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Introduction
The Cairn OptoLED is capable of driving two LEDs independently, over a
wide range of currents. For pulsed illumination, switching times of less
than 100 nanoseconds are achievable, and digital control inputs to
support this mode of operation are provided. The standard operating
current range, which is set either by a front panel control or by an exter-
nal control voltage, is 0-5 amps (A), but the unit can also be configured by
the user to supply maximum currents of 1A or 2A. This current is limited
by components in the interchangeable LED heads, above which the
current driver will temporarily shut down in order to protect the LED from
damage. However, since LEDs can be transiently over driven above their
continuous current limit (which is usually determined by thermal consid-
erations), the protection circuitry also allows a higher transient current
limit to be set. By incorporating the protection-setting components in
each LED head, they can be tailored to match the maximum safe levels
for that type of LED.
In addition, digital inputs are provided to switch each LED on and off
independently, with rise and fall times of less than 100 nanoseconds. The
OptoLED can be configured so that a single control signal can be used to
switch between the two LEDs, according to its logic level.
Each channel has a meter, which can be switched to display either
current (in amps) or power (in watts). Both readings are average levels,
so for example if an LED is pulsed on and off at 2A with a 50% duty cycle,
the meter reading will be 1A. However, since the current control is a
10-turn calibrated one, then its setting will correspond to the current
when the LED is on, so this parameter can also be read if required.
Although in the short term the light output from an LED tends to be more
stable than that from other sources, it is somewhat temperature depen-
dent. This effect can be particularly noticeable during pulsed operation
with
duty cycles of more than a few milliseconds, as the consequent varia-
tion in LED temperature through the cycle can cause the light output to
change during the pulse. To correct for this, the option of optical feedback
to stabilize the output is also provided.
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