1-2. The Model 1625A Active Current Shunt is 8 precision
AC/DC shunt. It incorporates seven decade shunt ranges fro*
200 uA full scale to 100 Amperes. Each shunt range has its
own current input terminals and a selector switch provides
access to the voltage output terminals of each shunt
resistor. A single set of binding posts conveniently
provides output voltage to the measuring voltmeter, the
performance of the si.unt resistors permits using the 1625A
as a laboratory standard as well as a general purpose
1-3. A. stable precision low noise instrumentation amplifier
is incorporated to buffer the voltage output terminals of
the shunt resistors* The amplifier has a precisely
adjustable gain of 10 so that the output potential from each
current shunt lay be used to drive a 200 ohms per volt AC/DC
transfer device. Sensed output leads fro* the precision
amplifier to the thermal transfer standard maintain accuracy
and measuring integrity from dc to well beyond 100 kHz
although ac flatness and accuracy is specified to 10 kHz.
The precision amplifier permits the shunts to operate at
lowest power and also drive an AC/DC transfer device.
Internal rechargeable Nicad batteries assure low noise and
permit off ground operation.
1-4. The availability of a precision amplifier permits all
the current input to the shunt resistors to flow without
bypassing measured current to the AC/DC transfer device.
This avoids the otherwise limiting condition which restricts
passive shunts to a low current of 2.5 mA when used with
thermal transfer standards.
1-5. The compliance voltage of the shunts is less than 0,25
volts at full scale for each range except the 100 Ampere
range which has a compliance voltage of only 125 mV. Each
shunt is a highly stable ac/do resistor connected in a four
terminal configuration, A compensated adjustable network
controls voltage output to a precise dc and ac current to
voltage conversion. The 20 A and 100 A ranges each
incorporate a non-inductive shunt resistor made of a
specially heat treated alloy developed by Ballantine, These
shunts are highly stable with a low temperature coefficient
of resistance of nominally +10 ppm per degree centigrade.
This.assures rapid settling time with best accuracy and
minimum measurement time.
1-6. Forced air cooling is provided by a fan mounted on the
rear panel. Outside air is pulled in by the fan,
passed over the shunt resistors, and exhausted at the rear
panel. DC power is provided for the fan. The fan is operable
when using 100, 120, 220, and 240 volts 50 to 400 Hz ac
mains power. The internal batteries will provide fan power
when operating the 1625A off line.
1-7. The 1625A is housed in a rugged all aluminum enclosure
which i$ fully shielded for EHI considerations. The
instrument has sturdy die cast end frames, side extrusions,
and vinyl clad covers. The enclosure conforms to EIA half
rack 5.25 inch high standards and may be rack mounted or
used on the lab bench or in portable applications.