How do I know whether a power supply is truely isolated

Discussion in 'General Electronics Chat' started by kevin.cheung19, Jul 19, 2012.

  1. kevin.cheung19

    Thread Starter New Member

    Nov 22, 2011
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    For awhile, I thought only Linear (transformer based) power supply are truly isolated. But I guess nowadays, since switching power supply is taking over. how to I know whether one power supply is truely isolated. Or to reword the questions for my knowledge sake, in a switching supply, how do I know whether it uses a transformer (isolated), or uses inductor (non-isolated)?
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    In industry we were taught to connect a suitable value resistor between two points to see if they were "isolated".

    If they are fully isolated the resistor will have no voltage across it, if they are not isolated the resistor will have a voltage and (if you have the right test equipment) you can put a 'scope across the resistor and see the waveform or voltage/current etc.

    In your case you could test by putting a 470k resistor between one of the mains inputs on the SMPS and the +12v output, then measure with a voltmeter. If the SMPS is fully isolated the voltmeter will read 0vAC and 0vDC across the resistor. And BE CAREFULL as this will make the resistor AND the output pins of the SMPS live and dangerous while the test is being done.
     
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  3. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Ask the manufacturer. Look at the schematic. There are numerous ways to tell.
     
  4. kevin.cheung19

    Thread Starter New Member

    Nov 22, 2011
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    The only thing I have is a spec sheet. It would be obvious that it has a transformer in there is a schematic was included, but unfortunately, the schematic was not there. The spec sheet stated "Isolation Voltage", and it is in the order of 4000VDC. Is this enough to tell that this is using a transformer in there? I need it to provide a 24V in series with a 200V DC. So I just don't want to make sure that I'm safe on that one.

    I was looking at the spec sheet below:
    http://www.cui.com/Product/Resource/DigiKeyPDF/EPSA_24W.pdf
     
  5. panic mode

    Senior Member

    Oct 10, 2011
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    it is isolated (or double isolated, see the double rectangle on the label? that means "double insulated").
     
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  6. kevin.cheung19

    Thread Starter New Member

    Nov 22, 2011
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    Oh, THANK YOU SO MUCH! This is exactly what I'm looking for. Now learned more about the signs on the label now!
     
  7. JMW

    Member

    Nov 21, 2011
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    Panic:
    Not hi jacking the thread, but where is the "double rectangle"?
    In the specs I see isolation voltages at for 10 ma current and Isolation resistance 100 M ohms at 500 VDC
     
  8. strantor

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    Oct 3, 2010
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    Why the resistor? Couldn't you take the same measurement without the resistor in place?
     
  9. ErnieM

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    Apr 24, 2011
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    I come from a Test Engineering background way back so I don't look at any spec as valid unless it has some numbers attached to it. If it is isolated, how much can it tolerate? 100 volts? 1000 volts? 1.21 gigavolts?

    And once it gives me a voltage it damn well better follow with either a current or a resistance. Either way you have a testable parameter.

    Otherwise I could connect two points with a one ohm resistor and claim an isolation of 1,000 volts (with a leakage current of 1,000 amps, but that gets hidden way down in the fine print).
     
  10. THE_RB

    AAC Fanatic!

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    Not really, "live" circuits tend to float due to capacitances and grounding effects etc.

    The resistor removes any floating voltage value that a high input impedance multimeter might detect, and gives you something quantifiable (as ErnieM suggested) because if it can develop a voltage across a resistor you not only know that isolation is compromised but it also gives you some real world figures what the voltage difference is and what current can be delivered.

    It was also common to have a little 5w mains voltage lightbulb with two wires and croc clips, this could be connected between two unknown points and indicate a voltage, or be used as a "resistor" when you put your voltmeter across the bulb. Some big equipment in industry could be "properly isolated" but still able to produce enough voltage/current to cause some visible light in a 5W bulb.
     
  11. ErnieM

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    Apr 24, 2011
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    If you read a voltage after adding a resistor between two points it does not mean the isolation is compromised. It means there is a voltage between those points. Your next mission (should you choose to accept it) is to track down the source of this voltage.

    When two points are isolated they are supposed to allow a voltage between them.

    Only when they are dead shorted together can there be no voltage between two points(because in that case it is the same point).
     
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    What you said is true ErnieM but it is nitpicking a little bit.

    In regards to my original suggestion which was to use a resistor from an AC mains pin to the 12v output of the supply with nothing else connected to the output (obviously) IF the supply is isolated there will be 0v across the resistor.

    The only exception to that would be if the PSU had an internal fault connecting those two points that were measured, and that very rare case could be tested by swapping the resistor to the other AC mains pin.

    Another test would be to put a variable PSU between the two points, and as that PSU is adjusted to different voltages there would be no current flow. Technically that's a better test than the resistor test but it's NOT a test that I would have suggested to someone! :eek:
     
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