20
ADDITIONAL TECHNICAL NOTES
This section includes useful technical information on these
topics:
• Reduce internal temperature or provide compliance
voltage limit
• Use Passive and Active interlocks
• Soft-clamp current limit
• Square wave response
• Remove the bail and feet from an instrument
•
Update firmware
•
Determine firmware version installed
REDUCE INTERNAL TEMPERATURE
OR PROVIDE COMPLIANCE
VOLTAGE LIMIT
At full current, the Maximum Compliance Voltage available
to the QCL is about 7.5 V below the maximum supply
voltage. This 7.5 V estimate decreases as operating current
decreases. To determine the best internal supply voltage for
your application, follow these steps:
1. Wire your QCL to the instrument and adjust the current
limit on the Limit screen and setpoint on the Control
screen for your application.
2. Press the Enable button to enable the output current. The
actual current displayed should match the setpoint.
3. On the Limit screen, select the blue MAX Supply Voltage
value and, using the Adjustment knob, adjust it down
slowly until the actual current starts to decrease. The QCL
current is now limited by the compliance voltage of the
instrument.
4. Using the Adjustment knob, now increase the MAX
Supply Voltage value until the current is no longer below
the setpoint. This is the optimal maximum supply voltage
for your setup.
5. On the Monitor screen, observe the QCL Voltage. This is
the voltage across the QCL at this operating current and
is the effective voltage limit to the QCL for your setup.
NOTE:
If you reach 18 V on the MAX Supply Voltage
input and the current is still not compliance voltage limited,
the instrument is operating at its lowest voltage and the
voltage limit cannot be reduced further.
USE PASSIVE AND ACTIVE
INTERLOCKS
The passive interlock is designed to trigger if Pins 1 and 2 on
the QCL D-SUB are not shorted. Use this interlock with doors
and enclosures that trigger an open circuit when opened.
The active interlock (ACTIVE LOCK BNC) can be used with
a temperature controller or other active system component to
disable the current output if the QCL exceeds an acceptable
temperature operating range.
SOFT-CLAMP CURRENT LIMIT
The QCL driver employs a soft-clamping current limit that
begins to act at a current lower than the calculated absolute
current limit.
The Limit Current is the
absolute
current limit, which is the
current that will not be exceeded under any circumstance.
Because of the soft-clamp circuit, the actual current where the
limit circuit begins to act is 20% below the absolute current
limit.
NOTE:
When operating at low currents around 100 mA
with the QCL500 LAB, the limit circuit begins to act around
32 mA below the absolute current limit.
If the output current is between this point and the Absolute
Current Limit, the output response to the analog input
setpoint is nonlinear. The result is that the drive current will
asymptotically approach the absolute current limit, and will
never exceed it even if the external modulation setpoint
voltage far exceeds the current limit.
illustrates the behavior of the soft-clamp current
limit. In this case the analog input voltage setpoint signal is
a triangle wave, with the amplitude set so that the driver will
attempt to overdrive the current limit.
Figure 22. Soft Clamping Current Limit Behavior