Hi All,

I’m getting back into electronics and have recently purchased a new DSO to replace an elderly CRT single channel one.
One of my areas of interest is diagnosing PSU’s. After much reading I’m drawn to looking at the safety and protection side of things.

I’m in the UK where my workroom is part of a 240v ring main circuit. The only protection is a single 32A fuse for the ring circuit.

I plan to make a small distribution board for my workshop consisting of:

a dual pole breaker http://cpc.farnell.com/pro-elec/aut2/incomer-100a-dp/dp/PL1066087

a dual pole RCD http://cpc.farnell.com/pro-elec/aur1-40a/rcd-double-pole-40a/dp/PL1066187

and several single pole breakers like these: http://cpc.farnell.com/pro-elec/aub1-1p-c16a/mcb-single-pole-c-curve-16a/dp/PL1159387


Other potential items for diagnosis are some old school power amplifiers from Bose, HH, Amcron and others. These are not soft start devices and have high inrush currents.

Another item on the shopping list is an isolation transformer for items under test. As funds are limited this will not be huge and will be limited for smaller items like small PSU’s.

The one(s) I’m looking are like this: http://cpc.farnell.com/schneider-electric/abl6ts40u/transformer-400va-1-x-230v/dp/TF01551

The cost of the isolation transformer is a bit of a concern and I wonder if there is a more economic solution out there.

Any thoughts and comments on my plans would be most welcome.

Many thanks.

 

I built my own isolation transformer. I used two microwave oven transformers. Removed the high voltage coils on both (which happened to be the same physical dimensions) and moved one 120 VAC coil onto the other transformer with the same 120 VAC coil. Kept the chokes and have used it a few times to isolate projects from mains power. I'm ABSOLUTELY NOT the expert on transformers, but what I built works. It's basically a 1 to 1 isolation transformer. Since you're in a region that operates on 240 VAC, perhaps you can find and scrap a couple microwave ovens in your area and remove the transformers as I did.

One note: I had to cut the welds from both transformers to be able to remove and replace the coils. When reassembled I had to re-weld them. If you don't have access to a welder then your transformer will be less efficient unless you can get a good magnetic bond between the E lam's and the I laminate. I mentioned the chokes, those were basically loose iron lam's that went between the two coils. Like I said, I'm not the expert. I put them in to limit the potential current from causing a catastrophic failure. Again, I'm not the expert.

The only other thing you need to be certain you do is to NOT damage the insulation or cause any of the wiring to make contact with the core. Doing so will place the core at some potential, and potentially a shock hazard.

Good luck with the rest of your endeavor. And again - I AM NOT THE EXPERT ON TRANSFORMERS. No doubt someone will correct something I've said. I welcome the criticism. It's one way I learn. The other way is by accidentally shocking myself. Don't learn that way; it's rather unpleasant. And potentially dangerous too.

 

I'm in a similar position to you @dansteely regarding an isolation transformer. I was looking at this which is a fair bit more expensive than the item in your list but is a complete unit with greater current capability. I still need to establish from RS whether this is a true isolation transformer (i.e whether there is no connection from earth to neutral or live or, if not, whether it can readily be modified to be a true 'tech' isolation transformer). I have however seen a few videos where people have used 2 identical , wired back to back (secondaries connected together) step down transformers. to achieve isolation. I wonder if anyone can identify any drawbacks to this approach as a DIY solution (e.g current handling). I could quite easily source 2 x 240V/12V transformers and enclose them (along with suitable fusing & outlet) far cheaper than buying a ready made 240V/240V transformer but I get completely confused about what VA rating of 240/12V step down transformers I would need to be looking for if I wanted to have an output of 2-3A.

 

@Standisher Again, I'm not the expert on this subject. But I believe with each transformer you lose a certain percentage of power. Going back to back with two transformers configured as you suggest will work, but you will also be losing something in the way of performance. Just how much - I don't know.

By nature a transformer primary is nothing more than a coil of wire wrapped around an iron core. The core itself is grounded for safety reasons. If the coil becomes shorted to the core then the core can become live power. Having it grounded means the breaker should trip. Your secondary is exactly the same thing - just a coil of wire. However, if you were to contact either leg of the secondary and should be touching ground at the same time you (in theory) shouldn't experience a shock. Why not? Because (in America) power comes to the home in the form of split 240 VAC. That means from line 1 to line 2 there is 240 V potential. But from either line ( 1 or 2 ) to neutral the potential is only 120 V. That "Neutral" is also grounded via the circuit breaker panel, likely the power mast (where power comes into the home), at the pole where the transformer is located and at a few more local grounding points. So if you should contact line mains and ground you can be shocked. Since the isolation transformer secondary is not a part of that circuit, contacting either leg of the secondary, you can not complete a circuit through your body. Meaning you don't get shocked. Even though it may be 120 volts. In short, you are isolated from line mains via magnetic coupling. Since magnetic flux can not conduct electricity - no, you are not in danger of a shock.

Now: Can you get a shock from the two leads of the secondary? Yes. If you contact both leads either directly or through your circuitry.

 

I feel a disturbance in the force. To wit: 16 amp breakers and an isolation transformer that can't deliver 2 amps. Methinks you are out of proportion in your endeavors. That aside, any two identical transformers can be connected back to back to achieve isolation if you happen to have some laying around. The problem with MOT's is that darn HV winding which causes more hazard than you started with, and breaking welds is beyond the kin of most people.

I hope I have been helpful.

 

One of my areas of interest is diagnosing PSU’s. After much reading I’m drawn to looking at the safety and protection side of things.

If the equipment you're working on has a transformer, you don't need additional isolation if you're working on the secondary side.

If it doesn't have a transformer, buy an isolation transformer if you don't know what you're doing.

 

On the 16A breaker front I can get smaller ones down to 1A. They go up in the following steps: 1, 2, 3, 4, 6, 10, 16 and upwards.

Obviously you could have many protected outputs in the steps mentioned above, but that doesn't seem that practical.

How is is this covered in the real world?

 

If the equipment you're working on has a transformer, you don't need additional isolation if you're working on the secondary side.
If it doesn't have a transformer, buy an isolation transformer if you don't know what you're doing.

I'm a bit confused with this statement. I am interested in using an isolation transformer for additional protection when using an oscilloscope to probe equipment under test. This video (from about the 9:00 minute mark) probably articulates, better than I could, the risk I'm trying to avoid. I suspect that all the equipment I work on will be powered through an internal transformer; so are you saying that this mitigates the risk described? The EEVblog video on this subject (from about the 10:22 minute mark) seems to say.... not necessarily. Hoping you can clarify.

 

I'm a bit confused with this statement. I am interested in using an isolation transformer for additional protection when using an oscilloscope to probe equipment under test.

Didn't watch the videos and refuse to watch anything from EEVblog.

When using a scope, it should always be properly grounded. You can probe any voltages you want as long as it's within the voltage limit of the probe. You just need to take care where you connect the ground lead because it's earth ground.

You can use the A+(-B) capability of the scope if you need to subtract signals. Assuming you have a dual channel scope with the ability to invert one channel and add with another.

 

How is is this covered in the real world?

Generally, the whole house has one massive circuit breaker, like 100 to 200 amps. Then a smaller breaker like the 32 amp breaker you mentioned is there to be sure the wires in the walls don't burn. When you get to the workbench, it is good to have a smaller breaker so you don't have to run to the mains box every time you make a mistake. My workbench can run on a couple of hundred watts, and the things I work on are generally less than 200 watts, but sometimes I want to fix a hair dryer or a room heater at 1500 watts, so a local breaker of 10 amps would be sufficient, but so would 15 or 20 amps. In addition, most of my equipment has an internal fuse of 1 or 2 amps and I have a 400 watt Variac which is very handy, and it has a 5 amp fuse because American houses use mostly 120 VAC.

 

As someone who has done electrician qualifications I can tell you imedialtley that your setup will break building regulations and is against the law.

Remember, its only illegal if your caught

 

I can tell you immediately that your setup will break building regulations and is against the law.

It can be done to code, but this guy is over-thinking it. If he does the scheme in the first post, it will require at least 2 sq ft just to mount the boxes.

 

@#12
It can be done to code but unless its signed off by a qualified electrician who is certified by a recognised body (elecsa for example) then the work is against the law. Otherwise building control needs to be informed and they need to inspect the work.

 

@#12
It can be done to code but unless its signed off by a qualified electrician who is certified by a recognized body (elecsa for example) then the work is against the law. Otherwise building control needs to be informed and they need to inspect the work.

I see. You're talking about the political side of the equation...which I avoid as much as possible.

In other words; If I got caught for every time I wired up some 120VAC or 240VAC...properly...safely...and to code, but without a permit and inspection, I would be in jail until bell bottom pants came back into style.

 

#12

If you where caught doing work that requires certification and did not get it done then Building Control will request that you remove the work and charge up to £50 per day until its fixed
https://www.stockton.gov.uk/environ...ens-if-i-contravene-the-building-regulations/

If you refuse to fix it then you can be jailed especially if the work causes harm to someone. For example, if you work caused someone to get electrocuted and they die then you get done for manslaughter. Otherwise you can face a number of years....

 

If you where caught

Yeah, I know. Good thing I did them all correctly, safely, and to code...(except for the political paperwork).

Seriously, I've been cheating the system for 40 years and never hurt anybody.
In fact, I never had even one complaint to the Code Department in 40 years!
I told my first boss on a trades job, "You will never get me to install a fire hazard at any price."
That idiot would have pinched the penny if I would have let him.

Enough, "Off Topic"?

 

@#12

Nah, off-topic can be helpful !

 

My cousin can bake a cake using just a 100 watt incandescent light bulb.

Should that light bulb be isolated from mains ? ? ? And if so, how much would it cost to have an inspector sign off on the metal box with the light bulb in it ? ? ?

MOT's. OK, yes, I agree there aren't a lot of us who can manage teardown of a MOT. (Microwave Oven Transformer) Let alone put one back together again. But I'm an odd sot. I have an angle grinder and several wheels that can cut the weld from the MOT. I also have a MIG welder and can put it back together again. I tend to think of others as being my equals when it comes to building stuff. Not saying I think everyone should be as smart as me, no. I'm saying I just assume my capabilities are common among the people here on this website. And I often consider my contemporaries here to be superior in knowledge on many subjects. And that has been prove time and again. I just see it as easy and assume that any average person (average like me) can do the same stuff I can do. I don't think I'm "Smart"; I just don't think I'm dumb. And I afford everyone else the same esteem. I don't hang around with dumb people. At least I don't think the people here are dumb.

On the subject of back to back transformers, you're going to need a substantial size transformer if you want to get some decent power output from it. If you grab something out of an old alarm clock radio - yeah, you'll be lucky to get 200 mW of power out of it. That's why I went with a MOT. Just so happens that a friend of mine who's a teacher helped me attain several isolation transformers used for training students in electronics class. I can't remember the exact sort of transformer but it has a capacitor attached to a tertiary coil and is supposed to maintain a steady output - even when the input can vary to some degree. Some sort of saturation transformer. Like I said early on, I'm not the expert on the subject. But I believe a MOT will give you enough power to handle most of the tasks you might work on at your bench. Can you use it to launch a space shuttle? Well, depends on the scale down size. But you're not going to launch the real thing using just a MOT. At least, mine works. When I repaired my Radio Shack Stereo Amplifier, I used my isolation transformer. I found a bad bridge rectifier. Lucky for me it didn't tear down anything else. And the bigger bridge rectifier I installed should be able to handle the load better than the original one. I still run the same fuse though. I NEVER upgrade a fuse without EXCELLENT reason. Though I have been known to downgrade a fuse from time to time. Especially when testing out a circuit that I don't have much confidence in. Fuses are cheap. My projects and equipment - well, that's a little harder to replace.

 

I don't see anyone else covering this aspect of isolation and like general safety so here are a few other things to consider when setting up a electronics/live line electrical components work area.

First is relative isolation for possible ground loops between possible power sources and tools/test equipment that you may be using. The top of my list for test equipment needing full line to ground and line to anything else isolation is Oscilloscopes.

Most have the input grounds/shielding of the test leads tied directly to the O-scopes own power line side earth ground which can be bad when doing tests on a piece of live equipment. Same issue when using more than one O-scope at a time. Make sure they have the capacity to be electrically fully floated and be independent of each other if needed. By isolating the test equipment that likely only draws a few 10's to hundreds of watts or less you can get by with substantially smaller isolation transformers.

Beyond that is the personal ground loop circuit issue which to be honest in all reasonable working environments is largely a non issue simply due to the typical types of clothing and also flooring used in such work areas. Dry socks, fair condition shoes and like on dry wood or even dry concrete flooring is a very good insulator capable of supporting basic voltage isolation way beyond what your local line voltages are in relation to the earth grounding the structure you are in has.

Add in the multiple layers they make when combined while also standing on wood or concrete that has a area rug or foot pads or even dry or painted concrete and getting electrocuted via a live line to earth ground short directly through your body and any or all of that is extremely unlikely to near impossible. Sweaty wet feet on dam bare dirt is the most likely scenario you would find that would carry a high possibility for full body current path electrocution so unless you're a barefoot hillbilly electrician working in a muddy dirt floor shack the odds are you not in any real danger for a floor based electrocution hazard.

So much so that in almost any normal case you could do an actual 1000 volt Megohmmeter test on yourself between you and your line power earth ground and pass with a reading that is so high its well above safe. I've actually demonstrated that many times to safety nazi coworkers over the years and rarely ever registered a measurable reading for it let alone one that gave a resistance reading low enough to count as potentially dangerous. (A 100+ mega ohms reading value doesn't get people killed. 100's of ohms reading values might.)

 

I also have a MIG welder and can put it back together again.

Would a stick welder work? (I can do that.)

a capacitor attached to a tertiary coil and is supposed to maintain a steady output

ferroresonant

I believe a MOT will give you enough power to handle most of the tasks you might work on at your bench.

A MOT is worth about a kilowatt but I found some surplus 150 watt transformers for $7.50 each. Most of the stuff on my work bench is so small that I have a 60 watt light bulb in a box marked, "heavy load resistors". When I need to get rid of the 'scope ground, I can isolate that instead of the project. Takes about 35 watts.