When an instrument is connected to a USB port, the hardware will be powered, resulting in an inrush current, which is higher than the nominal current. After the inrush current, the current will stabilize at the nominal current.

USB ports have a maximum limit for both the inrush current peak and the nominal current. When either of them is exceeded, the USB port will be switched off. As a result, the connection to the instrument will be lost.

Most USB ports can supply enough current for the instruments to work without an external power supply, but this is not always the case. Some (battery operated) portable computers or (bus powered) USB hubs do not supply enough current. The exact value at which the power is switched off, varies per USB controller, so it s possible that the instrument functions properly on one computer, but does not on another.

Older Handyscope HS3's (SN# 14470 and lower) are more sensitive to this problem.

In addition to the above, when the Handyscope HS3 generator is switched on, an extra circuit in the hardware is activated, resulting in more power consumption from the USB port. When the generator output is connected to a load with a (relative) low impedance, the generator will have to supply power to that load. This results in even more power consumption, meaning a higher supply current.

As explained before, a USB port can supply a limited amount of current to a connected device. When this maximum current is exceeded, the USB port will be switched off. As a result, the connection to the instrument will be lost.

The dimensions and polarity of this connector are:

Besides the external power input, it is also possible to power the instrument through the extension connector, the 25 pin Sub-D connector at the rear of the instrument. The power has to be applied to pin 3 of the extension connector. Pin 4 can be used as ground.

The following minimum and maximum voltages apply to both power inputs:

Note that the externally applied voltage should be higher than the USB voltage to relieve the USB port.

The Handyscope HS3, Handyscope HS4 and Handyscope HS4 DIFF are delivered with a special PS2 external power cable.

One end of this cable can be connected to a PS2 mouse or keyboard connector on the computer, the other end can be plugged in the external power input at the rear of the instrument. The cable also allows for the original mouse or keyboard connector to still be plugged in. The power for the instrument will partially be taken from the PS2 port of the computer.

In case a PS2 port is not available, or the computer still can't provide enough power for the instrument, an external power adapter can be used. When using an external power adapter, make sure that:

• the polarity is set correctly
• the voltage is set to a valid value for the instrument and higher than the USB voltage
• the adapter can supply enough current (preferably > 1 A)
• the plug has the correct dimensions for the external power input of the instrument