Aim TTi LD300 Fonctionnement page 19

Because the LEVEL controls set conductance, it is easy to get fine adjustment of the setting of
low resistance (high conductance) values in this mode. Conversely, Constant Resistance (CR)
mode is more convenient for high resistances. When the load is conducting, the WATTS &
OHMS position of the METERS switch can be used to view the equivalent resistance value of the
load.
The dropout voltage capability is active in this mode; if it is not required set the control to 0V.
Constant Resistance (CR)
In this mode, the load implements the equation I = (V – V
has a special effect in this mode: it acts as an offset to the start of the resistance characteristic.
Below the dropout setting, no current passes; above that voltage level, the current rises linearly
with a slope defined by the resistance setting. At low resistance settings, the action is similar to a
Zener diode with the threshold voltage set by the DROPOUT control and the slope resistance
defined by the LEVEL setting.
If the dropout voltage is set to 0V, this mode acts as a normal resistor with the load current
directly proportional to the applied voltage.
If the slow start circuit is used in this mode, the load will start at (nearly) zero Ohms and ramp up
to the set resistance value. This means that the initial current is higher than the final current,
which is probably not the effect required. For this reason the slow start circuit is of little use in this
mode; if slow start is required, use Constant Conductance (CG) mode.
Note that high resistance settings on the 400Ω range are primarily intended for use with higher
voltage sources (even at 80V the dissipation is only 16W). Small errors in current produce
relatively large errors in the effective resistance. The resistance can be adjusted to the required
value by using the WATTS & OHMS position of the METERS switch to view the actual effective
resistance.
Level Setting
The LEVEL A and LEVEL B controls set the two operating levels, which can be selected by the
LEVEL CONTROL switch, the transient generator or an external TTL signal. If only a constant
level is required, either may be used. The values can be read by setting the METERS switch to
the LEVEL A & LEVEL B position. The measurement units of these readings depend on the
operating mode, and are indicated at the top of the display.
Level Control
The LEVEL CONTROL switch provides the options of constant operation at either the LEVEL A or
LEVEL B setting, TRANSIENT operation (alternating between the two levels at a rate determined
by the internal transient generator), EXTERNAL TTL (alternating between the two levels under
the control of a logic signal applied to the rear panel terminals) or EXTERNAL VOLTAGE control
(when the operating level is set proportional to a voltage applied to the rear panel terminals). The
remote control facilities are described in a later chapter in this manual.
Dropout Voltage
The primary purpose of the DROPOUT Voltage setting is to protect batteries from total
discharge. It provides a voltage threshold below which the load will cease to conduct current. The
same voltage is also used as the threshold for the slow start circuit. When discharging batteries,
set the dropout voltage to the manufacturer's recommended end-of-discharge voltage. If the
dropout facility is not required, set the control to 0V (fully counter-clockwise).
Note that if there is any wiring resistance between the source and the voltage sensing point of
the load then there will be a soft entry into the dropout condition – as the current starts to fall the
series voltage drop reduces, so raising the terminal voltage measured at the load. There is a
possibility of instability in this operating condition.
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) / R. The dropout voltage setting
dropout