Every maker should have…[Pt.28] an Isolation-Transformer and a Variac
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- Опубліковано 12 жов 2016
- Roger explains why you should have an isolation-transformer for your personal safety.
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How does it work, what can you do with it and what is the difference to a Variac.
It´s cheaper and quite easy to build one of your own.
Wikipedia-entry for isolation-transformers: en.wikipedia.org/wiki/Isolati...
Wikipedia-entry for Variac and Autotransformer: en.wikipedia.org/wiki/Autotra...
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Very good, thorough explanation of the equipment and the dangers involved.
Thks for the reply Sir , I will keep in touch
Thanks! I have watched bunches of videos on isolation transformers, as well as variacs and I finally got the big picture. Oh, and I like your construction, by the way!
Got an isolation transformer that I made of two 230/24VDC 250VA transformers wired back to back. The variac is still in the old lab and I'm looking forward to getting that stuff.
One more thing comes in handy when testing mains-connected devices: a dim bulb in series with the device under test.
Nice video, there are variac's that are also isolated, Philips made a bunch of them, I have the 12NC242252900005 (newer version: 12NC242252900007). Very affordable second hand and really nice quality also.
grate video Sir
If you have collected some used/old low volt transformers over time, you can use them as a combined isolation and variac, by connecting all primaries in parallel to your mains source, and then swicth in/out the various secondaries in series as a variable output.
The secondaries should of course all match the minimum required output current, but may very well supply higher currents.
Tranformers may even have several voltage taps, which makes it more compact to build.
Yes. I have done that for my home lab. The variac is behind an isolation transformer.
The digital meter, is it not possible to power it off the input and so have it stay powered up the whole time?
After quite a long time I looked at this video. Thank you, all explained nicely and clearly. I'm a beginner in electronics, so such illustrative explanations help much.
But I suspect there is one dangerous idea in this video - grounding the variac. If you ground the variac (the neutral of it) and put it AFTER the isolation transformer as you proposed, you defeat both uses of the IT: the circuit is not any more (galvanically) isolated (it is referenced to the ground (and so neutral, null) through the variac) and also when trying to measure with Oszi you will again be at ground potential on the chassis and so said 100V would trip the fuse - or am I wrong? If you use IT it is so not so much important to know where the phase is, because both of the ends are "floating" - but it could be critical if not having an IT.
In this context also two beginner questions:
- is it not enough (if the DUT has its own powertrafo) just to lift (disconnect) the internal earth connection (or use a cable without ground (this is mandatory for IT and you could have mentioned it, although it can be seen in the video))? -This way the primary of the device is still referenced to the ground, but the secondary is floating (and we rarely service things in primary); I speculate you mostly need IT for the devices without the powertrafo
- in various US made videos we see people using a "current limiter" consisting of a lightbulb connected in series with a DUT: they use quite a big bulb. But we have different voltage here in Europe - what kindf of wattage would be suitable for a similar purpose here (or does that not work for 230VAC)
Oh, I saw your answer to my beginning remark in the comments below. But still, my questions remain.
On the variac what happens to the input neutral conductor? If it remains open circuit you are relying on the neutral to earth connection in the distribution box to complete the circuit. If you then use any local safety RCCD directly to feed your variac, I guess it will trip?
As I already explained in another comment: where I draw "earth" (not protective earth!) as the second connection to the variac, that was in fact meant to be the "neutral wire".
Because here in Germany many electronics people (not electricians) use the term "earth" interchangeable with "neutral wire" and know, that the neutral wire is connected to earth down in the distribution-box (or fuse-box as we call it here).
That´s were this confusion resulted from.
Hi there, what VA transformer do you recommend for using an analogue Tektronix oscilloscope 2430A and a Hantek DSO5210P, I cant seem to find how much current they consume. What VA is your transformer in your isolation transformer. Thank you
The isolation transformer shown in the video has 250 VA. That should be enough for nearly every piece of test equipment.
I do really like your series, lots of great advises! :-)
Just one comment, in some country's, can it be deadly to use an isolation transformer, because the way the electrical system is set up.
I think it include Deutchland (Germany), but guaranteed in Denmark where all power do cut off if you touch any live wire, unless you do use an isolation transformer, that keep delivering voltage until you dye or let go.
The ”cut off” is due to a GFCI, and those wont work if you use an isolation transformer, but instead you can touch either side of the secondary winding without getting a shock, since there’s no potential between the other side of the transformer and earth, the only way to get a shock is to touch both ends of the winding at the same time. So an isolation transformer is a lot safer when working on mains level electronics compared to an GFCI.
@@sstorholm In Denmark do vi call it HPFI but yes we are talking about the same:-) In Denmark, if you touch any phase, everything shuts off in a few ms. If you touch anything after an isolation transformer, nothin happens, unless you touch both wires and you get a ZAP as long you touch it. So to me it sounds safer to use the sstem that are build to shut off then a system designed to keep going if things happens:-)
Sebastian has explained it correctly. In your lab you absolutely want to avoid the shutdown of all electricity when accidentally touching the live wire. So e.g. even in industrial labs there are special AC-outlets without any protection like the "Ground Fault Current Interrupters" you mentioned and even without the Earth connection (for avoiding ground loops etc.). The chance of touching both wires accidentally with your left and right hand is minimal. Working without an isolation transformer in a hobbyists lab can give you either a potentially mortal shock or damaging your DUT by your uncontrolled movements when accidentally touching the live wire (with a bypassed GFCI or an older electrical installation without a GFCI) or you are suddenly standing in the dark when a GFCI is used.
@@KainkaLabs Thanks for your great comment. That is the other side of the coin, yes you will hate for all your lab gear to be shut off, it can screw up a lot of things. And yes you have to be extremely unlucky to touch both poles with each hands, and there are simply methods to avoid that. But my point is only safety, not all the other considerations. Some do play Lotto and do win and I was trying to say that from a safety point alone, a total shut off that happens automatically is safer then something that do not shut off. :-)
I am not saying that his advice is wrong or bad, just that the viewer may need to be told about what they have to choose between. That it is not black and white, that one solution is the one they have to use to be totally safe. :-)
When a great guy as him or you says something we know that it is right and worth listening to, you are both our mentors, so therefore my little comment.
Interesting way to measure with +100 Volt as a reference with an oscilloscope. You use an isolation transformer to float the device. Do you know that the former ground of the device is no longer floating (and at -100 Volt relative to earth) as soon as you connect the probe ground clip? This is because the clip ground is referenced to mains ground via the oscilloscope.
Would you describe this as safe?
You forget one thing in your train of thought: Because the connection to the live wire of the mains input of the DUT is interrupted by the isolation transformer, you would have to touch the chassis ground with one hand *AND* another voltage carrying pin with your other hand to get a (mild) shock.
Much more important (or better say disturbing concerning the measurement setup) in this way of measuring is the capacitive coupling of the primary voltage to the secondary of the isolation transformer which I described in another video.
Thanks for your quick reply. Are you sure? I assume in my argumentation that the person doing the measurement is referenced to earth. The +100 Volt inside the device is referenced to earth via the oscilloscope. So if the person touches the device ground, there should be a potential difference of 100 Volt. Not a very happy experience.
I have not yet seen the other video you are mentioning, but aren't the primary and secondary windings of isolation transformers wound in separate chambers to avoid the chance of an accidental isolation failure? That would also minimize capacitive coupling.
No. The DUT is floating relative to (protective) earth. That´s the basic purpose of an isolation transformer :-)
So (except for the stray current due to capacitive coupling in the µA-range) no current can flow from the device to earth when touching it.
.
Concerning the capacitive coupling see here:
ua-cam.com/video/YhDH7rOIuGg/v-deo.html
Even if they are wound in separate chambers there still is some capacity between them.
The stray currents can be minimized nearly down to zero by a shielding layer/winding between the 2 windings (then wound on top of each other). The shielding is usually a copper-foil which is externally connected to earth which leads the stray-currents to earth and protects from an accidental short between primary and secondary.
Thanks for the link!
I tried to ilustrate your point in following schematic: www.dropbox.com/s/762rg45bz6tyqh2/IT%20%26%20scope.pdf?dl=0 Am I wrong?
Why are the scopes even connected to PE?
Great video, after watching I got one question left; I don't want to ask stuppid question, I'm sorry for that, but I don't have an isolation transformer (jet), but can you do this trick, simply by grounding out the earth wire with a contra wall plug with simply making no earth connection between the device in test and the wall outlet? Thanks
No, because with the 2 "center"-holes in the wall-outlet you always have 1 as the "live wire" and the other one is always (local) "Earth". There is no way to directly connect a device to mains voltage without also having 1 wire as the live-wire.
KainkaLabs Thank you for reply. I'm Dutch, I mean also with a 2 prong plug connected(fase + 0= brown&blue) without the 3rd ground(greenyellow) connected like a early wall outlet without earthing?
The "0"-wire (Neutral-wire) is in fact connected to earth in your fuse-box. en.wikipedia.org/wiki/Earthing_system
So there is no way to get a "floating" connection to power in normal electrical home installations.
good after noon Sir , I will like to build my own isolated transformer 120 volts 5amp , can You help to build one or have a schematic to build one
If you need a "schematics" for an isolation transformer you perhaps shouldn´t start at all in building your own one.
Except for a primary and secondary fuse and a mains switch there really is nothing in it (as you can see in the opened case of my one in the video).
But if you are not familiar with building mains connected devices you better buy a readily built one instead of building your own :-)
I purchased an Bang Olufsen RT12 variac intended for repair work. Do you know how to check if it is an isolation transformer? I have found its schematic peel.dk/B&O/index.html website. Thanks! Eric, Sweden
With a multimeter in ohms-mode or continuity mode. Try all possible connections from primary to secondary. If only one of them gives a measurable Ohms-vlaue or even continuity it´s not an isolation transformer.
From the circuit diagram it´s clearly an isolationg-Variac, i.e. an isolation transformer with a Variac combined. So it should be safe.
KainkaLabs Thanks. That’s what I’ve heard from B&O fanboys but you don’t wanna risk it. Thanks for telling me how to check. Can I check all combinations on the input power prong to the output 2 prong right?
Two things that could have been explained better. Firstly, WHY is Variac usually an Autotransformer? It seems silly to not have the variable tap on an isolated secondary. Secondly, when you drew the autotransformer you made the input at the uppermost part of the windings. In actual fact, most variacs have the mains at about 90%, so there is additional windings "above" where the power comes in. This allows the small degree of voltage boost (eg on yours, you can output 260V).
You are right (and I think you know the answers :-)
First answer is of course that it´s by far the cheapest way because you only need a single winding.
Second answer is: I didn´t want to make it too complicated because people are new to electronics surely get confused when I make the drawing the way it is truly realized.
Thanks Roger - I'm a Variac owner and I also use one at work for testing the operation of small PSUs and LED Drivers under unusual voltages. I have always wondered why they make them as Autotransformer, and I've only ever seen them that way. I never even considered that price was the reason, since Variacs tend to be ridiculously expensive compared to the "normal" cost of a similar sized toroidal.
It would be nice if you would in English to an English video because more than 90% of the viewers are from outside of native German countries.
I didn´t draw "Protective Earth" but "Earth" which for better clarity should have been labeled as "Neutral Wire" (for not confusing it with "Protective Earth").
As you will know the neutral wire is connected to protective earth down in the fuse-box.
So Earth is not equal to Protective Earth.
Interesting. One thing to keep in mind on UA-cam, the US does not use EU/UK style RCDs. There very few commonly applied rules in the US on this topic. Our “national code” is just a suggestion to local governments. In no case are there any rules for whole house RCDs.
A huge number of video viewers are in the US.
Further, I have had endless and pointless debates on UA-cam with fools who have no concept at all about the huge variety of bonding issues. I am glad to see you avoided that swamp.
You should know one important point. The bonding between the current carrying neutral and the protective earth ground is in the service entrance.
If possible, avoid getting into this topic. It is sufficient to indicate that electronic isolation transformers do not carry the PE through to the circuit while medical isolation do. That accounts for the majority of bogus videos on the topic.
I watched your video and I think there's a dangerous mistake being said about working with an isolation transformer.
At about 10:45 you tell that "you don't have any life wire" and "not any phase", "you can't get bitten by mains voltage". This is not true!! This is only true when you touch a life connected point in the device with just one hand. But when you touch two different life connected points with both hands then current will flow through your body and it may kill you.
Of course there is still 230V. And it can be even the whole chassis that is life wired with 230V. This is often the case with old tube radios, tube guitar amplifiers, tube tv's, etc... With this kind of devices there's a 50% chance that the chassis will be connected to 230V mains power depending on how you plug the power cord into the wall outlet.
That´s right. But the risk is greatly reduced.
I’m glad somebody else commented about this, as I was wondering, too.
@@KainkaLabs, I’d encourage you to (at a minimum) pin this comment, as it seems to me that you potentially give a false sense of safety. Yes, it’s more safe, because the voltage can float... but you’re already talking about touching multiple points (with probes, anyway), which seems to imply that there’s no way at all to get a shock - which sounds like an impossible level of safety (because it is) to some, but may not be noticed by others, especially those who haven’t already built up an intuition about such things (or are finding things counterintuitive).
You say a variac is not isolating but then you draw one that is !!
You have confused the issue by your drawing of an isolated variac.
Why not just draw the auto transformer variac and not the isolated version?
Could you give the time of the video that you refer to.
That is explained from 19:38 on. Did you watch the whole video?
A variac is not per se nono-isolating. So the explanation of a variac starts with the general concept. And only later is the normally used auto-transformer explained. So I see no error in the didactics.
@@KainkaLabs Hi, Yes, 17:11 you draw an isolated variac
@@KainkaLabs Yes thanks, I watched the whole video.