ADInstruments PowerLab 2/20 Owner's Manual page 31

Figure A–5
The equivalent circuit of the
external trigger input, when
set up for a voltage level.
Figure A–6
The equivalent circuit of the
external trigger input, when
set up for contact closure.
Appendix A — Technical Aspects
2.9 V ± 0.25 V; once on, the trigger turns off at 1.8 V ±0.25 V, giving a
hysteresis voltage of 1.1 V. The external trigger input draws 2.2 mA at
5 V and 5.5 mA at 10 V.
The trigger input is optically isolated when set up for a voltage level,
as shown in Figure A–5. There is no electrical connection between the
external trigger ground and the ground of the trigger device. This
removes ground noise and current loop problems, and improves
static discharge immunity. In order for the external trigger to work,
though, a voltage must be applied between the outer ring and the
inner pin of the connector. Applying a voltage just to the centre pin
will not work. The trigger source must be capable of supplying at
least 1 mA of current to activate the trigger.
The equivalent circuit of the external trigger is basically an LED (light-
emitting diode) in series with a resistor (the second diode shown
below protects the optocoupler).
Centre pin of BNC
Outer ring
of BNC
In the external contact closure mode, the trigger input will respond to
a direct short between the centre pin and outer ring of the BNC. This
can be achieved with an external relay contact, a manual push-button,
or a microswitch. The trigger input is not electrically isolated when
set up for contact closure.
+5 V
470 Ω
1500 Ω
1500 Ω resistor
Optocoupler
Optocoupler
23