7
detectors can be recorded simultaneously. Or, by switching only one detector on, the desired
detector can be selected for measurement. The configuration is exceptionally useful for laser
scanning microscopes which normally use several detectors in different light paths.
Controlling Shutters
Shutters or other actuators can be controlled
via the high current switches of connector 2.
The switches can operate a voltage up to 20 V
and up to 2 A. The power supply for the
external load can be taken from connector 2 if
a current of 100 mA at +12 V or 200 mA at
+5 V is not exceeded. For higher currents an
external power supply must be used.
Since shutters and other magnetic devices are
inductive loads a flyback diode must be
connected across the load. For supply voltage
up to 12 V a resistor can be connected in series
with the flyback diode to achieve a faster turn-
off time. The value of the resistor should be
about the DC resistance of the actuator or
shutter coil.
The switches can automatically be shut down (i.e. set into the non-conducting state) when an
overload condition occurs at connector 1 or connector 3. Which of the overload inputs is used
for shutdown can be selected by jumpers on the DCC-100 board, see ‘Shutdown of High
Current Switches’.
Sometimes it is required to lock a shutter in the
‘closed’ state as long as a potential overload
condition persists. An example is a microscope
with a mercury or halogen lamp. When the
lamp is on, opening the shutter must be
inhibited. A solution is shown in the figure
right. The photodiode detects the light from the
lamp, and the amplifier sends an ‘/ovld’ to the
DCC-100. The signal can be connected parallel
to the /ovld from the amplifier.
As long as the lamp is on, /ovld remains ‘low’
and the shutter cannot be opened. This gives
additional safety against detector damage.
It must, however, be pointed out that an
absolute safety against detector damage cannot
be achieved in this way. If the lamp is switched on when the shutter is open it takes a few
milliseconds until the overload is detected and the shutter closes. This can be enough to cause
severe detector damage. Therefore, the best way is always an mechanical interlock so that the
lamp path is closed when the detection path is opened.
DCC
b0 Out
+12V
+12V
b1 Out
Shutter
Filter
Actuator
1
2
3
+12 to +20V
to switch
<+12V
to switch
Controlling shutters and filters
DCC
1
-
+
10p
100M
1k
BC850
AD820
BPW34
/ov ld
+12V
b0
Photodiode
in front of
shutter
3
2
HFAC26
+12V
shutter
Detector
Photodiode
light path
off-closed
/ov ld
Locking a shutter in a potential overload situation
