How can I stop the 851N powering down when not in use?

Your 851N will power off after approximately 20 minutes of inactivity. This feature is called Auto Power Down or APD. This can be adjusted to power down (in 5-minute increments) between 5 minutes and 2 hours.

If you want your unit to remain switched on until you manually power it off, this feature can also be disabled.

If you wish to adjust this setting; simply navigate to the settings menu and scroll down to Auto Power Down and set your preferred option.