HP 6235A - Page 11

3-17 Tracking Ratio
3-18 The TRACK VOLTAGE control can be used to
set a 1:1 ratio so that the voltage of the —18V supply tracks
that of the +1 8V supply within 1 % for convenience in vary¬
ing the symmetrical voltages needed by operational ampli¬
fiers and other circuits using balanced positive and negative
inputs. The TRACK control can also be used to set the
negative supply's output from a minimum of less than
0.5 volts to a maximum equal to the +18V supply's output.
Once the ratio is set, the ±18 VOLTAGE control will con¬
trol both output and maintain a constant ratio between their
voltages. To set the ratio between the —18V and +18V out¬
puts, proceed as follows:
NOTE
Any accidental movement or mechanical
vibration can vary the tracking ratio setting.
Greater accuracy in setting the controls can
be achieved by using a D VM to measure the
±18V outputs.
a. Set the 0 to +18V supply's output to the desired
value using the ±18 VOLTAGE control.
b. Set the 0 to —18V supply's output to the desired
value (equal to or lower than the magnitude of the +18V
supply's output) using the TRACK VOLTAGE control.
3-19 Overcurrent Protection
3-20 All three outputs are individually protected against
overload or short-circuit damage by separate current limit¬
ing circuits. The circuits for the +18V and —18V supplies
are factory adjusted to limit the output current to 0.255A
±1 5%. The circuit for the +6V supply is factory adjusted
to limit the output current to 1.275A ±15%. The current
limits are set by adjusting resistors R3 in the +1 8V, R13 in
the —1 8V and R23 in the +6V supply. (See paragraph 5-47
for current limit calibration procedures). No deterioration
of a supply's performance occurs if the output current
remains below the current limit setting. If a single load is
connected between the +18V and —18V outputs, the circuit
set for the lesser current limit will limit the output.
3-21 Operation Beyond Rated Output
3-22 The supply may be able to provide voltages and
currents greater than its rated maximum outputs if the line
voltage is at or above its nominal value. Operation can ex¬
tend into the shaded areas on the meter faces without dam¬
age to the supply, but performance specifications cannot
be guaranteed.
3-23 Connecting Loads
3-24 Connect each load to the power supply output
terminals using separate pairs of connecting wires. This
minimizes mutual coupling between loads and takes full
advantage of the low output impedance of the supply. Load
wires must be of adequately heavy gauge to maintain satis¬
factory regulation at the load. Make each pair of connecting
wires as short as possible and twist or shield them to reduce
noise pick-up. If shielded wire is used, connect one end of
the shield to the power supply ground terminal and leave the
other end unconnected. The 6235A's chassis ground term¬
inal is located on the rear of the supply.
3-25 If load considerations require locating output power
distribution terminals at a distance from the power supply,
then the power supply output terminals should be connected
to the remote distribution terminals by a pair of twisted or
shielded wires and each load should be connected to the
remote distribution terminals separately.
3-26 Parallel Operation
3-27 Two or more power supplies can be connected in
parallel to obtain a total output current greater than that
available from one supply. The total output current is the
sum of the output currents of the individual supplies. The
output voltage controls of one power supply should be set
to the desired output voltage, and the other supply set for
a slightly larger output voltage. The supply set to the lower
output voltage will act as a constant voltage source, while
the supply set to the higher output will act as a current-
limited source, dropping its output voltage until it equals
that of the other supply. The constant voltage source will
deliver only that fraction of its rated output current neces¬
sary to fulfill the total current demand.
3-28 Special Operating Considerations
3-29 Pulse Loading. The power supply will automatically
cross over from constant-voltage to current-limit operation
in response to an increase in the output current over the
preset limit. Although the preset limit may be set higher
than the average output current, high peak currents as occur
in pulse loading may exceed the preset current limit and
cause crossover to occur and degrade performance.
3-30 Output Capacitance. An internal capacitor across
the output terminals of the power supply helps to supply
high-current pulses of short duration during constant-volt¬
age operation. Any capacitance added externally will im¬
prove the pulse current capability, but will decrease the
3-3