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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.
Temperature Trip and Other Faults
If the safe internal temperature limit is exceeded because, for example, the fan vents have been
blocked, the output is automatically tripped off. The display will show a message indicating that a
fault has occurred and that the AC supply must be turned off and on again to reset the unit. If
over-temperature is suspected, the cause of the fault should be rectified and the unit allowed to
cool before the AC supply is turned on again.
Note that an over-temperature trip shuts down the primary side of the instrument and it is that
condition that is detected and causes the fault message to be displayed. However, other faults
may occur which would also cause the primary side to shut down and these will not be
distinguishable from an over-temperature trip. If the unit does not restart after being allowed to cool
then another fault might be present and the unit should be returned for service.
Logic Control Input and Output
Digital signal connections LOGIC IN and LOGIC OUT are available on the rear panel; press the
orange actuators of the screwless terminals, insert the connecting wire and release the actuator to
secure the connection. LOGIC IN is connected directly to the input of an isolating opto-coupler;
there is no series resistor. The input is active when an input current greater than approximately
1mA is forced between LOGIC IN and LOGIC COMMON. LOGIC IN can be set to enable the
output, disable the output, or be ignored when it is activated, see the Extra Functions section. Note
that LOGIC IN can only be used to enable/disable the output if the output has already been turned
ON with the front panel
key or by remote interface command; this permits the front panel
key to be used to turn the output off in an emergency, i.e. to override LOGIC IN. Note also that,
having turned the output ON with the
key, the ON lamp remains lit even when the output is
disabled by the LOGIC IN signal.
LOGIC OUT is an isolated rear-panel open-collector output that will sink up to 2mA when active
(‘switch closure’); the maximum voltage that can be applied to LOGIC OUT is 30Vdc. LOGIC OUT
can be set to be ‘closed’ or ‘open’ for output enabled or disabled, current limit (CI mode), power
limit (UNREG mode), or for any fault trip. The default condition is for LOGIC OUT to be ‘closed’ for
the Output ON; loss of AC power, which would deactivate the opto, is also reliably detected with
this setting. See Extra Functions section for further details.
Analogue Control
Rear panel connections V CONTROL and I CONTROL provide quasi-analogue control of the
Output Voltage and Current Limit respectively. The inputs are read 4 times per second by the
A-to-D converter and the output is set according to the selected input scaling. The input scaling for
both can be set as 0 to 10 Volts or 0 to 5 Volts for range maximum output. The Analogue Control
functions and scaling are both selected from the keyboard, see Extra Functions section.
The V CONTROL and I CONTROL inputs are differential, with a limited common mode range of
± 2.5Vdc with respect to the negative output. They should be driven from a ‘floating’ source such
as another power supply.
Analogue Control is activated from the Bus menu, see Remote Operation section. When
activated, the top line of the display shows which functions (V and/or I) have been selected and the
corresponding functions on the front panel (NUMERIC SET / JOG SET for V and/or I ) are inhibited
to avoid contention between Analogue Control and front panel operation.
Analogue Monitor
The output terminal voltage and output current may be monitored on V MONITOR and I MONITOR
respectively. The output scaling for both will be the same as that set for V CONTROL and I
CONTROL, see above section. V MONITOR and I MONITOR are always available at the rear
panel terminals and should be connected to a measuring system with ‘floating’ inputs such as a
DMM.
