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Double Insulated PSU, 14vdc output, 150vac potential

  1. Aug 15, 2012 #1

    I'm obviously new to the forum here so greetings to everyone. I am here because I have a problem with some equipment relating to my job and I'm trying to learn as much as I can about it and the effect that it is having on our equipment. I'm not looking for a 'solution' so much as just trying to understand the implications of the situation.

    Here is the outline of my problem:

    We have a device that runs off of a 14vdc external PSU. This equipment has experienced various problems in the past and has come to my desk as something to investigate. Unfortunately, due to the nature of this equipment I can not go into as many details as I would normally prefer to do. The end result here is that when I place an o-scope probe on the chassis I clearly see a 150vac peak to peak voltage. It's not a perfect sine wave but it is close with the exception that part of the top of the wave is cut off.

    Now, I do have some experience working with with electronics, but I am by any means an expert. I do know that this is a 'double-insulated' power supply and the because of that the 14vdc is 'floating'. I also understand that the reason I see this 150vac wave form is due to the floating nature of the PSU. My question here is how normal is it to have such a large ac voltage like this? How possible is it that this is causing some of the strange behaviors we've been seeing?

    I had also found that if the ac power plug is not fully inserted then the PSU will output pure ac just as if it's coming directly from the wall outlet, though I don't actually know how much current is there.

    I hope I've stated my problem clearly enough, but if there is anything that I need to clarify, please just let me know. I sincerely hope that someone here can help me to understand the situation.
  2. jcsd
  3. Aug 15, 2012 #2

    jim hardy

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    Science Advisor
    Gold Member

    No help yet?

    Welcome to the forum.
    A question well phrased is [itex] 2/3 [/itex] answered...

    where is the o'scope probe's 'ground' lead connected when you see this AC waveform?
    That's the little wire with small alligator clip.

    well, from what you describe, the 14 volt line probably "floats" with respect to earth but it surely has a return line for load current to get back into the floating supply.

    It might be perfectly normal if chassis is not tied to earth and you are measuring it with an earthed o'scope. Or it could mean trouble. You need to find out a little bit more about the supply.
    Does this thing have a two prong or a three prong power plug?
    If there's no safety prong and chassis is surrounded by plastic when machine is asembled, then you probably need to probe on the 14 volt line instead of chassis.

    BUT>>> you said it's got 150 volts on the chassis?
    Chassis is usually earthed for personnel safety.

    Get a GOOD ohmmeter and check for continuity between chassis and all the power cord prongs.
    I like analog Simpson 260 meter, or a Triplett 630, because they apply 9 or 15 volts on highest ohm scale which is enough to turn on a diode bridge, unlike the cheapie meters that only apply 1.5 volts or digitals that are typically 2 volts.
    I would expect continuity between chassis and third (safety) prong. Safety prong will be the longest one, it's for safety so is first one in and last one out.
    If there's no safety prong i would expect to find no connection at all between chassis and power prongs, but if there were one it would not be astonishing. The question then becomes "is it intentional?"

    If you find a couple diode drops, like a volt or two between chassis and one of the two power prongs on the plug, then chassis is tied to an input power rectifier bridge which explains your 150 volt waveform.
    Most switching supplies i've seen keep chassis away from those conductors so take a good look to see whether that connection(if it's there) is intentional or by by accident. That means visually tracing the tracks on the circuit board to see if one goes to a chassis mounting screw(intentional), or it's something accidental like a wire that got caught under a screwhead or a solder splash or the plastic film worn through on a capacitor that's laying against the chassis....

    here's how to read diode drops with an analog ohmmeter:
    1. Determine internal battery voltage by either looking, or -
    place meter on highest ohms scale probably RX10K or 100K
    connect it to input of a DMM, select DMM on DC volts and read battery volts on DMM
    ohmmeter should read input resistance of DMM probably 10 megΩ

    2. Connect ohmmeter to unknown device under test.
    Full scale deflection is zero volts across unknown
    midscale deflection is half battery voltage (hopefully 4.5 or 7.5) across unknown
    zreo deflection is all battery voltage across unknown
    ohm scale is non-linear so read on one of the linear scales and interpolate to find voltage across unknown

    Better DMM's have a diode position that'll put 2 volts across the unknown, some even 4volts for checking LED's.
    Get to know your test equipment and it'll be a great friend.

    do the above and you'll know a lot more about your supply.

    old jim
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