Floating ground

Discussion in 'General Electronics Chat' started by jonron, Jul 2, 2011.

  1. jonron

    Thread Starter New Member

    Jul 2, 2011
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    Hello

    Would be grateful for a little help please.
    I have a piece of kit that requires 9v DC that is ungrounded or floating ground.
    Whilst I have numerous 9v supplies I do not know how to test for floating ground. The manual for this kit states in big bold letters that anything other than a floating ground supply will have dire consequences, so I must get this right first time.

    Any help gratefully received.

    Regards to all.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Most (if not all) wall adapters I have encountered are floating.
    To check, use an ohmmeter set to the highest range and measure the resistance between the low voltage supply and the two or three prongs on the line cord. Your measurement should indicate an open circuit.

    Let me make this perfectly clear.
    Unplug the adapter from the wall socket.
    Measure the resistance between the low voltage side and the AC plug on the adapter (all three prongs).

    You never know who is reading these posts!
     
    Last edited: Jul 2, 2011
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Oops, you told them to unplug it AFTER you told em to measure it. That's at least 3 kids who fry themselves.

    I got a chuckle at your clarity, I could see it happening too!

    :p
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Judging from the posts I read on this forum I wasn't thinking of kids!
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    We don't know where jonron lives, but I suspect they might be from the UK area, or perhaps Downunder.

    In the USA, most wall outlets in "newer construction" homes (within the last 50 years) have electrical outlets designed to accept two-pronged or three-pronged plugs (two flat blades parallel to each other, and one round prong) and generally supply 115-120vac at up to 15 or 20 Amperes.

    The round prong is attached to earth ground. The wider flat blade is connected to neutral, which is connected to earth ground at the breaker panel/fuse box - but neutral is designed for power return, not a safety ground. The narrower blade is "hot", or line voltage.

    I am uncertain on how the wiring in the UK works, but I understand that it's generally 220VAC, with one side hot and the other neutral. I am uncertain if grounds are typically available, nor if the neutral is grounded as here in the States.

    Electrical wiring varies greatly worldwide. The US and Canada generally have 120/240 VAC split phase at 60 Hz, and much of the rest of the world uses 220VAC @ 50Hz.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Here in UK the domestic mains supply is (nominally) 230V, 50Hz. There are live (= hot), neutral, and earth connections. The usual mains socket provided here has three rectangular pins: two equal sized for the live and neutral, and one longer thicker pin for the earth.

    A ground is therefore available, but I notice that the majority of "wall-warts" and other small AC to DC adaptors have only two wire input. These things are generally isolated from the mains input (if not, they would be dangerous) so I would think that floating output is the most common. The OP would, of course, be well advised to check his particular supply.
     
  7. jonron

    Thread Starter New Member

    Jul 2, 2011
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    0
    Thank you MrChips, all checked and all working as it should. The way the manual was written making such a BIG point about floating ground I began to wonder if there was more to it.

    Regards to all
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    Yes, when the Sinclair ZX80 and ZX81 and similar computers came on the market, they came supplied with an RF modulator which you fed into the 300-ohm antenna connection on the back of your TV set. Most cheap TV sets came without mains transformers. Thus the chassis was live depending which way you plugged in the AC power cord.

    I hope you are not in a similar situation.
     
  9. radiohead

    Active Member

    May 28, 2009
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    I have an example of a floating DC ground. I had to troubleshoot a 12-volt device that was installed on a vehicle that used two 12 volt batteries in series for 24 volts. The client (unwittingly) installed the device across the terminals of the ungrounded battery. Still 12 volts, but it also created a 24v potential between the chassis of the device and the vehicle ground. He couldn't figure out why the thing kept arcing and sparking. Once it was explained that he should connect device to the grounded battery, the problem was solved.
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Good lord, I never noticed that! I hacked at my Sinclair over and over, adding a scraped Cherry Keyboard (thankfully white key tops so I could add in the extra symbols in pencil), some interface to use a "better" cassette deck for mass storage, and a hardware platform holding some wire wrapped RAM and even an EPROM (just 1 "E" back then) burner.

    I must have had a polarized plug on the little B&W TV I was using as a monitor as I never got zapped from this lash up.
     
  11. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The old computer or video to TV setup was normally not as dangerous as people are suggesting, because the antenna was typically coupled via low-value capacitors to limit the possible mains current. Otherwise to touch the aerial cable = death!

    Of course, the coupling caps could fail. :eek: Typically in the UK, TV antenna cables are 75Ω coaxial and sometimes connectors with uninsulated metal bodies are used. There must have been the odd nasty accident from time to time.
     
  12. WellGrounded

    Member

    Jun 19, 2011
    32
    2
    A "floating" DC ground is a PCB reference voltage that is somewhere between the total DC voltage potential of the whole circuit. For example an Op Amp circuit can have a -9 Volt and a +9 Volt power supply. The total voltage potential of the circuit is 18 Volts and the mid point is 9 Volts although the circuit chassis ground sees it as 0 Volts so that an AC signal can swing either way and become more - or more + than 0 Volts. The chassis ground "floats" at 9 Volts + above the lowest point with respect to the total voltage potential.

    Fire alarm panels are constructed the same way and the common building ground is attached to the floating ground of the PCB which is used as the chassis ground and is usually around 12 Volts(fire alarm panels operate on 24 volts). The most - point of the two 12 volt batteries in series is not attached to the building ground. A fire alarm panel can register a + or a - ground fault since most field wiring is near 0 volt potential or 24 volt potential, above or below 12 volts, so the the ground fault voltage is relative to floating ground voltage of 12 Volts. This method is used so that if a + or - pierced wire accidentally comes in direct contact with part of the building electrical system, such as an unrelated piece of electrical conduit it can be determined which wire, + or - is at fault.

    A car purposely has the metal chassis attached to the negative of the battery and the metal chassis now has battery negative potential and "ground"(0) Volts. By doing so if you wish to put power on a device you only need to run one wire from the battery positive, since the chassis will now be the - return if you can make a good metal contact to the metal chassis.

    "Earth" ground, "building" ground, "chassis" ground and "isolated" ground all have the word "ground" but have specific applications and the word "ground" is too often used as a generic term.

    As Sgt Wookie pointed out the main building electrical can be tied into an "earth" ground. As in the case of the fire alarm panel the "floating ground" on the PCB is actually a part of 3 different grounding circuits - earth ground, building ground and chassis reference ground.

    Danny
     
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