set (IC1 and IC2).
The ±5 VA is developed by one DC to DC
convertor circuitry: composed of Q103,Q104,
T102, D103, D104, IC102 for the negative
polarity. The other DC/DC convertor is
composed is composed of Q101, Q102, T101,
D101, D102 and IC101 for the positve polarity.
Summary of Contents for 640N
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