Keithley 244 Instruction Manual page 9

MODEL 244 HIGH VOLTAGE SUPPLY
OPERATION
2-5. OVERLOAD PROTECTION.
a. The Model 244 has an overload current limiting
circuit which limits the output short-circuit current
to a maximum 14 milliamperes. The circuit is adjusted
so that the OVEKLOAD Indicator will be lighted when
the output current exceeds 11 milliamperes. When the
overload is removed the Model 244 will automatically
reset to normal operation and the OVERLOAD Indicator
will turn off.
b. Current Limiting Characteristic.- The Model 244
current limiting is shown graphically by the typical
V-I characteristic curves in Figure 4. The OVERLOAD
Indicator is factory-adjusted to turn on at approxi-
mately 11 milliamperes. However the output voltage
remains constant for current exceeding 11 milliamperes
as shown. The current limiting occurs typically from
11.3 to 12.2 milliamperes depending on the dialed
voltage.
0
10
12
f .
I
14 mA
FIGURE 4. Current Limiting.
c. Model 2441 Overload Protection Option.- An
optional protection circuit is available which can
bz used to limit the current of the Model 244 when
w>nitored by a Keithley picoammeter such as the
I!zrdel 414A. Consult your Keithley representative or
t:,e Sales Service Department for further information
regarding the Model 2441 Option.
2-6.
CAPACITIVE La4ix.
a. When a capacitive load is connected to the out-
put, Model 244 will deliver a charging current up to
14 milliamperes maximum until the dialed voltage is
developed across the load. During the charging time
the OVERLOAD Indicator will be lighted as long as the
output current exceeds 11 milliamperes. The OVERLOAD
Indicator will turn off automatically when the charg-
ing current is reduced to less than 11 milliamperes.
b.
Before reducing the dialed voltage with large
capacitive loads (exceeding 1 joule of energy), set
the OUTPUT Switch to "OFF". Readjust the dialed
voltage and set the OUTPUT Switch to "ON".
NOTE
A resonant condition (with increased output
noise) could result for large capacitive
loads. If this should occur, change the
load capacitance slightly to remove the
resonant condition and reduce the output
noise.
2-7.
OUTPUT NOISE. The Model 244 output ripple and
noise is less than 0.5 millivolt rms above 5Hz with
maximum load. The output noise is typically 200 WV
rms ~with no load. Transient power line noise will
tend to increase the output noise.
2-8.
SHORT-TERM STABILITY. The Model 244 stability
is specified to be + 0.005% per hour worst-case after
a two-hour warm-up. After warm-up, the short-term
stability is typically 0.003% per hour for constant
line voltage, load, and ambient temperature.
2-9.
TEMPFJATURE
COEFFICIENT. The temperature co-
efficient is typically 30 PPM1 C (with a maximum
temperature coefficient not to exceed 60 ppml C).
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