I have a device (pyro-electric array detector) that connects to the Pc vis a usb. The device itself has an additonal power supply as well.
Now a strange thing happens: If I connect the device to a Pc-tower, it works. But if I connect to a Laptop it does not work. On my colleagues laptop it works but only if he plugs in his laptop's power supply.
My assumption was that there must be a difference in the volatage/current supplied by the individual computers. And indeed there are small differences.
4.7V from the laptop and
5.23V from the PC. The current seems to be low
0.16mA and therewith well below the USB max spec.
What I tried next is to connect an external power supply to be able to adjust the power to the device's need.
For that I cut the USB cable, connected the D+/D- cables for data and connected the power leading to the device to the external power supply. From a control LED on the device I can see that it is happy with that. Now, however, the device is not recognized on the PC any more (Windows device manager).
Any advice on how I can
a)power the USB device by an external power supply and
b) have it working on the PC?
Any hints are appreciated. Forgive me if what I tried is foolish. My last electronics lecture was about five years ago and I never tempered with USB before.