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Application Notes
This chapter is intended to give helpful information concerning practical applications of the unit.
All electronic loads are subject to the impact of source characteristics, interconnection inductance
and feedback loop characteristics, which can give rise to unexpected instability or poor dynamic
behaviour. The information given here will assist in understanding the factors involved. The initial
sections below cover general considerations, while later sections provide greater detail on the
particular characteristics of each operating mode.
Grounding the Current Monitor Output
A scope will often be used to view the voltage and current waveforms, particularly when using the
transient capabilities of the load to investigate the behaviour of a source. Take care to select a
suitable point to connect the scope ground, as voltage drops on the interconnecting cables
(particularly transients caused by inductance) can give misleading results. The Current Monitor
Output from the load is designed to avoid multiple grounds, as it provides common mode
rejection for differences from the load negative terminal up to a few volts. The negative terminal
of the current monitor must be connected to the load input negative terminal somewhere in the
circuit. If there is already another ground connection, then use that same point, otherwise the
best ground point is usually the negative terminal of the source.
Note that if the load is used with a source having the positive terminal grounded, then any
instrument attached to the Current Monitor negative terminal (and therefore also to the load
circuit negative) must be fully floating, to avoid grounding both terminals of the source.
Sources
Batteries are a low impedance source and, apart from the possibility of inductance in the
interconnecting leads, they are generally easy to use in conjunction with an electronic load. The
dropout facility should be used to protect batteries from being damaged by excessive discharge.
Electronic supplies have active feedback networks whose dynamic characteristics often interact
with the load. When that load (like this instrument) also includes an active feedback controlled
network, whose dynamic characteristics in turn depend on the nature of the source, it will be
apparent that the behaviour of the resulting system can be impossible to predict.
Source resistance
If a source has significant resistance (including the resistance of the connecting leads), so that
the voltage falls as the current rises, it is important to ensure that the voltage across the load
terminals remains at all times above the minimum permissible operating voltage.
The particular considerations concerning source resistance which apply in Constant Power mode
are discussed in the section covering that mode below.
Source inductance
Source and interconnection inductance has a major impact on the behaviour of the load: the
fundamental characteristic of an inductance is that it generates an emf to oppose any change in
current. As the current rises, the emf generated by the inductance reduces the voltage across the
load terminals, perhaps to the point where the load saturates. Whenever the voltage falls below
about 3V the transconductance of the power stage changes considerably; this changes the
damping factor of the feedback loop and alters its dynamic behaviour, possibly giving rise to
overshoots or even oscillation.
Whenever the load current falls, the inductor will generate a voltage transient which might exceed
the voltage rating of the load. The unit is fitted with varistors designed to absorb non-repetitive
transients up to 80 Joules, but repetitive energy up to only 2 watts. Even if the overvoltage
detector disables the load input, these varistors remain connected, so if either of those energy
limits is likely to be exceeded then some form of external protection must be added, such as a
catching diode across the inductor in the source.
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