Aim TTi LD300 Fonctionnement page 27

Zero Volt Operation
Although this unit is designed with very low internal resistance (less than 25mΩ) to enable
operation down to low voltages at high currents, there are occasions when a load is needed
capable of conducting the full current down to zero voltage. This can be achieved by connecting
an external offsetting supply in series with the load (with the opposite polarity). External voltage
sensing must be used, with the sense leads connected to the supply under test, outside the
series combination of the load and the offsetting supply.
An offset voltage of at least the operating voltage of the load plus any voltage drop in the inter-
connections is needed; the maximum permissible voltage is 6 Volts. The offset supply must be
capable of providing the full load current and must have a dynamic performance that does not
impair the stability of the combination; it should be protected against reverse voltages. If possible,
the use of batteries is recommended, but care must be taken to avoid full discharge or reverse
current.
Note that the combination of the offsetting supply and the load can apply a reverse voltage to the
source under test, so appropriate protection should be provided. It is strongly recommended that
a switch, capable of disconnecting the full load current, should be included in the circuit.
The load has a diode across the input terminals that will conduct current if reverse
polarity is applied, even if the INPUT ENABLE switch is released.
Multiple Unit Operation
It is possible to operate multiple loads in parallel in Constant Current mode. This will increase
both the current handling and power dissipation capability beyond that of a single unit. The
connections to the source should be matched as well as possible. Note that, because of the
additional feedback loops and inter-connections, stability issues are likely to be more
troublesome.
It may also possible to operate two units in series in Constant Voltage mode, but this should only
be used to increase the power dissipation capability, not the voltage handling. The open circuit
voltage of the source must not exceed the 80V rating of a single unit. It is, however, very likely
that instability will be a problem.
It is not recommended that multiple unit operation be attempted in Constant Power, Resistance
or Conductance modes.
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