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could be set such that is was above the actual output voltage but below the set voltage. This
could be used to trip the output under a fault condition which caused the load impedance to
increase and the actual output voltage to therefore rise above the OVP point.
Over-Current Protection
Over-Current Protection (OCP) can be set from 0·01A to 5·5A (QL355) or to 4.4A (QL564). If the
output current exceeds the set OCP the output is shut down (typically within 35ms).
To set OCP press SHIFT, OCP; the 10mA step JOG indicator will start flashing and the jog rotary
control can be used to increment/decrement the OCP setting in 10mA steps. Press OK to
confirm the new setting; to exit without entering a new value press ESCAPE. The factory default
setting is 5·50A (QL355) or 4.4A (QL564).
If the OCP is tripped the display shows the message
OCP triP
and the output is turned off.
Pressing ESCAPE at this point removes the message and the display now shows the preset
voltage and current limit. When the cause of the OCP has been removed (or the OCP limit
changed) the output can be turned on again.
Note that as with OVP, the OCP setting is not changed when the range is changed.
Note also that is possible and valid to set OCP below the set current limit. For example, the
power supply may be used to repetitively test a unit under test (UUT) which normally takes a
peak current of, say, 2 Amps. However, a faulty UUT would take a current of more than 2 Amps
and would be damaged by being left in a 2 Amp current-limited state. In this case the current
limit could be set to 2·1A, say, and the OCP set to 2·0A to ensure that a faulty UUT will trip the
supply off.
Output Protection
In addition to OVP and OCP for forward over-voltage and over-current protection, the output is
protected from reverse voltages by a diode; the continuous reverse current must not exceed 3
Amps although transients can be much higher.
Output Power (V x A)
If SHIFT, V x A is pressed the voltage display shows the product of measured output voltage x
measured current and the current display shows
UA
; the output power reading is continuously
updated at the normal measurement rate. Output Power mode is cancelled by pressing either
ESCAPE or V x A again. Jog is temporarily disabled (and the JOG indicators are turned off)
during the V x A display.
Temperature Trip
If the safe internal temperature limit is exceeded because, for example, the fan vents have been
blocked, the output is turned off and the display will show
OtP triP
. Pressing ESCAPE at
this point will do one of two things:
i. If the over-temperature condition has already cleared the message will be removed and the
display will show preset voltage and current limit. Assuming the cause of the over-temperature
has been rectified the output can be turned on again.
ii. If the instrument is still above the safe temperature limit the
OtP triP
message will flash
slowly ('lazy' flash) until the instrument has cooled, at which point the display will show preset
voltage and current limit again. Assuming the cause of the over-temperature has been
rectified the output can be turned on again.
Alarm Output
The recessed 2-pin connector on the rear panel is directly connected to an opto-coupled NPN
switching transistor (pin 1 emitter, pin 2 collector) which is turned on (i.e. switch 'closure')
according to the conditions specified in the Extra Functions section, see later. The default
condition is switch closure for any trip condition (OVP, OCP, SENSE or OTP). The maximum
open-circuit voltage permitted across the switch is 30VDC and the nominal sink current for switch
closure is 1mA.
