2.4 - VS [U3 Datasheet] | LabJack
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2.4 - VS [U3 Datasheet]


The VS terminals are designed as outputs for the internal supply voltage (nominally 5 volts).  This will be the voltage provided from the USB cable.  The VS connections are outputs, not inputs.  Do not connect a power source to VS in normal situations.  All VS terminals are the same.

Related specifications can be found in Appendix A.

For information about powering the U3 from a source other than USB, see the OEM information in Section 2.12.


What if some VS terminals are not working?  We hear this every so often, but since the terminals are all simply soldered into the same VS plane on the PCB it is almost impossible for some to work and some not.  Almost every time the actual problem is improper use of the screw terminals.  The only guaranteed connection with the screw terminals is when a conductor is securely clamped inside the terminal.  For example, touching DMM probes to the screw head of a loose terminal rarely provides a valid contact.




What current can be supplied by Vs? I want to power a device which draws 35 mA @ 5v.

That will be no problem.  You can usually draw up to 450 mA.  See the specifications in Appendix A.

Is this per line (or per device) or 450 is the TOTAL draw from all devices.

This comes from USB specs, which are per port.  For example, with a self-powered USB hub, each port is expected to provide 500 mA to each downstream device.

Do we have the ability to turn on/off VS in code?

No, VS is essentially a direct connection to the 5V power supply rail and cannot be turned off.  To do similar, you can use the DAC terminals, or you can use an LJTick-RelayDriver to gate 5 volt power.



I need the ability to keep my labjack u3 powered at all times even if the USB jack is accidentally unplugged or my Pc turned off for any reason. Can I use an adjustable dc dc converter and adjust it's output voltage to match precisely the Vs terminal voltage? I intend to connect my dc dc converter via a 200mA fuse.