Hum Bucker

You simply don't understand the whole concept of "earthing", do you?

By placing the electrical circuitry inside a metal box, which is connected to earth, and with one side on the a.c. supply joined to earth (at the sub-station), an electrical failure that results in the metal case becoming 'live' blows the supply fuse - thus saving the user from receiving an electrical shock. Therefore, to ALL test equipment should be earthed at all times in the simple interest of user safety.


You don't seem to understand the basics of SMPSUs, either.

If I wish to examine the waveform across the filter capacitors in a switch-mode PSU, where these caps. are fed from a bridge rectifier DIRECT from the a.c. mains, I MUST, repeat MUST connect the earth lead of my scope to the -ve of the caps.- otherwise, there is no reference for the 'scope to work to. To do this, a mains isolation transformer is ESSENTIAL between the a.c. supply and the PSU - otherwise there will be a dead short across the bridge.

There is no debate to be had on ANY of these aspects: they are simple, provable facts. Only the ignorant and mis-informed would disagree.

No more need be said. Period.

Hum Bucker.

 

Obviously far more than you do?.

That's the view of someone who doesn't understand the principles involved, or the alternatives available - a service department ISN'T a domstic environment, and forcing domestic standards on it makes it LESS safe, not more!. Hand on earthed scope, other hand on live wire - oops! - full mains straight across the heart. Maintaining an earth free service environment is considerably safer than an earthed one.

As you're blindly rattling off domestic earthing standards you're obviously far too young to remember the huge controversy over earthed and earth free environments?.

I'll fully agree it makes great sense in a domestic environment, particularly when you have washing machines, dishwashers, steam irons - anything that has water anywhere near electricity. But a service environment is a completely different affair!.

I might also mention I've been on training courses at huge international electronics companies, and in their training rooms there are no isolation transformers, and all scopes have their earth leads removed. I can't remember the last time I ever used an earthed scope?, at technical college all scopes were unearthed - again, for greater safety.

Mind you, at technical college they also had live bare brass 440V three phase terminals on the walls! - that was in the heavy machines lab, which we had a number of lessons in (by complete mistake as it happened).

Yes, you connect the chassis of the scope to the -ve of the caps, that's blatently obvious - and I do so on at least a weekly basis, and have done since SMPSU's first appeared in the Thorn 3000 series TV's. That's why you don't have an earthed scope in service departments - it's then perfectly safe if you use an isolation transformer or not.

Plenty of debate, only the ignorant and mis-informed would think otherwise!.

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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.

• If a an earth fault occurs on any live conductor, the other lives will float at 400V.
• If you're powering a device that uses an autotransformer to get a high voltage (e.g a neon sign transformer which runs at 20kV) and the live output developed an earth fault the neutral will float at 20kV which would damage the insulation in the distribution system and cause a fire.
• There's no earth path for high voltages induced from lightning strikes.

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.

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.

The only time I would agree with disconnecting a safety earth connection is when you're testing something that's got its 0V rail bonded to protective earth (PCs and some audio amplifiers). This is because failing to do so will result in a troublesome ground loop. This isn't dangerous at all as you're still earthing the appliance by connecting it to the scope's earth bonded chassis.

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.

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Maybe I should go to another forum; I did not mean to get something going. My bad.

I simply read an article on common reasons these power supplies blow fuses.

It described that it's highly typical for the diodes next to the filter cap/s to dead short for no reason. This seems to be true in my case but I want to make sure other components down the line after the diodes were not affected.

It was described that it is possible that the filter cap/s could go bad.

I am sure that this is typical but I thought I would just mention that there is a capacitor that crosses beside with the fuse. After installing a few fuses as I was trouble shooting I notice this capacitor would finally "pop" so to speak. Blow a little ceramic pc off it's self. I would assume this wouldn’t help as it is probably a result in the dead short.

Eric

 

Eric,
Some times you have to unsolder one end of a component in order to test it.

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Len

 

hi,
Ref earthing standards.

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

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

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).

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.

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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!

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These H&S courtesy visits are always much of a non event, I suspect its a case of getting their batting average upto par!..

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!

 

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

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Len,

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

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

 

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.

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Len

 

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.

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.

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.

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