15
Instantaneous Current Output
The current limit control can be set to limit the continuous output current to levels down to 10mA.
However, in common with all precision bench power supplies, a capacitor is connected across the
output to maintain stability and good transient response. This capacitor charges to the output
voltage and short-circuiting of the output will produce a current pulse as the capacitor discharges
which is independent of the current limit setting.
Power Limit
The maximum current at different voltage settings is limited by the power envelope illustrated
below:
The power envelope is set to give 80V/7.5A and 10V/50A under all supply conditions (both outputs
loaded); at lower output voltages the output power is restricted by the 50A current maximum.
When the power limit is exceeded, the status indication will change from CV or CC to UNREG. For
example, if the supply is set to 20V, with the current limit at maximum, and is connected to a 1
Ω
load, 20 Amps will flow and the supply will be in CV mode. As the voltage across the load is
increased, the power into the load increases until, at about 24V, the power limit is exceeded and
the supply changes from CV to UNREG.
Output Power (V x A)
If SHIFT, V x A is pressed the display shows the product of measured output voltage x measured
current in the form
nn.n VA
. . The V x A value is updated at the same measurement rate as for
V and I. Pressing ESCAPE cancels V x A mode.
Calculated Load Impedance (V/A)
If SHIFT, V/A is pressed the display shows the calculated result of measured output voltage
÷
measured current; this is a sampled result which can be updated by further presses of the OK key.
Pressing ESCAPE cancels V/A mode.
The display shows the calculated nominal V/A value with a resolution of 5 significant digits (10µ
Ω
resolution for V/A <1
Ω
). The accuracy is calculated from the uncertainty of the individual voltage
and current measurements and is indicated by showing the minimum and maximum possible
values (in brackets) beside the nominal. The uncertainty is worst when either (or both) V and I are
small.
This function is extremely useful for measuring low impedances at high currents, e.g. R
DS(on)
of
power MOSFETs. Always use the Remote Sensing function to eliminate the effects of connecting
lead and contact resistance when making low value V/A measurements.
Current Measurement Averaging
Press SHIFT,
I
AVG
to select current averaging;
I
AVG
shows in the top line of the display beside the
mode status. The AMPS reading is still updated at the same rate (4 times per second) but the
value displayed is the rolling average of the last 4 current measurements made.
QPX600D & DP
Power Envelope
