Operation - Main Outputs
Setting Up the Output
With the POWER switch on (
l
) and the
output OFF the output voltage and current limit can
be accurately preset using the VOLTAGE and CURRENT controls; the left-hand meter shows the
set voltage and the right-hand meter shows the set maximum current.
When the output switch is switched ON, the
lamp lights; the left-hand meter now shows the
actual voltage and the right-hand meter the actual load current.
Constant Voltage
The output voltage is adjusted using the coarse and fine VOLTAGE controls; the CURRENT
control sets the maximum current that can be supplied.
Constant Current
If the load resistance is low enough such that, at the output voltage set, a current greater than the
current limit setting would flow, the power supply will automatically move into constant current
operation. The current output is adjusted by the CURRENT control and the VOLTAGE controls
set the maximum voltage that can be generated.
The CC lamp lights to show constant current mode.
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.
Efficiency
To improve efficiency at low output levels the overhead voltage to the output regulators is
automatically switched by a relay to a lower voltage; hysteresis at the threshold point prevents
unnecessary switching when the output voltage is set at about that level. Apart from an audible
‘click’ the user will be unaware that the relay has switched; there will be no disturbance on the
output. The higher power 15V/5A and 30V/3A models use electronic pre-regulation, instead of
relay control, to maintain a low overhead voltage.
Protection
The output has intrinsic short-circuit protection and is protected from reverse voltages by a diode;
the continuous reverse current must not exceed 3 Amps, although transients can be much higher.
In common with all series regulated single-ended power supplies, the unit is not capable of
sinking current provided from an external source.
If a voltage greater than the set output voltage of the unit is applied from an external source, the
internal regulator will turn off, no current will flow, and the voltage meter will read the applied
voltage. No damage will result providing the applied voltage does not exceed the maximum
output voltage of the power supply by more than 20 Volts. Application of a voltage greater than
this is prohibited.
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