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Are there any adverse effects from isolating the AC power supply on my electronics bench?

4ringfan

happened to show me an Ebay listing the other day and it was like feeding candy to a baby.  My dad, he can be such an enabler!  Hehe!  So, the long and short of it is that I have acquired this BEAST (a Powervar 12 Power conditioner) and I am planning to modify it so that I can isolate and supply power to my entire bench.  The Powervac 12 is capable of supplying 12Amps at 120VAC, so the only thing I was thinking that I needed to be concerned with was possibly exceeding the supply capabilities of the transformer -- however I find the possibility of drawing that much power quite remote. So...


Here are my primary questions:


  1. Is there any reason or side effect that would contradict hooking the entire bench on the isolated secondary side of a transformer?
  2. Are there other things that I should be concerned with safety wise or otherwise plan for?
  3. Modification suggestions?
  4. In general, is this a good or bad idea?


I haven't found too many articles that talk about this, but here are links to a few that I have looked at:


isolation transformer - Electronics Forums


RadiolaGuy.com : Sonny's Tech Tips


http://www.signaltransformer.com/sites/all/pdf/IsolationTransformers%20Increase%20Safety%20of%20Electronic%20Systems.pdf


isolation - Why are we not always isolating the mains supply? - Electrical Engineering Stack Exchange


I have read the following and I take it that I should not connect any of my test equipment to the isolation, only projects that I am working on. What about power supplies?  What is the take on this?:

 

"DO NOT plug your test equipment into the isolation transformer, just the radio. I've seen folks wire their isolation transformer to a plug strip with all their equipment plugged into it and wonder why their "scope's" ground lead went up in smoke when they connected it to the radio chassis. If all your equipment is plugged into the isolation transformer, then nothing is "isolated"! Only the radio (or television) under test should be plugged into the isolation transformer!"


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  • 0

    Beast is right. Look at the coils for the transformer!image And it is bolted down so that it will not fly away under load!?!image

  • 0 in reply to clem57

    Yes, Clem, Hehe, that HUGE BEAST of a toroidal transformer is what sold me too!  I mean, how can one not love such a massive chunk of electromagnetic joy? image

     

    Come on people!  I'm looking for HELPFUL input on how to ISOLATE power to the bench, but do it SAFELY.  Don't let me do something STUPID!  Ok, well, please don't let me do something STUPID.image

    Lets just make this even MORE interesting!  image  What if I decided that I wanted to isolate my ENTIRE bench INCLUDING my test equipment!  What then?

    What PROTECTIONS should be in place to prevent DAMAGE to myself and my babies (test equipment, soldering station, power supplies, frequency generator, etc.)?

    Any issues to be worried about with SUDDEN load changes on the secondary?

  • 0 in reply to 4ringfan

    I'm guessing you don't want to listen to your father's advise  ... I'm sure he has been able to offer some good advise.

     

    Isolating transformers are really only designed to work with double insulated appliances.

     

    On a class 1 appliance any fault current is passed through the earth and trips the breaker.

    If you isolate your bench, then you remove that potential difference and hence the protection.

     

    Since this PowerVar maintains the Class 1 by connecting the earth to the sockets, how is the transformer wired?

    .. does it actually float the secondary or is it just providing a 120v/120v transformer and the neutral is joined both sides.?

     

    Also inside the device has a black box which contains ??

     

     

    The other issue I see if you isolate your bench is the phase voltage (and neutral) can float to anything, rather than it never exceeding the 120/240v.

    Switchmode type supplies have high harmonics, and its quite likely that the impedance of this device will not remove/smooth that component, which could then pass to other loads.

    It may also cause the secondary voltage to rise either as an AC or something closer to DC.

     

    Bench Isolation

    If you are trying to ensure you don't get a shock, then I recommend looking at RCD's. (You call them GFCI)

    A Residual Current Device detects an inbalance between the phase and neutral currents and trips which interrupts the power.

    Residual-current device - Wikipedia, the free encyclopedia

     

    They are available as portable, socket outlet, or switchboard mounted, and the medical/children version will trip at 10mA instead of the normal 30mA.

    They are mandatory on all NZ homes now. (except for some special circuits).

     

    Mark

  • 0 in reply to mcb1

    Mark,

     

    My 17 year old son wants to buy a motorcycle.  I've tried to convince him that it is a bad idea.  Death, dismemberment, high insurance cost, in ability to use it year round, weather, road hazards like animal carcasses, etc.  Nothing seems to be deterring him from his quest to do something potentially stupid.  I keep hoping that someone else other than me may convince him that it is a bad idea because God forbid he listen to his old man.  I think we all have a little rebellious teenager in us.  I'm no different.  My father is very intelligent and has a few more years of experience on me, so I should generally bow to his knowledge and wisdom.  Sometimes, however there is more than one way to skin a cat.  That is why I'm coming to the educated engineers on E-14 for alternative insight as to what should or should not be done.  He can't always be right... OR CAN HE?  Eeek!

     

    I will be modifying the PowerVar.  All primary filters, protection, etc will be left in place.  The secondary will be stripped down to basically the transformer windings.  I do not know what is in the black box as I do not have the PowerVar in my hands yet, but I would guess that it is a noise filter of some sort or other.  I don't want either of the secondary wires to tie to ground in any way.  I want to be able to stand in a puddle of water and touch either lead from the secondary.  My dad, John, mentioned possible capacitive coupling to ground just because of the proximity of the wires to the ground.  I will need to do some dielectric testing to see if that is a problem.  I suspect that I will also carry a good earth ground through to all of the equipment for shielding.

     

    You mentioned that the bench voltage could float to anything, possibly exceeding the 120VAC.  If the transformer is a 1:1 ratio, should the secondary floating voltage not track the incoming primary voltage fairly closely?  Where do you see this increase coming from?  Is the increase not an increase in total voltage, just that the output voltage can float higher than actual ground due to capacitive coupling within the transformer?  http://electronics.stackexchange.com/questions/150540/connecting-center-tapped-transformer-to-earth-ground-or-why-am-i-b…

     

    The collapsing magnetic field may also cause a voltage increase when the unit is turned off, but I believe there may be a way to mitigate that problem, but I'm waiting to see if anyone can tell me what that is before I divulge the idea.

     

    As far as the GFCI goes, could I just put that on the secondary?  It would be non-invasive and just detect any increase in current on one leg over the other.  I wasn't thinking about having the monitored ground resistor as part of the circuit, however.  Something similar to this:

    image

  • 0 in reply to mcb1

    Hi Mark,

    I appreciate your insights on this question. There are so many side effects to each of the choices that we make that it is always good to get input from the experts. I was not aware of any potential problems with switch mode power supplies when used on a isolated supply. These are good things to know.

    John

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