# How to check if a DC power supply is a floating source?

Imagine a laptop power supply with 2 prongs and I'm using this as a power supply for a circuit, and the output of this circuit is an input signal Vin for a data acquisition hardware.

How can I verify if this signal is floating? My idea is to apply continuity test between gnd of the Vin and neutral of AC in of the laptop power supply. Could it be said that then the power supply is a floating supply if continuity test doesn't beep?

• I would be very surpirsed to find one that isnt-... live mains at my laptops keyboard ugh – PlasmaHH Nov 1 '15 at 21:32
• what do u mean? – user16307 Nov 1 '15 at 21:33
• There's no way a two-prong power supply could be safety certified if it weren't floating. – Ignacio Vazquez-Abrams Nov 1 '15 at 21:37

There are a couple of ways that you can verify if the power supply is floating.

1) The easiest method involves using a piece of commercial test equipment called a "Hi Pot Tester". This is a device that provides a current-limited high-voltage (either AC or DC) across the device under test.

You simply short both of the output DC terminals together and connect to the Hi Pot Tester Ground lead, then short both input AC terminals together and connect to the Tester HOT lead. Apply voltage and ensure that the Tester does not show excessive current.

You should follow the test procedure that your country's Electrical Authority provides for the proper usage of the Hi Pot Tester. In Canada, our test voltage is (Two times AC Line Voltage) + 1000V for one minute or about 1250 Vac for one minute.

2) Simple method but not as thorough as (1) above: Connect a pair of identical resistors in series and connect the free leads of the resistors to the power supply output terminals. Connect the middle connection of the series resistors to earth ground.

Measure the voltage across each of the series resistors. The voltage should be identical across each resistor.

Pick the resistor value such that you get about 100mW dissipation in each of the resistors.

By UL standard 840 or IEC 61010-1 (I can't recall which one exactly) (if it is sold in America) it must be isolated from line voltage. The European union has basically the same rules for anything CE marked.

I would check on the case if it is Class II (with Roman numerals), as this refers to power supplies with either a double or reinforced insulation barrier between the input and the output. Class II supplies do not rely on an earth connection to protect against shock hazard. Many laptop power supplies are Class II. The symbol is a double square (seen below). You should be able to do your continuity test to make double sure.

= floating powersupply