current

Discussion in 'General Electronics Chat' started by h2opolo, Mar 28, 2014.

Aug 2, 2013
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I have some waterproof electrical components that will be powered by a dc power supply. The voltage will be from 12 to 35 v. The power supply will be away from the water but there will be two power terminals under water that cannot be insulated. The terminals are locate 4 to 5 feet apart from each other.

If the sum of the current from the electrical components and the current across the terminals from the water is less than the rated current on the power supply, will the circuit function properly?

Is there a way to calculate the current through the water as a function of distance and voltage? This will not be salt water.

2. paulktreg Distinguished Member

Jun 2, 2008
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Well your water, unless distilled, is going to be conductive but by how much is hard to say.

Why can't you insulate it? Large area?

If you are going to calculate the current through the water it I suppose it depends on how "impure" it is and must take into account the surface area of your terminals?

3. DerStrom8 Well-Known Member

Feb 20, 2011
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In order to calculate the current you would need to know the exact resistance of the water between the two contacts. This is next to impossible considering water will have different conductivity depending on how impure it is. Distilled water is not conductive, but impurities such as iron flakes, salt, etc. will change that. So unfortunately it is impossible to tell for sure what the current through the water would be.

As for the circuit working or not, it really depends. At the very best it will be unstable, which is not a good thing. Running uninsulated power supplies underwater is NEVER a good idea, you really MUST find a way to insulate the contacts.

Aug 2, 2013
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yes the surface area is too big to insulate. It's never going to be in salt water and it's also not distilled. It will be in contact with tap water and pond water. I would say the terminals have a surface area of 1.5 square feet.

5. DerStrom8 Well-Known Member

Feb 20, 2011
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Woops, looks like paulktreg beat me to it

Anyway, there's really not a good, safe way to do what you're asking. I would recommend exploring some different options. If you could make it a little clearer what exactly you're trying to do, we may be able to help you better.

Aug 2, 2013
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I was just trying to explore this concept because I thought it would be an interesting idea. I did some research and according to a source on harvard.edu the resistivity of ground watter can be anywhere from .5 to 300 ohm-meter.

I figured in the absolute extreme scenario you have .5 ohm-meter, a generously measured 1.5 square feet (.139355 m^3) of cross section, and an underestimated distance of 1.2192 m.

so... (.5)(1.2192)/0.139355=4.3744

v=i/R
v/r=i
max voltage = 35v
35/4.3744=8A ... which is not that bad at all

Now consider if we took the average ground water resistance.
(150 ohm-meter)
... the same calculation would be .03A which is very low

7. DerStrom8 Well-Known Member

Feb 20, 2011
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I don't know what you're using for a power source but 8A is quite high. At that current you would probably notice significant heating in the power supply and contacts/wires, etc. Not to mention your circuit would not get much current at all, most of it would be going through the relatively low-resistance (300 ohm) path.

Aug 2, 2013
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yes 8A is pretty high considering it is doing nothing but wasting electricity but what I meant by not bad is it will not over load my power supply. I would not do this if this were the case. But if I experimentally measure the resistance of the water with a meter and it turns out that at 35 volts its less than 1 A it may actually be plausible.

9. DerStrom8 Well-Known Member

Feb 20, 2011
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Again, 1A of wasted current is still quite high. Not to mention the entire system would be extremely unstable and inefficient. I still don't think it's a good idea. Period.

Aug 2, 2013
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It would depend on the water I need to test. It could very well be .04 A which on a 15 A power supply is only .3%

Aug 2, 2013
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but yes I agree, mixing electricity and water is never the ideal solution!

12. inwo Well-Known Member

Nov 7, 2013
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Anything over 100ma dc can be lethal. Certainly over 500ma.

Not to mention chemical problems.

Without more details this probably shouldn't even be discussed.

13. DerStrom8 Well-Known Member

Feb 20, 2011
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Heck, even 30mA can be lethal, or less!

@theOP It's not just that it's "not the best way to do things", it's a DANGEROUS and STUPID way to do things. Plus you would be lucky if your circuit even works when paralleled with even a 300Ω resistor. It's not that it "shouldn't be done", it really "can't be done" as you describe.