Keithley 2461 Reference Manual page 259

Section 3: Functions and features
Vext line
The digital I/O port provides a line (V
inductive circuitry such as relay coils or low-power solenoids. You can use the built-in 5 V supply or
an external voltage supply for these types of applications. The externally supplied voltage can be up
to +33 V.
Do not apply more than 50 mA (maximum current) or exceed +33 V (maximum voltage) on the digital
I/O lines. Applying current or voltage exceeding these limits may damage the instrument.
Refer to the following figure for a simplified schematic of a sample control circuit for a relay. You can
externally power a different device by replacing the relay coil with the other device. The relay is
actuated by configuring the corresponding digital output line. Most of these types of applications use
an active-low (set the bit to 0) to turn the relay on (ON = 0 V). In the low state (0 V), the output
transistor sinks current through the external device. In the high state, the output transistor is off
(transistor switch is open). This interrupts current flow through the external device.
+5 V line
The digital I/O port provides a +5 V output. You can use this line to drive external circuitry. The
maximum current output for this line is 500 mA. A self-resetting fuse with a one-hour recovery time
protects this line.
If you are using this supply to drive a relay, it should be connected to the V
protected by the flyback diode clamp.
3-88
Model 2461 Interactive SourceMeter® Instrument Reference Manual
) with a flyback diode clamp that you can use when controlling
ext
Figure 132: Digital I/O Vext (example external circuit)
line so that the relay is
ext
2461-901-01 A/November 2015