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| Isolation transformer - Voltage between output and ground |
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| tony359:
Hi all, I built my isolation transformer box - with light bulbs for limiting - some time ago. I use it very often even when it's not needed as I can switch on and off the device under test very easily from there. Some time ago I had a motherboard powered via the isolation transformer (via switching PSU) and I was wearing an ESD wrist-band. At some point I felt a tingling sensation where the wristband was touching my skin. I measured the voltage between what I was touching on the Mobo and mains ground and I could read 30V! Today I checked and indeed I read 30V between the output of the transformer and ground. With the box disconnected from mains, I have infinite resistance between the secondary and primary and also between secondary and ground. Ground - of course - is not going to the ground connection of the output which is then completely isolated. So between mains and the output there is the transformer, nothing else. The case of the transformer is grounded but I read infinite resistance between the metal part and the secondary. I thought that that was not possible? I thought that because of the isolation transformer in the way, I wouldn't read any voltage between the output and ground? What am I missing here? Picture of the transformer attached. Thanks! Tony |
| bdunham7:
The most likely possibility is capacitive coupling somewhere in the system. This is an example of why you should not break the ground connection by default with an isolation transformer. If you want to measure leakage, do it in µA not volts. You can put a 10k resistor across your multimeter and read it that way, 1V = 100µA. Just out of curiosity, how is your primary wired on that transformer--the picture is cut off, but it looks like you have a 400V primary? |
| tony359:
Thank you! It's an optical illusion! It's wired to 0 and 230V (I have 230V coming in). I can measure current with the multimeter. I thought I didn't want to have ground as that would add a reference to mains ground, making the isolation useless? |
| bdunham7:
--- Quote from: tony359 on February 10, 2024, 08:41:25 pm ---I can measure current with the multimeter. I thought I didn't want to have ground as that would add a reference to mains ground, making the isolation useless? --- End quote --- If everything is working right and you don't have to change the ground reference point from where it normally is to some other node, then leaving the ground in place is fine. The isolation is achieved because the secondary output of the isolation transformer is not referenced to that mains ground. Without the ground connection, the ground plane of the DUT can float away to any random voltage and if there is even a small leakage someplace, you will observe what you did. If you want to use a grounded measuring device like an oscilloscope for a measurement where both connections you want to make are not at ground potential, then you can break the ground. Say you had your MOBO + SMPS sitting on an insulated mat and you wanted to use a small shunt resistor to measure the +5V supply current at the high side. You would break the regular ground connection, install the resistor, connect your oscilloscope ground clip to the load side of the resistor and the probe tip to the supply side, energize and take your reading. In this case, even though you have broken the ground, it won't float away as it will always be 5V below the actual ground. Isolation transformers aren't a magical safety blanket. You have to think every step through. Take your small tingling as a warning! :) |
| EPAIII:
Lets get this straight. First you completely bypass the safety features of your local power grid by using an isolation transformer. Then you attach a grounded strap on your wrist. OK, it probably has a resistance built into it to limit the current, but still, IT IS GROUNDED. Then you are puzzled when you can feel a shock. Granted, a small shock, but still a shock. That resistance in the ESD strap is limiting the current to that small value. My first thought is YOU DO NOT UNDERSTAND ELECTRICITY! :palm: I spent over 45 years working professionally with all kinds of electronic and electrical equipment. Voltage levels from 5V or less to 40,000V or even more. I worked on high power transmitters and on flea power logic circuits and virtually everything between. I NEVER WORE ANY KIND OR ESD OR GROUND STRAPS. NEVER! Why not? Because I valued MY LIFE more than some static sensitive IC which cost mere pennies. If you or I touch a point that is hot, that ground (ESD) strap COMPLETES the circuit and current can then flow. But I worked with those static sensitive ICs and other components. In 45+ years how many did I destroy with static electricity discharges? ZERO! That's not an exaggeration. It really was zero devices destroyed with ESD. The need for protection against ESD is tremendously over stated. It is probably only needed in the warehouse where the ICs and other static sensitive devices are handled by people who have no idea of how things work. They are not working with energized circuits so the ESD protection devices are a lot safer to use there. Likewise, for isolation transformers. Again, in 45+ years of professional electronic work, I NEVER used them. Actually, looking back, I think only ONE SHOP that I ever worked in even had one. And these were professional shops usually with a number of very experienced technicians working in them. If you have an isolation transformer between the mains power and the device you are working on, you have DEFEATED any safety measures that your local power company and local electric code has WISELY incorporated in the grid up to the outlet your transformer is plugged into. And, if you bypass that transformer with a ground wire, then you may as well not have the transformer in the first place. Somewhere outside of your house or shop, your local power company has, GUESS WHAT. A TRANSFORMER! And it is configured in a manner that was determined by years, decades actually, of experience with electrical safety. You ALREADY have a transformer! Adding a second one is redundant and unnecessary! If you really want additional protection against electric shocks, I would consider installing a ground fault device either in your breaker box or at the outlet you are using in your shop. But that may drive you nuts with multiple trips if any of your equipment has a leakage path in it. DO learn how to work safely with electricity! But, in doing that DO understand how it actually works. PS: I also never liked the ESD mats for the work benches. If I was stuck with one, I DISCONNECTED the ground strap before it allowed me to get shocked. |
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