Design for an apparatus for more-safely experimenting with mains-powered project - Page 1
Forum section: electronics / projects

author: minime72706
author_rank: Frequent Contributor
timestamp: October 30, 2013, 05:05:52 am
content: Hey there -A few days ago I first considered the idea of using something more proper than an ungrounded equipment cable with an in-line fuse I installed. Current limiting would be wonderful, but I don't think there is a practical way to actually control or limit AC current in an active way. You see it in SMPS after the mains power is switched at a high frequency, but I've never heard of anything that'd fit this requirement other than a saturable reactor, which is not a sensible option.If no one has any magical wisdom on that topic, then would you agree that such an apparatus might have the following? :In-rush current limitingFiltering (X-cap and choke)Probably relay-switched (so I don't necessarily need to resort to a beefy switch among other reasons)Both a momentary button and either a ON/OFF button or toggle switch for turning the relay on and off (heh the momentary button is for when you utterly lack confidence in what you're about to plug in)Obviously a fuseBe properly grounded unlike my current "apparatus"That's all that comes to my mind and most of it is simple, but it'd be pretty awesome to have current limiting.Oh I'll probably include a current meter in the device too --- I have some ACS712 and ACS716 chips lying around.I may also just include a very simple and quick display indication using a bar-graph so one can really act quickly if they see something unexpected.I want to build something useful and to be proud of, heh...Thanks!

author: amyk
author_rank: Super Contributor
timestamp: October 30, 2013, 10:52:19 am
content: How about an isolation transformer.

author: dr_p
author_rank: Frequent Contributor
timestamp: October 30, 2013, 11:00:02 am
content: here's my take on that:https://www.eevblog.com/forum/testgear/smps-troubleshooting-aid

author: minime72706
author_rank: Frequent Contributor
timestamp: October 30, 2013, 12:43:37 pm
content: Quote from: amyk on October 30, 2013, 10:52:19 amThat came to my mind ~5 minutes after posting when I went to lie down to go to bed. It's a possibility.

author: con-f-use
author_rank: Unknown
timestamp: October 30, 2013, 01:13:59 pm
content: A Redidual Current protective Device aka RCD would be a good idea too.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: October 30, 2013, 02:02:10 pm
content: Why wouldn't a $5 GFCI work here (for the safety issues anyway)? Seems like it'd meet most of the requirements that you specified. What am I missing here?yes... yes.. I know... not nearly as sexy as your setup.

author: nowlan
author_rank: Frequent Contributor
timestamp: October 30, 2013, 02:45:14 pm
content: Variac?

author: Lightages
author_rank: Unknown
timestamp: October 30, 2013, 02:47:28 pm
content: Yes, technicians have been using a variac and an isolation transformer forever for troubleshooting. If you want to stick on a GFI it might help but it could also just trip all the time on iffy equipment and make it impossible to do anything.

author: pickle9000
author_rank: Super Contributor
timestamp: October 30, 2013, 02:56:10 pm
content: GFI's are good for "burn in". Variac and isolation xfrmr great for troubleshooting. Heavy items so if you can find them locally it's best.

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 30, 2013, 03:40:00 pm
content: Quote from: con-f-use on October 30, 2013, 01:13:59 pmExcept that a RCD (we call them GFCIs over here on this continent) isn't really compatible with an isolation transformer.The RCD shuts the circuit off when the current going out on the hot line is different than the current coming back on the return. In an ordinary grounded system, this could happen if current is coming back via some path through ground.If you put an isolation transformer before the RCD, the RCD is protecting against something that can't happen. Kirchoff's current law guarantees that the current going out one side of the transformer's secondary will be equal to the current coming back in the other side of the secondary. That's the whole point of the isolation transformer.If you put the isolation transformer after the RCD, the RCD only protects the primary of the isolation transformer, not the device under test.Neither the isolation transformer nor the RCD protect against faults between the hot and return lines.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 30, 2013, 03:46:17 pm
content: I actually purchased a 10A variac that should be here tomorrow - I think that's, psychologically, why I didn't mention an isolation transformer in my first post. (Yes, it is isolated)

author: minime72706
author_rank: Frequent Contributor
timestamp: October 30, 2013, 03:50:22 pm
content: Quote from: JohnnyGringo on October 30, 2013, 02:02:10 pmHeh, yeah, as you might expect - I want to build something. It might not be a terrible idea to include a GFCI for cases where I do NOT use the isolation transformer (variac). If I can fit everything I want in a PVC weatherproof dual-gang box, I will probably leave the option to use a GFCI outlet. The electrical box could then optionally connect to my variac or just straight into my power distribution strip.Quote from: dr_p on October 30, 2013, 11:00:02 amI kind of like the lightbulb idea - it'll have the nice added effect of making this thing look like an absurd prop.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: October 30, 2013, 04:35:02 pm
content: Quote from: minime72706 on October 30, 2013, 03:50:22 pmJust FYI, they also make GFCI breakers for the fuse box. It protects everything on that circuit.

author: nctnico
author_rank: Super Contributor
timestamp: October 30, 2013, 04:47:04 pm
content: Quote from: amyk on October 30, 2013, 10:52:19 amNo!Attaching a scope will ground the circuit. Better use a variac to lower the voltage to the bare minimum, a GFCI and a differential probe. Before attaching anything to the variac make sure its connected with the neutral on the pass through connection. The neutral is usually at earth level. If its connected like this and the variac is set to 0 then the voltage on the output is close to earth making the outputs 'safe' to touch (but don't touch them ever).And don't forget:A variac is not an isolation transformer!

author: con-f-use
author_rank: Unknown
timestamp: October 30, 2013, 05:14:22 pm
content: Generally related:https://www.eevblog.com/forum/blog/dave-suggestion-for-fund-friday-isolation-transformers-fuses-and-safety/msg313562/#msg313562Quote from: nctnico on October 30, 2013, 04:47:04 pmThat would make it about as unsafe as without an isolation Transformer once the scope is attached, right? It wouldn't fry the scope by itself or pose any more dangers? It's till better than nothing, as many things can be done with just a multi-meter. Or am I missing something?

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 30, 2013, 05:36:51 pm
content: Quote from: con-f-use on October 30, 2013, 05:14:22 pmIt depends. If you had a GFCI (RCD) on the circuit, followed by the isolation transformer, followed by a device under test that is grounded through a scope probe, then the isolation transformer may have a negative effect on safety. The isolation transformer would prevent the GFCI from tripping if a human touches a hot portion of the device while also touching a good ground. The GFCI will only see the primary of the isolation transformer, and in the scenario I described, the primary of the isolation transformer has no leakage, but is only consuming well-balanced power.Furthermore, if you have the isolation transformer and then clip the scope ground to an ordinarily "hot" portion of the device under test, you may have now put full mains voltage on the chassis of the device under test, while disabling the GFCI's ability to save yourself from your stupidity. Touch the chassis of the device under test while touching the grounded chassis of another device, and you could get a lethal shock which won't be stopped by the GFCI.If you took out the isolation transformer, then the GFCI could do its job. But then you're faced with the constraint that you must only hook the scope probe ground to a grounded portion of the device under test.The interactions between these safety devices can become complicated. It's definitely faulty logic to think that "isolation transformer is good, GFCI is good, therefore, isolation transformer plus GFCI is double good."

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: October 30, 2013, 05:48:00 pm
content: "A GFCI monitors the amount of current flowing from hot to neutral. If there is any imbalance, it trips the circuit. It is able to sense a mismatch as small as 4 or 5 milliamps, and it can react as quickly as one-thirtieth of a second."Wouldn't placing the GFCI after the isolation transformer still work? Since it's not depending on the ground, but between the hot and neutral.

author: Monkeh
author_rank: Super Contributor
timestamp: October 30, 2013, 05:57:25 pm
content: Quote from: JohnnyGringo on October 30, 2013, 05:48:00 pmBut without a ground reference, current can only flow from line to neutral. So it can never trip, there's no possibility of an imbalance.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 30, 2013, 05:58:47 pm
content: Quote from: JohnnyGringo on October 30, 2013, 05:48:00 pmThe hot and neutral are on the same winding of the transformer and are thus technically connected, at least through a variable impedance. GFCI outlet functions are effectively disabled by this.I haven't personally wrapped my head around the idea of where the current come from and goes to when, lets say, your DUT ends up coming in contact with earth ground. I bet the answer is simple...The isolation transformer will be an optional and separate function.Oscilloscopes are bullshit --- I can't even count how many times I've been irked by the fact that the inputs are not differential!MY PERSONAL SAFETY BE DAMNED!!!

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: October 30, 2013, 06:07:45 pm
content: Quote from: Monkeh on October 30, 2013, 05:57:25 pmAhhh... I think I understand. But if you or some object you are also touching is grounded, then there is a ground reference. Wouldn't the CFCI trip then?-Don't mean to be dense, it just comes naturally.

author: Monkeh
author_rank: Super Contributor
timestamp: October 30, 2013, 06:09:49 pm
content: Quote from: JohnnyGringo on October 30, 2013, 06:07:45 pmThat would not create a circuit.

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 30, 2013, 06:10:47 pm
content: Quote from: JohnnyGringo on October 30, 2013, 05:48:00 pmMonkeh is correct. To elaborate, imagine this:Isolation transformer -> GFCI -> device under testConnect scope ground to a "hot" portion of the device under test. Now, the chassis of the device under test may have mains power on it.Touch the chassis of the device under test while touching a water pipe. Result: shockThe GFCI only sees the output of the isolation transformer, which according to kirchoff's current law is always balanced. So the GFCI can never trip. The fault path sources and sinks equal current through the isolation transformer secondary.A GFCI relies on the fact that it's normally installed after the ground reference splits from the neutral wire. It detects current that goes "around" the GFCI, returning via some path involving the ground instead of returning via the neutral. But in the scenario we've described, the ground reference happens after the GFCI, so any fault involving the ground path will return current back to the isolation transformer secondary and keep the GFCI happy.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 30, 2013, 06:19:22 pm
content: Quote from: AG6QR on October 30, 2013, 06:10:47 pmI understand now --- I was thinking about the secondary too much.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: October 30, 2013, 06:20:48 pm
content: Ah, thank you for my edification. The transformer will just keep supplying as much current (within reason) and the GFCI will never know the difference.I've been cured! Thank god all mighty, I'm cured at last! Seriously, thanks for your explanation.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 30, 2013, 06:40:09 pm
content: This is somewhat unrelated to my own thread topic, but it's on the topic if ground, isolation and oscilloscopes. Some months ago I wanted to measure both the digital signal that drove an optotriac for switching a power outlet at the same time as measuring the mains voltage looking for transients. The product under test was experiencing commutation failure with a very low power inductive load, a very small water pump. I wanted to see where the transient was located in time with respect to the signal driving the optotriac.I decided that I could not make this measurement because the digital circuits were driven by a transformer while the triac was connected straight to mains through a fuse. What if I basically connected the primary and secondary of the transformer to effectively make it an autotransformer just for the one measurement? Would this work? It just popped into my head and I haven't given it a ton of thought.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 30, 2013, 08:31:14 pm
content: What I think I'll do is this:Probably a standard IEC receptacle for power input to my deviceFollow with a fuse, which may be built into the receptacle. --- Probably limit this to 5ANext in the circuit would be some MOVs (at least one across all "leads" of the circuit --- I have A LOT of MOVs to spare)Next in series would be a fairly beefy NTC resistorThereafter would be a relay (DPDT) to switch the input on and offOn the other side of the control relay would be an X2 capacitor and a common-mode choke (I have somewhat unknown common-mode chokes, which is one reason I'll limit to 5A)I had mentioned a current meter, which I have not fully described the implementation of. I think I ought to measure the current after all this protection junk because the characteristics will obviously be different than the mains input. I'll use an ACS712 and whatever else I decide I need to measure and display and control whatever I end up deciding on. I have rather compact AC/DC power supply modules.EDIT: The load will then be connected to a standard wall outlet in an enclosed outlet box. If it doesn't misbehave I may just make the outlet GFCI so that if I am earth grounded, it works as expected. --- Wait, will the thing be bothered by the circuits between it and the mains? I guess I'll just find out --- I can just get a regular outlet if this fails. I love dealing with annoying bullshit like constantly-tripping GFCI outlets.Would others agree that I should provide my logic circuits (and relay control circuit running on the same supply) with a decent-sized capacitor to keep operation predictable in case of a fault? I could also use a small 12VCT transformer for this.A lot of my reasoning for building this is because I have this crap lying around, I must admit.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 31, 2013, 01:34:41 am
content: Anyone want to call me stupid? or at least explain why one of my features/components is harmful and/or not useful?

author: minime72706
author_rank: Frequent Contributor
timestamp: October 31, 2013, 08:50:46 pm
content: Heh ... I must be kind of special. I ended up with a 10A AUTO-TRANSFORMER variac (so-called "contact voltage regulator") ... whoops.

author: Zucca
author_rank: Unknown
timestamp: January 27, 2014, 03:26:58 pm
content: Quote from: AG6QR on October 30, 2013, 06:10:47 pmAre you sure? Where is the loop for the shock if the 2ndary neutral is not involved? The water pipe is at ground potential which is the same of the device chassis and the hot 2ndary. There is no loop and no voltage difference.

author: AG6QR
author_rank: Frequent Contributor
timestamp: January 27, 2014, 03:53:05 pm
content: Quote from: zucca on January 27, 2014, 03:26:58 pmAssuming that the chassis and the intended "hot" portion of the DUT have mains power between them, as supplied through the plug (in other words, the chassis isn't designed to be isolated).We started with "Connect scope ground to a 'hot' portion of the device under test". Since the scope ground is connected to earth ground through the scope, the portion of the DUT that was supposed to be "hot" is now at ground potential, connected to the water pipe. The isolation transformer supply make sure that the chassis and what was supposed to have been the "hot" portion of the DUT have AC mains potential between them. So the chassis now has mains voltage with respect to the water pipe.

author: Zucca
author_rank: Unknown
timestamp: January 27, 2014, 04:43:54 pm
content: ThanksAG6QR, now it is crystal clear, at least I hope.I attached a zuccaCAD schematics.In my little experience I didn´t know the chassis can be connected to the neutral,....

author: Monkeh
author_rank: Super Contributor
timestamp: January 27, 2014, 04:46:33 pm
content: The concept of line and neutral have no place with an isolation transformer. A neutral is created by grounding that conductor, the entire point of the transformer is to not have a grounded conductor.

author: cybermaus
author_rank: Frequent Contributor
timestamp: January 27, 2014, 04:47:47 pm
content: Quote from: AG6QR on January 27, 2014, 03:53:05 pmThat is assuming there is a connection between the chassis and the "neutral" line. Which is only the case if there is a major fault in the wiring, because no approved household devices anywhere in the world, regardless of class or if they are grounded or double isolated, or have an polarized plug or whatever, purposely connects either hot or neutral to the chassis. (keyword household. Industrial may be a different matter)So unless if there was a wiring isolation error to begin with, which you would track down with a multimeter (or a megger) not with a scope, you will not get shocked by the chassis.However, if you do accidentally grab the hot or neutral wire, because the scope will have re-grounded the circuit, you would get a shock, and indeed, this time the GF device would not trip. So there is something true in that under special conditions an isolation transformer can increase risk, namely if you defeat the isolation by re-grounding it. (with a scope or any other means) So don't do that!How about connecting *both* scope and device under test after the isolation transformer, and not grounding the scope?

author: Zucca
author_rank: Unknown
timestamp: January 27, 2014, 04:55:05 pm
content: Quote from: cybermaus on January 27, 2014, 04:47:47 pmThankscybermausto take me back to real life on planet Earth.Quote from: cybermaus on January 27, 2014, 04:47:47 pmYour suggestions is surely interesting but I don´t know the consequences of not grounding the scope. On the paper it seems reasonable.

author: cybermaus
author_rank: Frequent Contributor
timestamp: January 27, 2014, 05:26:02 pm
content: Quote from: zucca on January 27, 2014, 04:55:05 pmNot sure either. I guess the noise may creep up.What are you trying to achieve anyway? A multimeter is usually enough to troubleshoot that part. Or are you building/fixing primary switched power supplies?Anyway, while a isolation transformer may add some protection, I do feel a simple and proper Ground-Fault switch is very safe and fast. I think isolation transformers were more of a thing from when GF was not normal or even required yet. Like back in the 70ies. Maybe 80ies in some parts, but the're required here for quite some time now and it is one goverment requirement I agree with. (I used to wire houses as a side job when I was in college)Also note that if you did have a good isolation transformer, even if you did not accidentally ground it, if you somehow managed to grab *both* hot and neutral, then on an isolation transformer you would not trip anything, the burning would continue till it carbonized. But if you had grabbed both wires from the wall socket, the GF would have tripped. So one could argue GF is safer then isolation. GF protects against grabbing both wires, isolation only protects against grabbing one.Of course, one could also argue that if you manage to grab both wires, good old Darwin should apply anyway.

author: Zucca
author_rank: Unknown
timestamp: January 27, 2014, 05:37:50 pm
content: Quote from: cybermaus on January 27, 2014, 05:26:02 pmI was interested on the reasons to connect one cable of the 2ndary of an isolation transformer to ground/earth.See my post here:https://www.eevblog.com/forum/beginners/isolation-transformer-2ndary-neutral-bonded-to-ground-or-not/

author: Co6aka
author_rank: Unknown
timestamp: January 27, 2014, 11:12:35 pm
content: Quote from: cybermaus on January 27, 2014, 04:47:47 pmI've done that, and it works -- potential is relative -- but it's too easy to "forget" so you've gotta be very mindful. I "solved" the problem by getting a differential probe, but have realized that I should have gotten two, or perhaps four. (Damn scopes should have differential inputs to begin with!) Don't do a lot of work (yet) where I need it though.FWIW, over the years I've seen these on benches:Sencore PR570......and I have one of these on my bench:Kill-A-Watt PS...For smoke tests and physically probing around I have a isolation transformer, variac, and a mounted lightbulb base. (Long live the incandescent bulb!) I didn't install a bypass switch for the bulb because of the "forget" problem, but after re-thinking about it maybe I'll add a momentary switch.