30th April
written by Todd Harrison

Tutorial using a Tripp-lite IS1000 isolation transformer.

  1. What I bought for my tech bench.
  2. Some models and prices of isolation transformers.
  3. How to alter a public version into a tech version.
  4. Helpful tips on using your oscilloscope safely with and without an isolation transformer.
  5. How to be a little safer when working with high voltage live mains


These would be good tech isolation transformers that don’t need to be modified:
4.30A isolation transformer, 115/115VAC, 500VA
2.50A isolation transformer, 120VAC/120VAC, 300VA
1.25A isolation transformer, BK Precision BK1604A
0.43A isolation transformer, 115VAC, 50VA


  1. 01/05/2011

    Hi Todd, Great video!

    FYI, I posted it to Dave Jones forum and Tweeted it.
    You really should join us techies on Twitter. :-)


  2. 01/05/2011

    Thanks John! I will.

  3. 01/05/2011

    Don’t know if you’re on Dave’s forum, but here’s a direct link to the post if you’re interested:


  4. [...] Harrison] has released a ~50 minute video covering everything you want to know about Isolation Transformers for protecting yourself and your costly equipment. Admittedly I have not given the subject much [...]

  5. Carlos

    36:07 Those unpopulated components seems to be varistors, not variable resistors.

  6. 02/05/2011

    @Carlos, I wonder why they left them out? I bought the HG grade. I hope they didn’t cheat me with a non-HG board. :(
    But then the board has voltage test points marked 2v 5v 7v and such. Maybe its just populated for testing maybe for some other product and they are just multi-purposing the some boards.

  7. Telford Dorr

    That’s good, as far as it goes. On my unit (built as a large breadboard), I also added a standard Edison light bulb socket in series with the secondary output of the transformer. Normally I keep a 15 amp screw-in circuit breaker in this socket (it’s a big transformer), but I can also screw in light bulbs and heater elements of various sizes, which act as short circuit current limiters. This is useful when testing unknown devices or developing projects which run off of line power, in that if there’s a problem with the device or you’ve made an error in wiring, it won’t pull more than a limited amount of power. Saves on the arcs and sparks…

  8. 02/05/2011

    @Telford, That’s a great idea! I will have to move the outlets to the front of my unit to do that. Can you send photos and/or part numbers of the screw in breakers (.5 to 8 amp) and heater elements. That would come in real handy.

  9. telforddorr


    Circuit breaker is a Mini-Breaker 15 amp unit, available at Home Depot in the fuses drawers. Have an email address to send the pictures to?

  10. Ian

    Why wouldn’t you power the scope via the isolation transformer, to remove the problem described at the end of the video?

  11. 11/05/2011

    @lan, If you mean the part about making your circuit dangerous again by strapping the secondary to earth mains ground through your scope’s ground clip or shorting out the secondary through two prob ground clips at two different potentials on the circuit under test, then plugging the scope into the transformer wouldn’t change anything. Both conditions would still exist. If you are not referring to that part please be more specific or give a time start/stop in the footage. Thanks.

  12. Ian

    You seem to go to all the trouble of removing the earth connection on the secondary of the isolation transformer, and then effectively re-connect it via the scope.
    If both the unit under test and the scope were powered from the secondary of the isolation transformer, there would then be no single point on the unit under test that would be at a dangerous potential relative to earth. What am I missing?

  13. 12/05/2011

    @lan, Your missing the fact that the scope is not floating. It has a ground pin and even if you connect it to the transformer the transformer passes this gound pin through to earth mains ground. Therefore when you connect your ground clip to any point on the circuit you just strapped your secondary windings, as of that point, to ground as well. This makes every other point at a different potential a high voltage to earth mains ground reference. At which point if you touch something you shouldn’t and you’re grounded by your feet or in any other way to earth then you will get zapped by the path introduced by the scope’s ground back to the secondary windings.

  14. Ian

    OK, fair point, so why wouldn’t you disconnect the earth pin, so the scope and unit under test is entirely isolated and floating? The way you are suggesting use the transformer seems to have little benefit, once you plug in the scope (with earth connection) you lost any isolation benefit.

  15. 12/05/2011

    @lan, That would work just as you say and I have been tempted to float my scope more than once in my life. I did touch on that in my video some, but here are my full thoughts on doing just that:

    1) My way “as in the video” does still have a huge benefit. You can connect your scope to any part of the circuit without the risk of turning your scope into a welder and without expensive differential probes.

    2) Even if you did as you say, you can still short your secondary through your scope with a 2nd probe so please be mindful of this if you do such a thing.

    3) You gain nothing doing this with the transformer. You can do this with your scope, just the same, directly to mains.

    4) Scope manufactures like Tektronix warn heavily in their documentation not to float your scope and here is just one reason. It’s not a safe practice to start floating equipment. Why not remove the ground from all your equipment; your scope, power supply, function generator, washer, dryer, stove, microwave, frig; why bother right? Well, it’s not safe that’s way, you know that. It would work just as you said, but floating equipment like that can get you in deep dodo when you make it a practice, get complacent or comfortable. One day you will misuse your setup or some kind of unforeseeable colossal mistake occurs or something unknown and unpredictable goes faulty at high voltages right where you wouldn’t ever expect. I can’t predict nor explain when or how this could occur. If I could I wouldn’t let it happen. But the unexpected still seems to find a way to occur. When, if, this ever happens in whatever unimaginable way then your last chance at safety is that your equipment has a chassis with a hardwired path to ground, instead of it being you to ground. You may see sparks, it may go pop and a circuit breaker may trip but you know for sure the fault was there and the hardwired ground saved your life. When you don’t expect something is when it always bites you in the ass.

    5) If you float your scope or remove the ground from your circuit under test then PLEASE be even more aware and even more alert to all possible dangers.

  16. 12/05/2011

    @lan, A second reason for part (4), if you need one:
    From the Tektronix Fundamentals of Oscilloscope Measurements:
    Under the section: “Floating a Conventional Grounded Oscilloscope”

    “WARNING This is an unsafe and dangerous practice and should never be done! Failure to follow safety warnings can result in serious injury or loss of life.

    Although this technique is a method to use existing equipment to make floating measurements and remove ground loops on lower frequency signals, it is an unsafe and dangerous practice and should never be done.

    At higher frequencies, severing the ground may not break the ground loop as the line-powered instrument exhibits a large parasitic capacitance when floated above earth ground.

    The floating measurement can be corrupted by ringing.

    Floating oscilloscopes do not have balanced inputs.

    The reference side (the “ground” clip on the probe) has a significant capacitance to ground. Any source impedance the reference is connected to will be loaded during fast common-mode transitions, attenuating the signal. Worse yet, the high capacitance can damage some circuits. Connecting the oscilloscope common to the upper gate in an inverter may slow the gate-drive signal, preventing the device from turning off and destroying the input bridge. This failure is usually accompanied by a miniature fireworks display right on your bench.”

  17. taw

    I have two of the black Trip Lite ISO’s (IS250 and IS1000) and after opening them up I am a bit confused. On both of them there are several gound connections. One is from the mains input and is tied to the chassis. Another is from a green wire coming out of the secondary winding. Number three is attached to one of the secondarys outputs and the last one is tied to the output outlets ground connection.

    Should I remove all of these except for the mains input chassis ground?

  18. 23/06/2011

    @taw, Only remove the wire(s) that connect the secondary windings to a ground point. Leave all other grounds connected but remember to test everything before you trust your work.

  19. robert

    Couldn’t you just use 2 isolation transformers? One for the test equipment and one for the DUT. Everything would be floating.

  20. 23/08/2011


    *”Floating” a ground referenced oscilloscope is the technique of defeating the oscilloscope’s protective grounding system – disconnecting “signal common” from earth, either by defeating the grounding system or using an isolation transformer. This allows accessible parts of the instrument such as chassis, cabinet, and connectors to assume the potential of the probe ground lead connection point. This is dangerous, not only from the standpoint of elevated voltages present on the oscilloscope (a shock hazard to the operator), but also due to cumulative stresses on the oscilloscope’s power transformer insulation. This stress may not cause immediate failure, but may lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation!

    Not only is floating a ground-referenced oscilloscope dangerous, but the measurements are often inaccurate. This results from the total capacitance of the oscilloscope chassis being directly connected to the circuit under test at the point where the common lead is connected.


  21. Ron

    If I was to use an isolation transformer like the GIS-500-R from Jameco would I just use a 3 prong to 2 prong plug converter when plugging a unit to be tested into the secondary outlet. And is there any advantage of the Tripp Lite over this unit.

  22. 18/01/2012

    Yes, you would have to use a 3 prong to 2 prong converter.

    The Jameco would be just fine. I would have picked the GIS-500-R for my lab but I needed 8amps on the secondary and the GIS-500-R only does 4amps. But really 4amps is quite a lot for most jobs that don’t use big motors.

  23. Sean

    I built an isolation transformer using two ex-APC battery backup transformers. It works great, but I’m wondering if I should be concerned with the phasing of the two transformers. In other words, should the input voltage to the isolation unit, and the output of the unit be the same phase? I haven’t tested it yet, but I’m guessing I can swap the leads that connect the two transformers together and change phase? Thanks Todd!

  24. 11/03/2012

    No, your secondary will be fine as is. It is an isolated system on the secondary.

  25. Sean

    Fantastic. Thanks again!

  26. Kelly

    You mentioned a list of isolation transformers in your original post that do not need to be modified. Please note that the BK Precision 1604A should NOT be on that list. I just purchased one and the secondary ground is connected to mains ground through the case. It is however connected with a standard screw terminal that is easily disconnected.

  27. 23/05/2012

    Kelly, Thanks for that tip but I don’t think you are understanding the concept. The ground on the output sockets SHOULD be connected to mains ground. That is simple grounding safety and you should reconnect your BK’s output grounds to the mains ground. What you don’t want is the secondary WINDINGS of your transformer connected to ground like in my example in the video. To test, unplug your transformer and meter the output socket’s HOT and Neutral to ground. You should get an open for both tests. If you get a short or near short in ohms then you have an safety issue and need to modify the transformer to make it a tech isolation transformer. But I’m sure you will not have to modify your BK 1604A as this is a well documented tech “service” isolation transformer. See the 1604A manual.

  28. brian

    If there are so many dangers and problems with floating scopes, how do handheld multimeters and handheld scopes get away without a connection to earth ground?

  29. Gary H

    Hi Todd,,I have a BK 1655a Power supply which is touted as an Iso xfmr,,would this be configured as needed for a tech version

  30. 25/05/2013

    @Gary, Yes the output would be in isolation.

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