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Switching power supply issue.

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hi,
Ref earthing standards.
 
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Hero999 said:
I'm only 25 and I've heard about it and to me it's common sense why the earthed, mainly because if it isn't earthed it can float at whatever voltage it wants to.

Any argument which claims 'common sense' as a point for it, hasn't understood the problem sufficiently well :D

However, on a controlled environment like a test bench, it's normally only single phase and you know what you're connecting to it so the above problems don't occur.


Not if you're using an isolation transformer as there's no return path to earth.

Only from any source connected via an isolating transformer, and assuming it's insulation doesn't break down.

No, it isn't safe if you don't have an isolation transformer. The -Ve pulses up an down at mains potential with respect to earth so as soon as you touch that scope you're a goner. You're better off blowing a fuse or rectifier then learning from it and buying an isolation transformer.

Why would you be a 'goner' - to get a shock you need to complete a circuit, operating in an earth free environment ensures that isn't possible - apart from directly between neutral and live (which would be the same with an isolation transformer as well).

When was this?

In the 1960s may be, but not in 2008 and just because you've seen it done before it doesn't mean it's safe.

1970's - and because modern practices have changed, doesn't mean it's any less safe than it was back then. There are significant advantages in an earth free service environment - which are obvious if you consider the mechanisms by which you can get a shock.
 
hi.
As an employer I had to work within the H&S safety guide lines.

If an employee/or a third party is killed or injured due the employer/manufacturer not adhering to the H&S standards, it leaves him open to prosecution and compensation claims.
If you are shown to be negligent your insurance company is not liable for the compensation claim.

Ref the isolation transformers, IIRC any work/test bench must have a distance of 1.5 metres from another work/test bench that is using an isolation transformer.
 
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Last time we had the factory inspectors round, did they want to see the isolation transformers? NO, did they want to see the low voltage soldering irons? NO, did they want to see the isolated aerial sockets? NO, were they interested in an earth free safe environment? NO.

What did they ask? - "DO YOU HAVE ANY TOILETS" (yes we do, this is the 20th/21st century), and "DO YOU HAVE H&S INFORMATION LEAFLETS ON THE AEROSOLS YOU USE" (yes we do, all H&S leaflets, information, and everything we've ever received is chucked in a suitably labelled box file).

They went away EXTREMELY happy, we had toilets, and we had instant access to all H&S information, in a nicely labelled box.

Personally I thought they were a COMPLETE waste of time, however the senior inspector was a really cute young woman, which made it all worthwhile! :D
 
These H&S courtesy visits are always much of a non event, I suspect its a case of getting their batting average upto par!..:rolleyes:

Its when things go badly wrong they roll out the big guns, then they will go thru your company with a fine tooth comb. The companies insurers will then also give you the once over.

I think most of us cut corners on safety from time to time in order to get the job done, in hindsight thats not a good idea.

I would ALWAYS advise that the Statuary regulations regarding safety be followed at all times, especially, if I am advising an inexperienced person.

The danger is, many of the OP asking for help have no experience, so in reading some of our bad habits and practices, it could place tham at risk.

I have seen you on a number of occasions, caution and warn an OP not to attempt certain repairs/projects.

There's a young guy currently asking about 'dc fault finding',, It sounds like a SMPS problem,
My advice will be not to attempt the repair.
 
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since you have 2 of them, you could short the leads on one PSU and see what blows up, then follow down what blew up, and it should be the same problem on your other. Of course if 2 failed, they were probably el cheapos with every party not being worth a crap, so in that case, throw it in the recycle bin and send it back to china!
 
crusty said:
since you have 2 of them, you could short the leads on one PSU and see what blows up, then follow down what blew up, and it should be the same problem on your other. Of course if 2 failed, they were probably el cheapos with every party not being worth a crap, so in that case, throw it in the recycle bin and send it back to china!

Phew!
I'm glad you are around to give out such HELPFUL technical advice...:eek:
 
Len,

Yea I have removed each component as I have tested.
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Shorting out the unit is not going to give me the same results as a component that shorts out internally, as far as what does and doesn’t become damaged.

Eric
 
dbtoutfit said:
Len,

Yea I have removed each component as I have tested. Good
------------------------------------------------------

Shorting out the unit is not going to give me the same results as a component that shorts out internally, as far as what does and doesn’t become damaged. Agreed. You don't know which component failed first, and even if you did, the result may be different if you tried to simulate.

Eric
Without a circuit to look at, I can only guess.

Are you able to trace out the relevant part of the circuit?

I have a circuit of the SMPS of my daughter's VCR. I could scan it and post here. It won't be identical to your's, but it should give you some idea of what a SMPS looks like.
 
Nigel Goodwin said:
Only from any source connected via an isolating transformer, and assuming it's insulation doesn't break down.
That's very unlikely if it conforms to the safety regulations, I can't remember the exact British standard but it states that the secondary shall have double insulation from the primary and that it shall be able to withstand spikes of 2kV.

Why would you be a 'goner' - to get a shock you need to complete a circuit, operating in an earth free environment ensures that isn't possible - apart from directly between neutral and live (which would be the same with an isolation transformer as well).
No, it won't be a completely earth free environment because the earth is bonded to the neutral unless you use an isolation transformer.

Also, the scope case only needs to have single insulation between from the mains so if it developed a fault then you'd be totally knackered.

1970's - and because modern practices have changed, doesn't mean it's any less safe than it was back then. There are significant advantages in an earth free service environment - which are obvious if you consider the mechanisms by which you can get a shock.
I disagree, if I went for a job anywhere and discovered they used the cowboy practises you promote then I'd walk out on the first day, after taking photographs and reporting them of course.

I don't know why the scope manufacturers grow a brain and make double insulated scopes, they'd be so much more usefull and safer too.
 
Hero999 said:
No, it won't be a completely earth free environment because the earth is bonded to the neutral unless you use an isolation transformer.

Yes it will, because there's no other earth source available - so you can't get a shock between live and 'earth', only between live and neutral - EXACTLY as you could with a transformer.

Your isolation transformer scheme is far more dangerous - you isolate the incoming mains, so you have two 'lives' with no earth reference. You than clip your scope to one of them, earthing it - making that neutral again, and leaving the other a full 240V live - where you can get a shock to earth exactly as if the transformer didn't exist.

Where as my earth free environment makes this impossible under all circumstances.
 
Ideally yes but how is an earth free environment practically possible?

I don't see how it is.

Your soldering iron's tip is earthed.

Even the concrete floor will conduct electricity to some degree (it absorbs water).

The metal bench you're working on is earthed.

Your chair is earthed.

Even the screws on the plug socket are earthed!

What happens if a conductor shorts to the scope's cause?

The case will either become neural or live with respect to the other circuit.

The isolation transformer isn't dangerous, you obide by normal safety practice and don't touch the circuit being tested.

Your floating scope's at mains potential is a death wish, as soon as touch it the current will find a path to earth where you leaset expect it and kill you.

The only way you can safely not connect your scope's case to ground it if you run both it and what you're testing from an isolation transformer but in practice, it's only really important you run the device under test from an isolation tranformer.
 
Hero999 said:
Ideally yes but how is an earth free environment practically possible?

I don't see how it is.

Your soldering iron's tip is earthed.

Only if you earth it!.

Even the concrete floor will conduct electricity to some degree (it absorbs water).

That's assuming a concrete floor?, which it isn't, and being at ground level?, which it isn't. The floor is a Pitchmastic type of substance (not exactly sure what?), but it's non-conductive, and has rubber matting over it as well.

I can stand on the floor and hold a 240V live wire perfectly safely - and usually do a couple of times a year.

The metal bench you're working on is earthed.

Who uses metal work benches? - I certainly wouldn't entertain them!.

Your chair is earthed.

Why would a chair be earthed?, why would you assume it's even made of metal?.

Even the screws on the plug socket are earthed!

Possibly! - obviously depending entirely on the wiring, and for an earth free environment they wouldn't be.

What happens if a conductor shorts to the scope's cause?

What happens if it shorts to some where else? - you can't just randomly assume something is going to happen like that, and there are plenty of random conductors shorting to things that are more dangerous in an earthed environment.

But if it does short to the scopes casing nothing happens, because it's not earthed.

The case will either become neural or live with respect to the other circuit.

But is completely of no consequence, as there's no earth reference to get a shock from.

The isolation transformer isn't dangerous, you obide by normal safety practice and don't touch the circuit being tested.

Normal safety practice doesn't allow you not to touch the circuit in a service environment - although you obviously take great care in everything you do. Keeping one hand in a pocket is always a good idea, but earthed or not makes little difference to that (apart from you're more likely to get a shock if you touch a live conductor in an earthed environment).

Your floating scope's at mains potential is a death wish, as soon as touch it the current will find a path to earth where you leaset expect it and kill you.

There is no earth, so that isn't possible - why do you find that so hard to understand?.

The only way you can safely not connect your scope's case to ground it if you run both it and what you're testing from an isolation transformer but in practice, it's only really important you run the device under test from an isolation tranformer.

You appear to have been 'brainwashed', you've been told there's only one possible way to do something, and can't think 'outside the box' for alternatives.

There are normally multiple ways to do most things, with varying advantages with each - you are stuck in a single minded approach and can't conceive that there may be alternative methods just as good, and possibly better.

In an attempt to prove your single minded point of view you have made loads of completely incorrect assumptions in your last post - just think of what earthing is for, and what it's supposed to do, and why it's done like it is. It's not perfect, it has major flaws, but for a domestic environment it's probably the best choice - I wouldn't deny that!.

My only reason for repeatedly posting in these threads is to try and make people understand that blindly following a single idea isn't always the best way - use your brain, consider the alternatives, don't be a sheep!.

Like I've said everytime this has come up though, if you don't understand it, then DON'T DO IT!.
 
I can see your point, I just think that relying on something like an earth free environment isn't good enough to protect you from electrical shock. All that needs to happen is that you come in to contact with something that's earthed.

What happens if someone plugs your unearthed scope in to a non-earthed environment and there just happens to be an earth fault?

Even in a non-earthed environment, the scope's case will become earthed if the neutral acidentally touches the case.

I understand that if there are no electrical paths to earth and you touch a live conductor you won't get shocked. I'm just not stupid enough to trust that there are no paths to earth.
 
But there are less chances of a shock than with an earthed scope and an isolation transformer - particularly bearing in mind as soon as you connect the earthed scope to the primary side of the power supply, you're earthing the secondary of your isolation transformer - making it completely useless, and it may as well not be there!.

Like I've said all along, we run all benches via their own isolation transformers, but we still maintain an earth free environment and don't earth the scopes - this provides the maximum possible amount of safety.
 
What about ESD?

Adequate ESD measures require the bench and you to be earthed which is totally incompatible with what you're suggesting.

I don't know why the scope manufactures don't just make double insulated scopes.

Nearly all power tools are double insulated these days.

They could still ensure the unit has adequate EMC screening by wire mesh on the inside of the case, there's not excuse.
 
Hero999 said:
What about ESD?

Adequate ESD measures require the bench and you to be earthed which is totally incompatible with what you're suggesting.
ESD earthing is done via a 1M resistor for safety reasons.

There is no need for a direct earth since any charge will leak away quickly via a 1M resistor.
 
ljcox said:
ESD earthing is done via a 1M resistor for safety reasons.

There is no need for a direct earth since any charge will leak away quickly via a 1M resistor.

Yes, we use ESD mats and wristbands where applicable - as you say, no direct earth requirement, so still no shock hazard.
 
What if someone borrows your un-earthed scope with an internal fault and plugs it in someone else whilst touching the metal chassis and not stood on a rubber mat? They could potentially receive a fatal shock.

Removing the earth from a scope could potentially cause a fire risk if an internal fault develops, you may as well short out the fuse whilst you are at it, that’s just as dangerous.

Don’t Techtronic make battery driven scopes?
 
I agree, just use an isolation transformer when testing mains circuit on an oscilloscope, it's the only really safe way of doing so.
 
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