User manual

Page 26
ELR/ELM 5000 Series
www.elektroautomatik.de
[e-mail]
EA Elektro-Automatik GmbH
Helmholtzstr. 31-33 • 41747 Viersen
Germany
Fon: +49 2162 / 3785-0
Fax: +49 2162 / 16230
3.2.3 Resistance regulation / constant resistance
Not available with load modules of series ELM 5000.
3.2.4 Power regulation / constant power / power limitation
Power regulation, also known as power limitation or constant power (CP), keeps
theDCinputpowerofthedeviceattheadjustedvalue,sothatthecurrentowing
from the source, together with the input voltage, achieves the desired value. Power
limitation then limits the input current according to I
IN
= P
SET
/ U
IN
as long as the
power source is able to provide this power.
Power limitation operates according to the auto-range principle such that at lower
inputvoltageshighercurrentcanowandviceversa,inordertomaintainconstant
power within the range P
N
(see diagram to the right).
While the DC input is switched on and constant power operation is active, the condi-
tion “CP mode active” will be shown on the graphic display by the abbreviation CP,
as well it will be stored as internal status which can be read via digital interface.
Constant power operation impacts the internal set current value. This means that the maximum set current may
notbereachableifthesetpowervalueaccordingtoI=P/Usetsalowercurrent.Theuserdenedanddisplayed
set current value is always the upper limit only.
3.2.5 Dynamic characteristics and stability criteria
The electronic load is characterised by short rise and fall times of the current, which are achieved by a high band-
width of the internal regulation circuit.
In case of testing sources with own regulation circuits at the load, like for example power supplies, a regulation
instability may occur. This instability is caused if the complete system (feeding source and electronic load) has too
littlephaseandgainmarginatcertainfrequencies.180°phaseshiftat>0dBamplicationfullstheconditionfor
an oscillation and results in instability. The same can occur when using sources without own regulation circuit (eg.
batteries), if the connection cables are highly inductive or inductive-capacitive.
The instability is not caused by a malfunction of the load, but by the behaviour of the complete system. An improve-
ment of the phase and gain margin can solve this. In practice, a capacity is directly connected to the DC input of
theload.Thevaluetoachievetheexpectedresultisnotdenedandhastobefoundout.Werecommend:
80 V model: 1000uF....4700uF
200 V model: 100uF...470uF