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Model 721A
Sect. II Page 1
SECTION II
OPERATING INSTRUCTIONS
2-1
OPERATING CONTROLS
Figure 2-1 shows the function o'Peach of the controls
and is normally self explanatory. There are a few
additional facts to be considered which may be im¬
portant in some applications.
2-2
METER RANGE SWITCH
This switch connects precision internal resistors
into the meter circuit to obtain the various voltage
and current ranges. When measuring current, the
meter shunt resistor is in series with the output
terminals. The meter shunt resistance adds to the
source impedance of the supply which is normally
less than 0.2 ohm in series with less than 30p,h.
The Table 2-1 lists the additional resistance for
each current range.
Where minimum source im¬
pedance is important, the METER RANGE switch
should be left on one of the voltage ranges except
when actually measuring load current. An accidental
short circuit can damage the meter if it is on one
of the lower ranges and the SHORT CIRCUIT CUR¬
RENT switch is set to a high value.
Short time
overloads of two times full scale will not damage
the meter movement.
2-3
SHORT CIRCUIT CURRENT SWITCH
This switch controls a circuit which adjusts the peak
current output capability of the supply. The cali¬
bration is nominal. The actual value may be read
by shorting the supply and reading the value on the
monitor meter.
The clipping action is gradual. Consideration should
be given to this characteristic if pulse type circuits
are being supplied.
The average current may be
within the supply rating (150 ma) but peak currents
may be high enough to cause the supply to clip. If
the switch is set to a low peak value this situation
can occur at low average current levels.
When
pulse circuits are being supplied, this switch must
be set to a value which is greater than the peak cur¬
rent requirements of the circuit.
The output circuit has 24
jif
capacity shunting it,
which helps supply high current peaks,.providing
they are of extremely short duration.
The value
of any external capacity added will improve the
peak current capability but will decrease the safety
provided by the SHORT CIRCUIT CURRENT switch,
since the external capacity will provide high surge
currents. The surge currents may destroy external
components before the average current increases
sufficiently inside the supply to cause the limiting
circuits to operate.
TABLE 2-1.
ADDITIONAL INTERNAL
RESISTANCE
Monitor Meter Current
Added Internal
Range (f.s.)
Resistance
10 ma
10 ohms
30 ma
3.33 ohms
100 ma
1 ohm
300 ma
0.33 ohm
2-4
SERIES OPERATION OF SUPPLIES
When operating the Model 721A at a voltage more
than a few volts off of ground be careful not to
accidentally short circuit the external circuits so
the Model 721A is subjected to voltages of reverse
polarity or high voltages of the same polarity. To
do so will instantly destroy the electrolytic capa¬
citors in the power supply and possibly the transis¬
tors.
When a number of supplies are operated in
series, be sure the SHORT CIRCUIT CURRENT
switch on each supply is set to the same (or higher)
value than maximum peak current desired.
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