isolation transformer

[pete g]

hi, I would like info on how to use an isolation transformer. I have an rca and b&k iso transformer. I going to want to measure waveforms in a plasma tv. I'am going to be probing using both the hot and cold grounds. a friend of mine said I need to use two transformers one for the scope and one for the dut. is that correct? I thought I needed to isolate the scope only. thank you.

 

[hevans1944]

@pete g, you must either be very patient or have other resources! I see you joined the forum almost five years ago, waited almost three years to post one pretty simple question about voltage and current metering but did not receive a satisfactory follow-up answer... and now here you are again two years later with a question about isolation transformers for measuring waveforms in a plasma television. I hate plasma TVs because of the broadband RF interference they generate that interferes with amateur radio reception, but I will try to help you out.

The most basic use of an isolation transformer is to isolate something from the mains supply, one terminal of which is connected to earth ground. Usually your measuring instruments also have one terminal more or less connected to earth ground also. This means you cannot "float" that measuring terminal to measure anything that is at a potential above or below earth ground. An isolation transformer ahead of the measuring equipment will "isolate" its "ground" measuring terminal from earth ground and allow you to float that terminal... with some caveats that are very important.

The main caveat is the isolation transformer, while providing galvanic (DC) isolation between the power line ground and the measuring equipment ground, does not provide perfect AC isolation. There will be leakage capacitance between the primary and secondary and between the "isolated" secondary and earth ground. Some isolation transformers incorporate an earth-grounded Faraday shield between the primary and secondary windings that helps to alleviate the problems caused by this parasitic capacitance, but you should be aware that it always exists and can affect your measurements.

Even if your measuring equipment is battery operated, and therefore completely separated from the mains supply, there will always be some stray capacitance between the measuring equipment "ground" or common terminal and the device-under-test (DUT) common terminal. The effect of the stray capacitance depends on the waveform and the circuit you are trying to measure with your "floating" measuring instrument. Waveforms with high-frequency components are more likely to be affected than low-frequency waveforms. The best "recommendation" I can give you is to try it to see if the measurement results make sense. The alternative is to use a floating "true differential" measurement, but this is very much dependent on the common mode rejection capabilities of the measuring instrument.

As for using a second isolation transformer on the plasma television... this is probably not necessary, but it probably won't hurt either and will certainly be safer. You may have to connect a capacitor between earth ground and the plasma television chassis to provide an AC "ground" but that pretty much defeats the idea of using an isolation transformer to power the plasma television.

Many test equipments, such as oscilloscopes, can be "floated" by simply isolating the third-wire "green safety ground" to power the o'scope from just the line and neutral. The power transformer in the o'scope then serves the function of "isolation transformer". I have done this quite often, but there are limitations on how much above or below earth ground you can "float" the chassis of an oscilloscope before bad things happen to the o'scope. The o'scope manufacturer will generally not tell you this for obvious product liability reasons, so you do this at your own risk. I only do it as a last resort if an isolation transformer to power the o'scope isn't handy.

 

[Harald Kapp]

Many test equipments, such as oscilloscopes, can be "floated" by simply isolating the third-wire "green safety ground" to power the o'scope from just the line and neutral. The power transformer in the 'oscope then serves the function of "isolation transformer".

Hop, in my opinion that's no good advice. While technically this will work, it is unsafe. The safety ground is there for a reason (otherwise the manufacturer could have omitted it in the first place - and would have done it as it costs money) and should never be isolated. Using a separate isolation transformer designed for that task is a much preferable option.

 

[hevans1944]

Hop, in my opinion that's no good advice. While technically this will work, it is unsafe. The safety ground is there for a reason (otherwise the manufacturer could have omitted it in the first place - and would have done it as it costs money) and should never be isolated. Using a separate isolation transformer designed for that task is a much preferable option.

I agree, it is "bad advice" to even suggest this to a beginner who is unaware of the problems it can create. So, I went back and edited to color in red that part where I said you do this at your own risk.

I should perhaps mention that the first time I did this I was working on line-powered equipment and needed to measure the gate-to-cathode voltage pulses applied to an SCR with the cathode at line potential. I sweated bullets while making the measurement, fully aware that my oscilloscope chassis was "floating" at line potential with the full faith and credit of our local electric utility behind it. If the "ground" lead of the test probe had accidentally come into contact with a real earth ground there would have been a spectacular explosion and I would have had to go home to change my underwear. So, all you beginners out there and wannabe electricians: DON'T DO THIS!

Thanks, Harald, for spotting that.

Hop

 

[ver chan]

im sorry if you find it dangerous and impractical.

 

[hevans1944]

im sorry if you find it dangerous and impractical.

As @Harald Kapp reminded me, it is dangerous. Neither of us said it was impractical.

Electricity is dangerous, but that doesn't mean we can't use it safely. Defeating the green-wire safety ground means you are then responsible for the safety it provided. Not everyone, especially beginners, are fully aware of exactly what this responsibility requires if they are to remain safe.

Here in the States, when the third-wire plugs were first introduced, and before homes could be re-wired to comply with safety grounds, many consumers simply hacked off that prong with a saw and continued on their merry way, plugging the cord into two-prong outlets, blithely unaware that they had just defeated something that was designed to save their lives in the event of an electrical insulation failure in their appliance. Perhaps a few even died as a consequence, but the manufacturers came up with a solution: double-insulated appliances that didn't require a safety ground. Of course this doesn't protect idiots that drop their appliance in the bath water, but we now also have GFI (Ground Fault Interrupter) protected outlets and plugs to prevent that type of Darwin selection.

 

[pete g]

hi heavens, firstly, thank you muchly for your prompt reply. I want to make sure I understand. so, I should use the iso trans for the scope only? do not defeat the safey prong on the scope. does the dut need an isolation trans also? sorry, if I sound somewhat dense, but iam 77 years old, and definetly not as sharp as I used to be. thanks again.

 

[hevans1944]

@pete g you only need the isolation transformer on the o'scope. Its only purpose is to prevent the oscilloscope chassis ground from interfering with the television chassis ground through a mechanism known as a "ground loop." Do not defeat the safety prong on the line input side of the isolation transformer.

The oscilloscope chassis ground is normally connected to the "green wire" prong of its power cord, as well as to the common or "ground" lead of the oscilloscope probe. Use an ohmmeter to check for continuity between the probe "ground" and the "green wire" prong. If any continuity exists you must float the chassis of the oscilloscope on the secondary side of the isolation transformer by defeating the oscilloscope's "green wire" prong with an adapter plug.

Once you do this, the "ground" lead of your oscilloscope probe, as well as the oscilloscope chassis, will "float" at whatever voltage you connect it to. Ideally, this will always be the circuit common of the television and that will be at earth ground potential by virtue of the "green wire" prong on the television power cord.

I hesitate to recommend connecting the "ground" lead of the oscilloscope probe to anything other than the "green wire" ground of the television set. Remember to avoid touching the oscilloscope chassis when it is "floated" by the isolation transformer lest you become part of the "ground" circuit.

It will do no harm to connect the DUT (the television) to an isolation transformer appropriately sized for the power drawn, but I can't imagine a troubleshooting scenario where it will help either. The whole point of using an isolation transformer on the oscilloscope is to isolate its chassis and probe "ground" from the power line ground. There can be a potential difference between "grounds" in the internal wiring of a building. If these "grounds" are connected together a ground loop current will flow that can and will interfere with any measurements you try to make, especially low-level signal measurements.

You "restore" the oscilloscope ground connection at the DUT when you connect the oscilloscope probe "ground" to a suitable common or ground point at the DUT. However, no ground-loop current flows because the isolation transformer interrupts the ground-to-ground path between the oscilloscope chassis and the DUT chassis.

Note that modern televisions use a coaxial signal feed from the antenna or cable modem to the television. The outer shield of this coaxial feed should be connected to a good earth ground, either by you or the company that installed the cable. It is this coaxial cable shield that should be be connected internally to the "green wire" safety ground of the television power cord, and eventually to the oscilloscope chassis via the "ground" lead on the oscilloscope probe.

 

[pete g]

@pete g you only need the isolation transformer on the o'scope. Its only purpose is to prevent the oscilloscope chassis ground from interfering with the television chassis ground through a mechanism known as a "ground loop." Do not defeat the safety prong on the line input side of the isolation transformer.

The oscilloscope chassis ground is normally connected to the "green wire" prong of its power cord, as well as to the common or "ground" lead of the oscilloscope probe. Use an ohmmeter to check for continuity between the probe "ground" and the "green wire" prong. If any continuity exists you must float the chassis of the oscilloscope on the secondary side of the isolation transformer by defeating the oscilloscope's "green wire" prong with an adapter plug.

Once you do this, the "ground" lead of your oscilloscope probe, as well as the oscilloscope chassis, will "float" at whatever voltage you connect it to. Ideally, this will always be the circuit common of the television and that will be at earth ground potential by virtue of the "green wire" prong on the television power cord.

I hesitate to recommend connecting the "ground" lead of the oscilloscope probe to anything other than the "green wire" ground of the television set. Remember to avoid touching the oscilloscope chassis when it is "floated" by the isolation transformer lest you become part of the "ground" circuit.

It will do no harm to connect the DUT (the television) to an isolation transformer appropriately sized for the power drawn, but I can't imagine a troubleshooting scenario where it will help either. The whole point of using an isolation transformer on the oscilloscope is to isolate its chassis and probe "ground" from the power line ground. There can be a potential difference between "grounds" in the internal wiring of a building. If these "grounds" are connected together a ground loop current will flow that can and will interfere with any measurements you try to make, especially low-level signal measurements.

You "restore" the oscilloscope ground connection at the DUT when you connect the oscilloscope probe "ground" to a suitable common or ground point at the DUT. However, no ground-loop current flows because the isolation transformer interrupts the ground-to-ground path between the oscilloscope chassis and the DUT chassis.

Note that modern televisions use a coaxial signal feed from the antenna or cable modem to the television. The outer shield of this coaxial feed should be connected to a good earth ground, either by you or the company that installed the cable. It is this coaxial cable shield that should be be connected internally to the "green wire" safety ground of the television power cord, and eventually to the oscilloscope chassis via the "ground" lead on the oscilloscope probe.

 

[pete g]

hi hevans, thank you so much for your patience and help. I understand your advice and will follow your recommended suggestions. pete g.