You're under 40, aren't you?
www.tek.com/dl/51W_10640_1.pdf
“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.
TEKTRONIX RECOMMENDS ONLY
THOSE MEASUREMENT TECHNIQUES
THAT COMPLY WITH
SAFETY ENGINEERING PRINCIPLES
AND ENSURE ACCURATE MEASUREMENTS.
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Actually I am not. But this explains a few things. You are probably referring to Tektronix old tube equipment. They had a few models were the transformers started to fails after 30 years or so. There is a guy I I think in Florida that rewinds these transformers for collectors.
Rayma, that document does not apply to low voltage power supplies. Hence the Tektronix product for floating the scope on low voltage supplies. And this is why you will not be able to come up with one verifiable case were the scope burned up on a low voltage circuit.
Don't I? Though not identical with using the isolation transformer, it is similar in the fact that (once the potential difference between wires and person have equilibrated) there's no current flowing. It's current that kills, not voltage, though a higher voltage will create a higher current as Ohm put down in his laws.
You still feel something when using an isolation transformer and that's because of paracitic capacitance which will accommodate leakage current. So care still must be taken at all times.
Not neccessarily. Double isolated equipment (class II) has no connection to safety earth and is still safe.
Most commercial audio equipment is class II without safety ground. This can only mean that the secondary side is not mains earth referenced and could float at any potential with respect to it. Connection to a class I appliance is still possible, though. And that is because there's no loop the current can flow through.
When that loop is closed is when you get hum: a groundloop. Now the voltage differential causes a current.
A PC is a class I device that must be earthed to be safe.
An audio system may not be earthed if it consists of class II devices only. In that case a groundloop is not possible. When there's at least one other earthed appliance in the chain, a groundloop will occur. Depending on the surface of the loop (the larger, the worse) that may cause hum.
Once the power supply ground is connected to safety earth, it becomes mains earth referenced. Now there's no longer a separation from earth!
Earth has become part of the secondary side's return, as a result a current can flow through you to earth from the secondary side you thought was galvanically isolated. And again, if the voltage is high enough, the current through your body can become lethal.
As I explained above, that's because the connection to safety earth means a current can flow from the hot secondary side through your body to ground.
That's why isolation transformers (at least the ones I know) have a three prong inlet and a two prong outlet. It only works if the DUT is completely floating with respect to mains and earth.
Exactly the circumstance to use an isolation transformer on the DUT!
Sorry to say this to Tek aficionados but company I used to work for had a lot of Tek equipment. Then we discovered Agilent and found that they were in another league. Better equipment and service for the same kind of money... We never looked back. Perhaps nowadays Tek has caught up, but the damage is done.
Good post Jitter. I searched and searched for a schematic of a class II power supply for audio use and found nothing. Double insulated I can understand. But I do not understand why the secondary would not float up to random voltages due to transformer leakage current without being tied to safety ground. It seems to me that when you connected your interconnects on class II equipment you would have an initial voltage mismatch and a surge when the two class II devices were connected and their voltage level reached equilibrium.
I did find some information back here at.
http://www.diyaudio.com/forums/diya...udio-component-grounding-interconnection.html
That referenced other documents like AN004.pdf from Jensen. I did not thoroughly read the article, but I found on page 5 first paragraph it talked about devices being disconnected from ground can float up to 60v.
Do you or anyone else here have experience with the class II audio devices and explain the voltage mismatch or how it would be prevented on the class II devices.
I did find some information back here at.
http://www.diyaudio.com/forums/diya...udio-component-grounding-interconnection.html
That referenced other documents like AN004.pdf from Jensen. I did not thoroughly read the article, but I found on page 5 first paragraph it talked about devices being disconnected from ground can float up to 60v.
Do you or anyone else here have experience with the class II audio devices and explain the voltage mismatch or how it would be prevented on the class II devices.
It isn't prevented. Class II devices can and do float up to around half the mains voltage. You just have to be careful to switch off before changing connections, especially those which connect signal before 'ground' (such as RCA).
You are unlikely to find DIY schematics for Class II audio (or anything else). Safe Class II is best left to the professionals. Many DIYers have trouble implementing the much easier Class I safely!
You are unlikely to find DIY schematics for Class II audio (or anything else). Safe Class II is best left to the professionals. Many DIYers have trouble implementing the much easier Class I safely!
DF96 thanks for the clarification, now that I understand the characteristics of the class II power supply I will keep an eye out for them and remember to power them down before I change any connections.
It's unlikely you will find one. With a bit of common sense, it wouldn't be too difficult to build something that would comply with class II, but how can you be sure? DIYers haven't got the equipment to prove that their contraption won't put lethal voltages on exposed conductive parts if a fault were to occur.
The only way for us to be safe is to use safety earth in our projects.
There are also different levels of isolation. Interesting to see is this teardown of a medical grade power adapter (or plugpack as its called in that vid). From about 19:00 a closer look is taken at the transformer and then compared to a regular one. The differences in isolation are enormous.
So, if a mains transformer is of unknown isolation class, we must assume the lowest and use safety earth.
Oh but it does. As a test I took out and old DVD-player with a two-prong plug that I have here on the table with nothing connected to it. I'm going to measure some AC voltages with a high impedance DMM (about 20 MOhm).
The power ground and circuit ground are connected together the case, as usual. I'm in 230 V/50 Hz land.
Wrt = with respect to or referenced to (black lead/COM of the DMM).
L wrt N: 230 V (obviously, but to make sure my power strip is on).
Case wrt N: 66 V.
Case wrt L: 176 V.
I took the measurements a few times, they are steady and repeatable. They are the result of leakage, but it's only because of the high impedance of the DMM that I can measure them. If I put 1 MOhm between N and the case, the voltage drops to 27 V and if I make it 100 k there's only 3 V difference.
Oh, and touching it, I didn't feel a thing. This I could not say from a CD-player I have and why I started this thread.
Measurements repeated but now with an RCA lead to my DAC which is mains earth referenced:
Case wrt N: 0 V.
Case wrt L: 230.0 V.
Exactly as expected. The low impedance connection to safety earth makes it a god reference for measuring the voltages on the primary side, which the floating case obviously was not. It makes no difference for the secondary voltages, BTW.
Probably why manufacturers instruct to connect devices with the power off.
A very good article, I read it a couple of years ago. It mainly deals with groundloops in balanced and single ended equipment. It's not about how to safely measure in/on mains powered devices.
It also deals with groundloops and tells you not disconnect safety earth to cure them. They use isolators (transformers) instead. I did this once between a class I PC that was earthed to mains and my A/V-system that was earthed to the cable TV's coax shield. That created a huge groundloop that I broke with a transformer for audio signals. Later I went digital and used the optical output (Toslink) to achieve that separation.
Not really, but I don't consider it a problem as long as you don't put them in a loop. That way they all equilibrate to the same voltage potential (like the helicopter and the power wire) and that's it. But once a loop is created, those voltage differentials would probably drive a groundloop current (read: cause hum).
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Jitter i watched that video on the medical power supply and standard class II tear down. I wish he would have completely disassembled the transformers. I was under the impression that class II power supply transformers had to have a fuse/thermal fuse embedded in the transformer. Otherwise good clear closeups and lots of detail shown.
Perhaps for 50/60 Hz transformers, but I wonder about SMPS transformers. I would expect that the control circuitry also handles the protection and no thermal fuse is needed.
On another forum I came across this article on floating measurements with oscilloscopes. Worth a read.
I also made one crucial mistake in this thread: floating the DUT and then connecting the scope's earth lead to it also turns the DUT in a mains earth referenced device. Again, a shock can be received from the DUT (if the voltage is high enough), despite the isolation transformer.
Making safe measurements on higher voltages with an oscilloscope forces the use of differential probes or one of the other methods in the article. Unfortunately, floating the DUT is not enough and floating the scope is not a safe option.
I also made one crucial mistake in this thread: floating the DUT and then connecting the scope's earth lead to it also turns the DUT in a mains earth referenced device. Again, a shock can be received from the DUT (if the voltage is high enough), despite the isolation transformer.
Making safe measurements on higher voltages with an oscilloscope forces the use of differential probes or one of the other methods in the article. Unfortunately, floating the DUT is not enough and floating the scope is not a safe option.
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