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Limiting current

Posted on 2016-09-20
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Last Modified: 2016-09-29
Hi there,
I have a small project on the goal to de-rust some old car parts.
I have setup a small Electrolysis tank for this and I am now trying to get my head around the relationship of Volt to Ohms and Ohms law etc.

I need to provide it with a 12v supply and allow it 2-300mA. I have either a power adaptor which can output 300mA or a big 12v battery.

If I connect the leads direct to the tank it draws around 5 amps. I need to limit this to 2-300mA so I thought I could use a resistor. I put a 100ohm resistor in line with the positive lead and whilst it did limit the current, it also get very hot which I didn't think was right. It also only allowed around 2v through when measured.

What is the best method to limit the current off the battery?

I want to do this outside without the need for mains power but if I did use a mains adaptor 12v 300mA power supply I presume it would not do any harm to the adaptor? The draw of 5A from the battery concerned me but I gather the adaptor would push the 300mA rather than let a much higher load be pulled through it.

Many thanks,
Jack
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Question by:JackHodson
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20 Comments
 
LVL 98

Assisted Solution

by:John Hurst
John Hurst earned 248 total points
ID: 41807836
The test you did means the tank does not run at 12 volts. It runs at less than 12 volts. So the battery is supplying too much current ONLY because the tank is not rated for 12 volts.

If you get the voltage correct, that is, supply the rated voltage for the tank, it will draw the correct current.

Because the battery voltage is too high, using a dropping resistor as you did lowers the voltage too much.

What is the rated voltage of the tank?
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LVL 33

Expert Comment

by:Paul Sauvé
ID: 41807845
i did a quick check on Google (i was a chemist in another life)

it seems that most techniques use a battery charger

for example:
Cleaning Rusty Tools; Electrolysis Made Easy―http://www.rickswoodshopcreations.com/miscellaneous/rust_removal.htm
and
0
 
LVL 49

Expert Comment

by:dbrunton
ID: 41807884
>>  It also only allowed around 2v through when measured.

You put the resistor in a series circuit.  That will limit the voltage.  Not the current.

You'd need to put the resistor into a parallel circuit.  That will limit the current.

I recommend going the adapter way.  You can always measure the current draw and voltage it puts out as a check.
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Expert Comment

by:Lim HC
ID: 41807936
By putting the resistor into a parallel circuit will limit the current. Do understand that you have to get a higher wattage type of resistor to withstand the heat .
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LVL 22

Accepted Solution

by:
CompProbSolv earned 1008 total points
ID: 41808024
If you are connecting 12V to the tank and it is drawing 5A, then it is showing a 2.4Ohm load (Voltage=Resistance* Current or Resistance=Voltage/Current).

If the electrolysis tank is a truly resistive load (not likely) then you COULD limit the current with a series resistor.

To get 300mA from 12V then you need a total load of 40 Ohms (12V/300mA).  Since your tank acts a 2.4 Ohm you would need a resistor of about 37.6 Ohms.

Keep in mind that Power=Voltage*Current=Resistance*Current^2.  Under these circumstances you'd be dissipating 0.216W through the tank (2.4 Ohm *0.3A*0.3A) and 3.4W through the resistor (37.6 Ohm *0.3A*0.3A).  That would cause the resistor to heat up significantly.

If you put a resistor in parallel with the tank it will pull current round the tank, but that will only change the current through the tank if you end up pulling down the battery voltage.  This is not a good approach.

The 12V 300mA adapter MIGHT work, but not likely.  It is designed to put out 12V up to 300mA.  Your tank will try to draw more than 300mA and what happens to the adapter depends on how it is designed.  In any case, you are using a device that is intended to put out a specific voltage when you want a specific current.

The comment "I need to provide it with a 12v supply and allow it 2-300mA" is not really accurate.  Between voltage, current, and resistance you only get to pick two.  The third will follow.  My bet is that the tank wants a regulated 200-300mA and that it shouldn't take more than 12V to get it.  What you probably want is a power supply that has the current regulated to 200-300mA and not the voltage.

You can construct a current limiter to connect to the battery to ensure that you get whatever you want in the 200-300mA range being delivered to the tank (as long as 12V is enough to accomplish that).  It will end up acting like the 37.6 Ohm resistor that I mentioned above, but will vary itself as needed.  In any case, it will have to dissipate significant power just as the resistor would.

The critical question is: does the tank want 12V (not likely) or 200-300mA (more likely)?  The design of the power supply is different in those two cases.  While most sophisticated devices are designed to run from a specific voltage, I'm betting that this wants a specific current as it doesn't have its own internal regulation system.
0
 
LVL 93

Assisted Solution

by:nobus
nobus earned 248 total points
ID: 41808120
the best solution is to use a controllable power supply with a current limit;
buy one or make ine yourself :  http://www.instructables.com/id/PC-Power-12-V-Current-Limiter/
0
 

Assisted Solution

by:Lim HC
Lim HC earned 248 total points
ID: 41808159
with the controllable power supply current limiter. should add a VR to control the current.
New-Picture.png
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LVL 22

Assisted Solution

by:CompProbSolv
CompProbSolv earned 1008 total points
ID: 41808183
@Lim:
That circuit looks as if it will regulate voltage.  From the original posting I think it is current that needs to be limited.  There's no assurance that the tank is a fixed resistive load.
0
 
LVL 22

Assisted Solution

by:CompProbSolv
CompProbSolv earned 1008 total points
ID: 41808190
@nobus: as it states, that circuit is intended to allow "tens of milliamps of current" which is not what is needed here.  It's more of a short-circuit protector.

One has to keep in mind that if a 12V supply is going to deliver 300mA, there will be about 3.6W to dissipate somewhere.   Based on what we were given, nearly all of this will be dissipated in the current regulator.  Some sort of heat dissipater is going to be needed.
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LVL 88

Assisted Solution

by:rindi
rindi earned 248 total points
ID: 41808264
As far as I know, for electrolysis to work, there isn't much of a voltage or power requirement. Why are you limiting yourself to the values of 12V and 300mA?

Theoretically you could also use a lower voltage, like 5V, and I have an old nokia cell phone powersupply here that outputs 5V and supplies a current of 350mA, which would be close to your current... That way you wouldn't be burning up all your waste energy by using resistors. The process would just take longer. Even if you are using a 12V Powersupply that also supplies more then 300mA the electrolysis should work fine. With more Amps it would be faster. Just make sure you don't use your car battery as source. They usually don't have any protection circuits that protect the battery from over loading it.
0
 
LVL 93

Expert Comment

by:nobus
ID: 41808523
i know - it's only an indication to get him started, if he wants to do it himself
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LVL 4

Author Comment

by:JackHodson
ID: 41812397
Thank you everyone for your great replies - I didn't expect so much.

I have just assumed that 12v is the requirement for a DIY electrolysis tank - 300mA would be a better way to describe the requirement.

rindi, your suggestion of the adaptor is probably my easiest bet but as CompProbSolv says if I put  12v adaptor with a 300mA output it could  get damaged by the tank wanting more (as in the 5A it pulled from the battery when I connected that without any resistors). I was hoping that the tank would just take what it is given and won't try to draw more current than is supplied.

I have an old PSU available too. I would use a car battery charger but the type I have is the newer digital type and I don't want to ruin it.

So, I need the best way to get a steady 300mA.

Many thanks.
0
 
LVL 93

Expert Comment

by:nobus
ID: 41812553
the best way is a power supply with current regulation, as sshown here :  http://www.electronics-lab.com/project/0-30-vdc-stabilized-power-supply-with-current-control-0-002-3-a/

you can also buy them ready made
0
 
LVL 98

Expert Comment

by:John Hurst
ID: 41812588
You should really check the true voltage requirements as it would not draw 5 amps if it only needs 300mA
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LVL 4

Author Comment

by:JackHodson
ID: 41812596
Thanks - I will give that a go.
@John - it doesn't have any specific requirements - it is just a big bucket of water with washing soda and sacrificial plates of steel in it.

 Thanks again all.
Jack
0
 
LVL 88

Expert Comment

by:rindi
ID: 41812692
Only if you are using a battery would could it get overloaded. A normal powersupply just won't deliver more than it is specified for. For a large tank I don't think 300mA will be close to enough, except if you have years to wait.
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LVL 98

Expert Comment

by:John Hurst
ID: 41812760
Ohms law is just that - LAW. And as an outcome, current is pulled and not pushed.

So if the battery runs the tank at 5 Amps and you are sure about the 12 Volts, then a power supply providing 300 MA will run the tank at about 2 Volts.

You need to sort out a proper supply.
0
 
LVL 22

Assisted Solution

by:CompProbSolv
CompProbSolv earned 1008 total points
ID: 41812770
The key here is whether the process needs a constant current (likely) or a constant voltage (less likely).  The resistance of the box likely changes with different operating conditions so it's not enough to assume that a specific voltage will give you the desired current.

You COULD use a supply of 0.72V which would give you your 300mA of current (based on 12V giving 5A) but the current would change if the resistance of the box changes, which it probably does.

So.... if the process wants a specific current, you need a power supply with current regulation.  The example that nobus gave would likely do the trick well.
0
 
LVL 88

Expert Comment

by:rindi
ID: 41812778
Electrolysis doesn't require a specific current. It'll just work faster if you have more current available and the liquid can draw it. If it can't draw the current, the current will automatically be less.
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LVL 93

Expert Comment

by:nobus
ID: 41813433
if you use a controllable, voltage and current supply - you can set all like you wish
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