Scrapbox transformers, simple car battery chargers, (scary) computer PSU (I don't recommend this, especially not at 200A !) Lidl were selling suitable 5A PSUs last week for under a tenner.
Watch the voltage. Over 12V is too much. Too much gas generation, no better for de-rusting.
Good way to kill yourself. Anything over 2A is unusually specialised and only to be attempted with extreme care.
Moderately thorough dissertation:
I don't think 2A is at all unusual, we handle 200A DC regularly on our chargers, albeit at 29V, but 50A at 110V is a regular (had one in today from EWS) and 150A at 600V has been the highest so far.
Current won't hurt you, it's the voltage behind it that does the damage.
Going back to the electrolysis issue, there's a lot of info on one of the American websites, I'll check out some url's tonight at home.
Current against area matters, if you have a large area to derust then you will need extra amperage unless you are prepared for a long wait.
Peter
-- Peter A Forbes Prepair Ltd, Luton, UK snipped-for-privacy@easynet.co.uk
I'm sorry but I have to comment on such a dangerously wrong statement, it takes as little as 10mA of AC current through the heart to cause fibrillation which usually results in death. Voltage is the driving force behind current but it is the current that kills, if the body is in a particularly low impedance state such as when the skin is wet it only takes a relatively small voltage to produce a potentially lethal current, on the other hand if the body is dry the same voltage could be harmless as it produces little current. It's the current that matters not the voltage, and the path the current takes is fundamental to the danger, a current through the heart is very dangerous but one across the hand for example is far less dangerous.
Greg
Greg:
You can't 'have' 10mA of current if there is no voltage behind it to drive it through the resistance of the body.
My original statement stands.
Peter
-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:
And is total and utter bollocks, I've just explained that it's the current through your heart that's likely to kill you, in fact it's almost always the cause of a death by electrocution, the other being severe burns caused by heating due to current, yet you stand by your statement that "Current won't hurt you", hmm.
Yes a current needs a voltage to drive it but it's the current that does the damage, if it was the voltage then a static shock which is typically 10kV -
20kV would kill you which, oddly enough, it won't because it doesn't produce enough current. On the other hand a mere 230V shock from the mains can, in certain circumstances, produce enough current through the heart to kill you. It's not the voltage, it's the current.Greg
Greg is quite right, but I can see Peter's contention. The voltage is required for the current to exist, but it IS the current that is dangerous.
Voltage is potential energy whereas current is kinetic energy. A 100 ton weight suspended above your head is potential energy, but will not kill you unless it is released. It is the kinetic energy, i.e. the velocity of the falling weight that will kill you.
With good insulation to ground you can withstand many thousands of volts, but standing on a wet floor in bare feet even US 110 volts can kill.
Drop the resistance and what was previously a safe voltage becomes dangerous because the CURRENT has achieved a dangerous level. The voltage does not need to change for that to happen. It is when the CURRENT achieves the critical value that death occurs.
This is all basic Ohms law
Alan
I'm inclined to agree with Greg here. My understanding is that the current damages the body's tissues, not the voltage. But in order to have a current you have to have a voltage to drive that current through the body. What current is dangerous, and what voltage is needed to drive that current, are very difficult questions to answer. Even if someone can come up with answers to these questions, it's doubtful that the answers are very useful, as the severity of a shock depends very much on the environment and circumstances, and you don't want to push your luck with electricity. There is a pretty well verified story of a chap in the UK who survived a shock from an 11 kV pole transformer, but there are also several stories of people who have suffered fatal shocks from arc welding equipment, which usually has an open circuit voltage of 50 to 80 V.
What is clear is that if you have a power supply capable of supplying an infinite current, it becomes more dangerous with increasing voltage. Conversely, if you have a power supply with an output current limited to a very low value, say less than 1 mA, it will never be dangerous, even if it can supply that current at a very high voltage. Although my battery charger can supply 75 A, it can't necessarily drive 75 A through the human body. A few experiments I looked at online suggest a hand-to-hand resistance of between 1000 and 10000 ohms. Even at 1000 ohms it would take 75000 V to drive 75 A through the body. If the battery charger supplies 14 V it would be capable of driving 14 mA through a body with a resistance of 1000 ohms. As Greg says, this might just be enough to be harmful, but it would require stupidity e.g., one wet hand on each charger terminal, and even then it's unlikely to be fatal.
I still think that the battery charger will be safe to use for electrolytic de-rusting. Think about it this way: is a 75 A battery charger any more dangerous than a truck battery which can supply a peak current of 750 A?
Whatever I do, I'll be careful. I'll let you know when I've tried it.
Best wishes,
Chris
I'm not going to get into one of your, now well-known, 'pedant mode' arguments, Greg, you have your opinion and I'll have mine.
You worry about getting electrocuted by a PP3 battery, I'll carry on with our high voltage DC equipment.
No further discussion will be entered into.
Peter
-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:
Very well put Alan, far better than I could muster late last night 8-)
Greg
The wiring regs TRY to make this assesment, for example the much more stringent requirements for a room with a bath or shower that means people are likely to have wet hands AND feet, compared to a kitchen where only their hands are likely to be wet. There has also been quite a lot of research into the affects of current on both the human and animal body, but as you say the reality is so complex that it's all a very grey area.
And conversely there was recently a case of a couple of roofers carrying an alloy ladder through a wet field with an 11kV line over it, two lumps of charcoal were found by a dog walker some days later 8-(.
By coincidence I happen to have researched the potential short circuit current of batteries in my work recently, with a view to protecting FETs, a typical 24V truck battery can supply around 10,000A to 20,000A which can vaporise the metal forming the short thus producing severe burns. 24V on the other hand is considered a Safety Extra Low Voltage (SELV) by product safety standards and is thus safe for the public to touch.
Greg
My "opinion" is based on a professional knowledge of the Wiring Regulation and on European and International product safety legislation, and yours is based on?
To get back to the original issue, the big danger of using a low voltage mains derived power supply to remove corrosion from metal is not the limited voltage or current, but the hydrogen gas that it could liberate, just as for charging a wet call battery the liberated gas has must not be accumulated in an enclosed space.
I would, however, caution that if a large battery is used as a source it should have a suitable fuse fitted as close to the battery terminal as possible in case of a short circuit. An inline automotive blade type fuse holder is readily available and easy to fit. If it was me I would also fit a fuse if using a welder as the supply.
Greg
And just to prove that it's the current that kills, here's the results of an unfortunate experience with a 9V battery.
John
Yes it is possible, but fortunately very, very, rare. Our outer covering of dead skin cells is a good insulator and protects us from such things, but when wet or pierced the resistance of the body can be very low, add to this the fact that some are very much more susceptible than others and it can happen.
Anyone who doubts that a small battery has enough energy to kill should consider what a portable defibrillator is powered by, and it's job is to deliberately stop the heart and allow it to restart in a normal pattern...
It's interesting to read that the US Navy teaches that 100mA is needed to kill yet it's a documented fact that as little as 5mA AC through the heart itself can cause fibrillation, of course if the voltage is applied externally then only a proportion of the current will flow through the heart so a larger current is needed. The standard RCD in this country is calibrated to trip at 30mA and ones with lower ratings are used in some situations.
One think rarely mentioned is that it takes a very much smaller current through the brain to cause severe effects, hence a 9V battery applied to the wrong parts of the scalp can cause a severe shock. FOR GODS SAKE DO NOT TRY THIS AT HOME!!!!!
Greg
Moving back to the original topic of the thread, I just made another addition to my collection:
Chris
I suspect this isn't a complete picture. I believe defibrillators contain circuitry to boost the voltage considerably.
Best wishes,
Chris
I must try and coller an old Ferraris (sp?) disc meter to do some experiments with grid exited induction generators.
BTW. as you are obviously into such things, what did you make of this ebay item?:-
Yes that's right they do, the point I was making was that people don't realist that even a small battery contains enough energy to kill.
Greg
Oh I'm sure one wll crop up at a junk sale sooner or later - anything I can tinker with will do.
It was the two brushes at right angles to each other that got me - as I asked at the time, would a four pole repulsion motor have that configuration?
They come up on eBay pretty often. Usually they go for less than £10. I think the seller who I'm purchasing this one from had another which didn't sell if you're interested. I also have a spare myself which I was saving for a project, but I'm not sure I'll get round to it. If I don't get round to it soon you can have it. Let me know if you're interested.
That's odd. It looks to be a motor rather than a dynamo. It's unusually quaint. I'm not sure what to make of it. Either it is a very early motor as the seller suggests, or it's a motor made for demonstration purposes. As soon as the electrical engineering industry was established in the UK almost everything carried a data plate. If I had to guess I'd say it probably is a very early motor.
Chris
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