Fluke PM2811 User Manual page 106

D - 2
This set-up will cause no problem as CH 1 supplies a constant current (CC) of 5A
and CH 2 supplies the additional current needed in constant voltage mode (CV
indicator).
A problem will arise when the current needed by the load decreases below 5A, for
example when the load resistance increases. This is illustrated by figure D.2.
Vset = 12V
Iset = 5A
CH1
CC
Vset = 10V
Iset = 5A
CH2
CV
Figure D.2 Channels connected in parallel to a load. Both channels are in
Constant Voltage mode.
CH 1 will now enter the Constant Voltage (CV) mode, as the current needed by
the load is less than Iset (5A). This implies that the voltage across the load will
change to the highest Vset (12V). As Vset of CH 2 is 10V, its down programmer
will become active, so that CH 1 will supply 3A plus the current the down
programmer of CH 2 will sink. So this set-up is not suitable for load currents, which
can be lower than Iset of the Constant Current channel.
For this case there are two solutions. You can set the current of CH 1 to a value
below 3A so that CH 1 will enter the Constant Current mode as indicated in
Figure D.1. Or, you can set both channels to the same voltage, so that there will
be a constant voltage across the load. When the voltages are exactly equal, the
down programmer will not become active and the current through the load will be
equal to the sum of the currents of the individual channels.
Be careful in using the Standby and Disable modes
Consider the following case. You have connected two channels in parallel. You
don't need the second channel to supply current, because the first channel can
supply enough current. You disable the second channel to switch it off. The down
programmer of this channel will try to sink its output to 0V. As the first channel still
supplies a voltage, the down programmer of the second channel will continuously
sink current. This causes power dissipation inside the case, which must be
prevented as the Power Supply is not designed for this use.
3.5A
12V
12V 3A
-0.5A
12V
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