Importance of isolation Transformers

Discussion in 'General Electronics Chat' started by automagp68, Mar 25, 2012.

  1. automagp68

    Thread Starter Member

    Nov 13, 2011
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    So i am fairly new to the tube world and i have been slowly learning tube amplifiers.

    I have heard from several people that using a variac for test amps is not a good idea because it is not isolated.

    I should use an isolated power supply.

    I was just an an electronics show and picked up one of these

    I do not think it is isolated however. I only paid $20 for it so no real loss i guess.

    Can someone shed some further light on this.
    Why is it good to test with isolation transformers? Do i really need one?

    Should i not use this thing?

    If i need an isolation transformer then what good is a variac?

    All i really wish to do is upon start up for the first time of an unknown tube amp or a freshly made home brew i want to be able to slowly turn it up. and check things out before we get to full wall ac voltage. As opposed to just plugging it in the wall and hoping the circuit is ok.

    Any ideas?
    Thanks
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    The problem with conventional AC is it is connected to the ground you stand. It is possible to get a lethal shock by grabbing one wire.

    A variac does what the name suggests, it varies the voltage of the AC. It doesn't isolate however.

    If you have a situation where the power company is giving you low (or much more rarely, high) AC voltage a variac can be handy.
     
  3. automagp68

    Thread Starter Member

    Nov 13, 2011
    81
    0
    Hi

    Ok so i was just doing some more research.

    A transformer used for a variac does not have a primary and secondary winding as we traditional think of it.

    It is just one winding tapped off in the middle. Hence it is not isolated.

    Where does the rheostat come in?

    IS the rheostat traditional pre or post transformer?

    I was looking at the schematic but it does not do a good job of showing it.

    An isolated would have a traditional transformer therefore it is separated my magnetic induction.

    Do i have that right? or am i wrong?

    So alot of amplifier techs of course just do there testing using a wall socket. In my case my wall voltage is very high. Almost 130.

    Is there any danger to me using a non isolated supply other then loosing the benefit of the isolation?

    So in essence its the same as doing testing right out of the wall except now i can control the voltage? Like a dimmer control for your dinning room light.

    I must exercise the same caution as if it were plugged into a wall though obviously.

    So if all of what i just said is true, how does having an isolated supply increase safety?

    The isolated supply basically is like a virtual ground. Its not really connected directly to earth ground other then though magnetic induction. And that is safer?

    I hope what i said makes sense. Please correct me on any of the above assumptions i just made.

    Thanks

    p.s im just an EE student so still learning my AC theory
     
  4. mlog

    Member

    Feb 11, 2012
    276
    36
    I may be wrong, but I thought a variac was an autotransformer with a variable tap on the transformer rather than a rheostat.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    A Variac does indeed have just one winding. But it doesn't have a rheostat. The knob controls a brush slider which moves along the exposed winding to vary the tap point and thus the output voltage.

    Yes an isolation transformer isolates the earth ground from the transformer output ground, thus reducing the danger of touching the hot AC while also being in contact with earth ground.

    An isolation transformer is only needed if you are poking around in the electronics while it is operating. An un-isolated Variac is ok as long as you are just using to to power up the electronics and not touching any of the electronic innards.

    A good safety precaution is to always power the electronics from a GFC outlet when troubleshooting any equipment. That will protect you from any shock between the mains hot wire and earth ground.

    I actually tripped a GFC once myself when I accidentally touched the blade of a plug that was partially out of a live socket. But I still received a pretty good jolt before the breaker blew.
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Doesn't the tube amp have an input transformer built into it? If so I can't see a problem with using a Variac to power the amp at a lower voltage. BUT all safety precautions should still be applied when in side the tube amp.
     
  7. ectmlk

    Member

    Feb 11, 2008
    21
    0
    I've been researching isolation transformers recently as well. In fact about to post a question about sizing one shortly.

    This video I found invaluable in understanding not just the isolation transformer as has been explained above, but the risks and hazard you could create if your not careful when using and not using an isolation transformer.

    http://www.toddfun.com/2011/04/30/isolation_transformers/

    It is long but worth taking the time. You should be able to identify if you purchased an isolation transformer from the video.

    Hope this helps

    mlk
     
  8. automagp68

    Thread Starter Member

    Nov 13, 2011
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    0
    Thanks for all the great replies and the clarification on the Variac.

    That makes perfect sense. Therefore the variac output ground and the wall socket ground are shared.


    Here is an interesting thing. I took the power supply i just bought apart.

    This thing has a Giant Transformer inside. And then a rehostate connected also. I think the rehostat is more for DC side.

    Not sure how it affects the AC side. The diagram for the trasnformer suggests single coil. As that is what is pictured. However it appears a normal transformer with with primary inputs and secondary outputs.
    Im not exactly sure how this thing is wired.

    For the fun of it i took an ohm meter and stuck it in one of the blades on the AC output and then touched that to one of the blades on the power cord for the actually power supply and it did show continuity. i.e no isolation there.

    So im still not exactly sure how this thing works.But at the moment although there is a transformer im assuming no isolation.

    Having the benefit of isolation of course does not allow you to go stick your hand on a 400v cap or something though lol correct?

    haha if so i think that is a miracle.
    Assuming you still cant go latch on to a 400v cap with your finger what is the real benefit?
    I get what it does, but if you can still be killed by a lethal voltage by touching the wrong thing even if you do not share the same ground then whats the point.

    If someone could correct that last paragraph that would be great.

    I think that is my primary question.

    You can still get a lethal bite with or without it.

    Does one have less of a chance of bite?
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    One consideration is the path for the shock. If the path is across you fingers then it's not likely to be lethal. If it goes through you body to the mains ground then it's more likely to be fatal.

    Also consideration is that 50-60Hz AC is more likely to cause a heart arrhythmia when going through your body then if DC does (DC pulses from a defibrillator are actually used to stop arrhythmias) . So it's important that you minimize the chance of AC going through you body by isolating it from the mains ground.
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    In the early days of electronics 'tube' equipment was sometimes built with a live chassis, or chassis parts at worst, or at best some live metal parts, exposed internally. Insulation relied upon the case, often wooden.
    Alternatively the metalwork was not 'live' but was not grounded either so could become live in a fault condition or acquire a high voltage from pick up.

    An supply via an isolation transformer is just one vital safety measure when dealing with high voltage equipment that is on the bench out of its case.

    Another is keeping your spare hand in your pocket so you don't inadvertantly touch something 'hot'.

    A variac is often used in servicing and prototyping to 'run up' kit from a lower voltage so any inherent short is limited. However tube circuits do not take kindly to this treatment and will not work at much reduced voltage.

    go well
     
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