The PSU seems meaty enough.
Please tell me its not a Switch Mode Power Supply though.?
You would have to put the DVM test leads in series with the positive stub of your PSU and the positive lead supplying the U3S.
Switch the DVM to current scale.
There is usually two scales for the DVM. A socket for the red test lead that will measure milliamps and depending on the DVM, 10 amp isn't unusual for the other socket.
Unless its an auto ranging meter and that would cure that problem.