12
Set Current Limit
With the output off, the standard status display shows the set current limit; with the output on it
shows the actual output current in all modes (CV, CC or UNREG). Both the actual current and
the current limit are shown to a resolution of 10mA.
The current limit can be set directly from the numeric keypad: press the NUMERIC SET I key,
enter the new value using the numeric keys and confirm by pressing OK. The broad principles of
keypad entry are explained in the Initial Operation chapter, which should be read by new users.
When SET I is pressed the AMPS display changes to show the present current limit value in a
smaller font, with the message
Enter I or ESC
below it, and the OK lamp starts flashing.
The new current limit is then entered from the keyboard, e.g. 12·34A is entered as 1, 2,
·
, 3, 4.
As it is entered it is displayed directly below the present setting, replacing the prompt message.
The minimum current setting is 0·01A; the maximum setting is 50.00A, i.e. there is no over-range
capability.
Pressing OK at any point will set the current entered with any remaining digits set to zero,
e.g. 1, 2
·
, 3, OK will set 12·30A; 1, OK will set 1·00A. When OK is pressed the current limit is
changed and the display reverts to the standard status mode.
Pressing ESCAPE at any time during the sequence, or making no key press within 10 seconds of
the previous one will cause the display to return to its original reading before SET I was pressed.
Entering a value outside the range maximum (including trying to enter 3 digits before the decimal
point) or trying to enter more than 4 digits will cause the buzzer to beep; the last key entry will be
ignored.
The current limit can also be set using the rotary jog control. Pressing JOG SET I will illuminate
the I JOG SET lamp and the AMPS display will again change to two lines. With the output ON
the top line continues to show the actual output, but the additional lower display now shows the
present current limit; in the lower display, the JOG indicator ( ) is positioned under the digit that
was last jogged. Whilst the I SET lamp is lit, each further press will move the JOG indicator
one digit to the left; the selection 'wraps round' such that when the largest value of jog increment
has been reached the next press returns it to the lowest. The default position at power-up is
under the LSD, i.e. the lowest jog increment is selected. The jog steps that can be selected are
10mA , 100mA and 1A.
With jog enabled the current limit can be incremented or decremented with the rotary jog control
with a step resolution indicated by the position of the JOG indicator. The output immediately
follows the setting, i.e. no OK is required. With the output ON and in CV mode the upper line of
the display showing actual current will be a lower value than the current limit being ‘Jogged’ in the
lower display; however, if the output goes into constant current mode (indicated by the CV status
message being replaced by CC) the upper display, showing actual current, will follow the current
limit value being ‘Jogged’ in the lower display.
Note that the dual display during JOG SET I is primarily of use if the output is ON; with the output
off both lines of the display show the
Limits
value and change simultaneously as the current
limit is jogged.
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.
