HP laptops and thin clients do use a "sense" pin in its (3-wire) power supplies. Unlike Dell 3-wire supplies which use a 1-wire serial protocol on the 3rd wire to communicate supply capacity, though, HP devices apparently sense a DC voltage as indicative of power supply capacity. An interesting article on this subject is here:
HP Thin Clients PSU
The gist of this article is that HP laptops and thin clients use a resistor between the sense pin and the power rail to indicate the power capacity of the power supply; e.g., 65W. By setting the resistor size, you can spoof the laptop/thin-client into believing that a DC supply has any desired capacity.
As a last note: inexpensive power adapters for these devices, with the required blue-tip and supply capacity, are readily available. I bought several (
Power Supply AC Adapter For HP t740 Thin Client Desktop Power Cord Cable Charger | eBay) and they work fine on my T740s.
