What is floating supply?

[Vtile]

Like tittle suggest, what actually is a floating supply. What is a definition of a such.

This is more tricky question as it first seems, because all supplies are leaking in various ways and as such are not floating.

So the the definition is more through case to case functionality isn't it.

[capt bullshot]

In general I do call any supply a floating supply that isn't bound to a circuits or systems reference point / ground / earth.
Can be anything from an off-line linear transformer (secondary not grounded / earthed) or SMPS, a seperate secondary on a transformer, output of an isolated DC/DC converter, even a bootstrapped gate driver supply is considered a floating supply. Any kind of bootstrapped supply would do.

Usually there is a more or less conductive path to ground, but the individual floating supply reference point (whether it is positive or negative) is "floating" against the circuits / systems reference point. Can be everything from a DC offset to high voltage and / or high frequency stuff.

So it isn't floating in the sense of "totally isolated from the rest of the world".

[Berni]

Yes there is always some amount of leakage on a floating supply. What makes it called a floating supply is that the output has no on purpose made low resistance connection to any other part of the circuit.

If one tried to use a non floating supply where a floating one was needed then the act of connecting the supply to the load would cause a short circuit somewhere.

Also anything that is floating will have some limits imposed on how much can it float. Something like a battery might seam like it has no limits. But if you for example tried to float a battery at +100 megavolts obviously there would eventually be a high voltage arc over to something else making it no longer floating. Similar restrictions apply to isolated mains power supplies. Sure you have a floating 5VDC coming out of your wall wart but if you float that output more than a few kV above mains voltage there will be an arcover between the primary and secondary of the transformer or some other component (including the PCB)

[PA0PBZ]

[schmitt trigger]

 

I hope that they have a properly working GFI!!

[Ian.M]

[Brumby]

To me, a floating supply is one that can be connected to any circuit without an abnormal or problematic current flowing.  The only caveat being that there are no breakdown conditions involved.

Capacitive coupling of any circuit to any other exists between every circuit in the world (and beyond) - but the important factor is that the impedance of these is very high in comparison to the circuits of interest and when one point on each of two such supplies are shorted together, there is zero voltage difference and effectively zero current flowing.

[David Hess]

Floating power supplies almost always include a 3rd or 4th ground terminal so ground may be strapped to any of the other terminals as required.  For a bipolar supply with 3 terminals and a ground terminal, the middle terminal is common as shown below.

[Richard Crowley]

The ultimate floating supply is a battery.  The only leakage to the Real World is what you accidentally allow.

The simple, generic answer is simply a power source which has neither end hard-wired to some other circuit reference (like "ground").  Of course, beyond that it is a matter of what you are doing and how much isolation do you NEED vs. how much isolation does the power supply allow.

And there are two ends of the spectrum for "isolation".  At the low-end, how much leakage is there between the floating circuit and the rest of the world.  And at the other end, how far above (or below) ground can you take the  floating output without breakdown failure?  Perhaps most hobby or commercial bench supplies may be capable of several 10s of volts (or maybe even 100V or more?)  But, of course there are special experiments that require much higher isolation than that.

I am reminded of a high-voltage experiment that used a Van de Graaff generator to create the thousands of volts required for the experiment.  But they needed an isolated small voltage at the top to power some instrumentation circuit.  But how to get it?  Some creative person thought of simply using a small DC motor/(generator) as the top pulley for the belt. So the motion of the belt created the very high voltage, and also turned the little generator at the top. Brilliant!