DC current leakage problem

[dmr]

6 minutes ago, WotEver said:

Because it’s being bypassed by the capacitor straight to the hull. 

Have we ever decided if AC ripple causes stray current erosion? 

But the disadvantage of considering them to ‘Switch on’ is that the reader can imagine a dead short like a switch, exactly as SeaDog did. Analogies rarely work well in electrical descriptions. 

Did hear that during an undergraduate power electronics lecture the lecturer referred to the saturated power transistors as "hard on" and an overseas female student put up her hand up and said "I don't understand, what is a hard on? "

..............Dave

[WotEver]

8 minutes ago, dmr said:

What about a meter connected via a bridge rectifier made from Schottky diodes?????

That’d work

Getting back to capacitors, it’s difficult to determine an ideal size. A 2V 1,000 uF cap would limit the AC current to about 10mA but it can’t be an electrolytic and I’ve never found an AC capacitor much above 10 uF. And anyway, is 10mA sufficiently low?

There’s a company here who sell a GI capacitor but they don’t mention its value. 

http://www.yandina.com/acrobats/GICapacitor.pdf

[nicknorman]

Two polarised electrolytic capacitors in series facing each other?

but anyway, I don’t think a large value is required, it’s just a bit of energy leaking from some suppression circuitry somewhere.

Edited November 17, 2017 by nicknorman

[dmr]

I think the real issue is that its all very complicated and I certainly don't know nearly enough about it. A meter will only respond to an average value so a reading of 100mV could be due to a dc voltage of 100mV, or ask Nick says, it could be from one diode (pair) going into conduction so producing 1.4 volts but only during the peak of one half cycle so with a mean DC value of 100mV. A diode might actually do better here because I think I remember that the human eye will see a rapidly flashing ED as almost as bright as a constantly lit LED??? 

But will a pulsating current be as destructive to a boat as a steady DC current.???

Off grid CCing makes life a whole lot simpler.  but then again if I was a marina dweller I think I would get a transformer.

...............Dave

[Keeping Up]

Some random thoughts from my wandering ponderings about GI's, especially after my hull corroded away last year:

As asked above, does anybody really know whether a slight ac leakage is significant towards corrosion or not?

Would the slight current through a GI monitoring meter be a problem, especially if it's DC? If so, diodes are better because they pass no current until they conduct (if you see what I mean)

In our tests in 2009, ChrisW and I measured that the dynamic impedance around the GI, from boat to shore earth via the hull and canal, was less than 3 ohms. Thus any ac leakage into the boat's earth/hull would inevitably by-pass the GI (unless it was a massive current) hence a capacitor would make no difference except maybe on a GRP boat. This was on a 67ft steel boat with the hull blacked but not the baseplate, in 3ft of water against plastic piling.

I discovered that my Sterling battery charger did not have any significant leakage of AC or RF to the mains earth itself. But it did however have about 20mA of AC leakage from the 240v side to the 12v side when it was working. I had not noticed it before because I had tested it on a bench and not on the boat when trying to work out why the RCD had started tripping. It went out for re-cycling.

 

[dmr]

4 minutes ago, nicknorman said:

Two polarised electrolytic capacitors in series facing each other?

but anyway, I don’t think a large value is required, it’s just a bit of energy leaking from some suppression circuitry somewhere.

Do some electrolytics work ok reversed biassed as long as the voltage is not too big?

And would you want an electrolytic in a safety critical circuit?

..................Dave

[WotEver]

26 minutes ago, dmr said:

Do some electrolytics work ok reversed biassed as long as the voltage is not too big?

And would you want an electrolytic in a safety critical circuit?

..................Dave

As to the first, not to my knowledge. They go pop. 

And for your second question, I wouldn’t because they tend to go pop anyway. 

[nicknorman]

With 2 electrolytics in series facing each other, any reverse bias current is limited to that required to charge the forward biased electrolyic. Its not the same as simply reverse biasing a solitary electrolytic. Thus if the biasing is DC and prolonged, the forward biased capacitor ends up with all the voltage across it, the reverse biased one with none (so it's no longer reverse biased).

There's no safety issue. If the capacitor goes open circuit - no hazard. If the capacitor goes short circuit - no hazard.

Edited November 17, 2017 by nicknorman

[dmr]

1 hour ago, nicknorman said:

With 2 electrolytics in series facing each other, any reverse bias current is limited to that required to charge the forward biased electrolyic. Its not the same as simply reverse biasing a solitary electrolytic. Thus if the biasing is DC and prolonged, the forward biased capacitor ends up with all the voltage across it, the reverse biased one with none (so it's no longer reverse biased).

There's no safety issue. If the capacitor goes open circuit - no hazard. If the capacitor goes short circuit - no hazard.

Unless it goes off with such a bang that it severs the earth wire!!!!!. We found a big collection of very old and very big electrolytics in the lab back in my University days and a Friday afternoon entertainment was to put them the wrong way about on a bench power supply. They fizz for a bit first, then a really big bang.   Best done inside a steel waste paper bin. Dunno if the modern ones are as much fun.

................Dave

[nicknorman]

1 hour ago, dmr said:

Unless it goes off with such a bang that it severs the earth wire!!!!!. We found a big collection of very old and very big electrolytics in the lab back in my University days and a Friday afternoon entertainment was to put them the wrong way about on a bench power supply. They fizz for a bit first, then a really big bang.   Best done inside a steel waste paper bin. Dunno if the modern ones are as much fun.

................Dave

But the point is, they won’t fizz or bang as the only reverse current flowing will be that required to charge up the other one. Not enough energy to cause much heat or banging!

[dmr]

5 minutes ago, nicknorman said:

But the point is, they won’t fizz or bang as the only reverse current flowing will be that required to charge up the other one. Not enough energy to cause much heat or banging!

I was only been silly, I think it required a pretty hefty power supply to get the more satisfying explosions, but then nothing like as good as filing a balloon with a mixture of oxygen and acetylene!  I have of course grown up and this was all in the past.

...............Dave

[Dr Bob]

10 hours ago, smileypete said:

Wonder if it's an issue with the hull earth bond or the earthing relay in the combi.

Can you easily and safely access the earth connection from the galvanic isolator on the combi side? (This is why I don't like 'inline' galvanic isolators as it makes this harder to do)

If so try connecting earth the connection from the galvanic isolator on the combi side (not the shoreline side) securely to the metalwork of the hull and see if the LED stops lighting.

The last page of posts hasnt really taken me forward very much but excellent discussion. I can try smileypete's suggestion when back on the boat next week. I have got someone coming out to have a look, but I will try and establish if the earth connection is broken by the GI even though the LED (a red one Nick)  is glowing slightly.

[Flyboy]

2 hours ago, nicknorman said:

With 2 electrolytics in series facing each other, any reverse bias current is limited to that required to charge the forward biased electrolyic. Its not the same as simply reverse biasing a solitary electrolytic. Thus if the biasing is DC and prolonged, the forward biased capacitor ends up with all the voltage across it, the reverse biased one with none (so it's no longer reverse biased).

There's no safety issue. If the capacitor goes open circuit - no hazard. If the capacitor goes short circuit - no hazard.

Totally agree with you, electrolytics in series face to face have been used for ages to make a non polarised one. Nowadays non polarised electrolytics are widely available.

[dmr]

17 minutes ago, Dr Bob said:

The last page of posts hasnt really taken me forward very much but excellent discussion. I can try smileypete's suggestion when back on the boat next week. I have got someone coming out to have a look, but I will try and establish if the earth connection is broken by the GI even though the LED (a red one Nick)  is glowing slightly.

Did you follow the link that I posted? Its a bit vague but it does suggest that a glowing LED can be normal with some battery chargers.

..............Dave

 

[Sea Dog]

8 hours ago, WotEver said:

 

But the disadvantage of considering them to ‘Switch on’ is that the reader can imagine a dead short like a switch, exactly as SeaDog did. Analogies rarely work well in electrical descriptions. 

You have misunderstood me, Sir; I was not imagining anything of the sort.  I said 'switched on' expecting it to be easier to comprehend for those readers without an electronics background.  I put it in inverted commas specifically so that those who have a better understanding of diodes, like yourself, would recognise that I was deliberately using the term out of context. I also worked on the drop being 0.7V whereas you used 0.6V, which I assume we can live with? 

Sweeping generalisation alert: Although the basic stuff is a long way in my past, I am a Chartered Engineer, and I must say that I have rarely found a basic electrical issue that's not better explained to a layman in terms of a plumbing analogy! 

I should add that your explanation does cover why the led still lights when there is a fault current.

Edited November 17, 2017 by Sea Dog
Add.

[WotEver]

3 minutes ago, Sea Dog said:

I also worked on the drop being 0.7V whereas you used 0.6V

It depends on the current

It’s between 0.6V and 0.7V (ish). 

[Sea Dog]

2 minutes ago, WotEver said:

It depends on the current

It’s between 0.6V and 0.7V (ish). 

See also my footnote which crossed in the post.

[WotEver]

5 minutes ago, Sea Dog said:

Sweeping generalisation alert: Although the basic stuff is a long way in my past, I am a Chartered Engineer, and I must say that I have rarely found a basic electrical issue that's not better explained to a layman in terms of a plumbing analogy! 

Hah! So find the plumbing analogy to describe battery charging complete with an explanation of efficiency and sulphation

I’ve tried (and failed) as have others. A sponge analogy only goes so far, as does a shallow bucket with a hole in it. Silting up doesn’t really cover it either.,,

[Sea Dog]

3 minutes ago, WotEver said:

Hah! So find the plumbing analogy to describe battery charging complete with an explanation of efficiency and sulphation

I’ve tried (and failed) as have others. A sponge analogy only goes so far, as does a shallow bucket with a hole in it. Silting up doesn’t really cover it either.,,

Oi! I did give you a "sweeping generalisation alert", did I not?

Edited November 17, 2017 by Sea Dog
Hmm, so if the bucket had one of those car wash grid things in the bottom, and we were using the 2 bucket wash method, but eventually the sponge was clogged after doing the wheels, and the rinse water was now muddy, and... Oh sod it, no-one understands b

[WotEver]

1 minute ago, Sea Dog said:

Oi! I did give you a "sweeping generalisation alert", did I not?

And hereby the rule is proven

[Sea Dog]

3 minutes ago, WotEver said:

And hereby the rule is proven

Perhaps, but such things are not to be aired in front of a mixed audience methinks...

[MtB]

On 17/11/2017 at 20:45, Sea Dog said:

Sweeping generalisation alert: Although the basic stuff is a long way in my past, I am a Chartered Engineer, and I must say that I have rarely found a basic electrical issue that's not better explained to a layman in terms of a plumbing analogy! 

 

Ok I'm curious now.  Which plumbing component is analogous for an electrolytic capacitor?!

[nicknorman]

45 minutes ago, Mike the Boilerman said:

 

Ok I'm curious now.  Which plumbing component is analogous for an electrolytic capacitor?!

That’s easy - an expansion vessel. The sort with no bladder. When you put positive pressure on it works as expected. When you put negative pressure on it sucks air in through the Schrader valve. But I suppose it doesn’t explode nor emit smoke when you do that!

[Sea Dog]

9 hours ago, Mike the Boilerman said:

 

Ok I'm curious now.  Which plumbing component is analogous for an electrolytic capacitor?!

^^^^ wot he said above. Or maybe a rubber thing on a stick. 

[dmr]

13 hours ago, WotEver said:

It depends on the current

It’s between 0.6V and 0.7V (ish). 

I think you will find that it depends on temperature, it can't really depend on current because almost no current flows till the threshold is reached, but yes, once the diode (or transistor) is conducting the volt drop will increase with increasing current.

...........Dave