Need some electrical engineering assistance

[heathicus]

My son is participating in his school's 8th grade science fair. He wants to use electrolysis (aka "Electrolytic Cleaning") as his experiment and test how the change in electricity effects the speed of the process.

The plan is to set up 3 small tanks, with as identical as possible setups so we only have one variable. I've got some pieces of rebar that are pretty rusty and will be our test pieces. We'll have all three running at the same time, in the same size container, with anodes that are alike, an electrolyte solution that was made in one batch and distributed to all three, etc. I don't know if we can manage to set up a live demonstration of the process for the science fair, but if we can, we will.

They didn't give us much time. They first mentioned the science fair Friday, sent home an information packet Tuesday that didn't really give any guidelines, he had his project approved today (Thursday), and the fair us a week from tomorrow (Friday). Between his homework and me getting home late from work, we really only have this weekend to perform the experiment.

We've been planning on using" wall wart" DC transformers of various voltages with the same amperage and test the change in voltage. So I stopped by Goodwill today and picked up 3 transformers - 5v/550ma, 9v/500ma, 12v/500ma. But now I'm wondering if that is even enough amperage to get any results and if we should be testing change in amperage instead. What say you electrical experts?

And if that's the case, what is the best method, given our limited time, materials, and budget? Can we keep voltage the same and only change amperage? If so, how?

[JPG]

Current is what is doing the 'work'.

The following will affect the current.

Surface area of the anode and the cathode(sacrificial and derust electrodes).

The spacing of the electrodes.

The electrolyte.

The voltage.

The surface conductivity.

The type of voltage(pure dc vs pulsating dc[filtered vs unfiltered]).

Soooo the easiest way to vary the current(all else being equal) is to vary the separation.

That would allow using the same power supply for all three.

Make sure the combined load does not exceed the power supply rating.

As a control, make sure the three sets are indeed experiencing different current.

Take photos of before and at periodic time intervals.

Even with small currents, a week should provide enough time to demonstrate the effect of different currents.

Might be a good idea to document the current at each photo op. I predict as the surface changes, the current will also.(gut feel!)

P.S. When taking pix, capture the side of the cathode that faces the anode. The current flow is primarily 'line of sight'.

[heathicus]

Can I measure the current with a multimeter?

I understand that adjusting distance changes the current, but for the purposes of a scientific experiment, why wouldn't it be better to keep everything the same except for the current applied?

We can't really run the experiment for a week. We basically have just this weekend for it. We're just not going to have time next week. He will still have to assemble his display and write a report. That's another reason I wanted to run all 3 tanks simultaneously.

[JPG]

Can I measure the current with a multimeter?

I understand that adjusting distance changes the current, but for the purposes of a scientific experiment, why wouldn't it be better to keep everything the same except for the current applied?

We can't really run the experiment for a week. We basically have just this weekend for it. We're just not going to have time next week. He will still have to assemble his display and write a report. That's another reason I wanted to run all 3 tanks simultaneously.

You do not apply current. You apply voltage and current results.

Since you are on a tight time schedule, you might consider using a car battery as a power source. Far greater current capability.

If the multimeter does not have a DC current range, inserting a small resistance and measuring the voltage drop across it would suffice. Small means less than 10 ohms. You could use one resistor across the meter leads for all three measurements(one at a time).

[dusty]

I would consider a battery charger for the power source. I use an old Sears charger when I do this. With the charger I use, there are at least two different charge rates (trickle and full).

[heathicus]

Well, we're not trying to achieve good cleaning, but to isolate the effect that increased voltage (or amperage) has on the speed of the process. I don't have a good enough battery charger. Just a trickle charger I use for my motorcycle and sometmes it works for electrolysis and sometimes doesn't, but I don't feel it gives me control over the variables. Plus, I don't have 3 of them to run simultaneously. With the wall warts, I can run them at the same time, and I know the exact voltage and amperage they provide. I can keep everything else the same.

My question is just whether 500mA is high enough to get results, and whether we should be testing change in amperage rather than voltage. If 500mA is not enough, how much is enough? If we should be testing amperage, is there an easy way to do that given our limitations and requirements? Wall warts of the same voltage but different amps? How many amps minimum? How much difference between steps up in amps?

[dusty]

I'm sorry, Heath, I can not answer your questions. I have never measured the current so I do not know. However, I believe that with only milliamps of current the process will work but very slowly and I do mean slowly.

I'd go set up and take some measurements but I can't do that either. I broke the bucket that I used and I don't have a suitable container to reassemble the setup.

Sorry. I have been of no help at all.

[BuckeyeDennis]

Heath, I don't know much about electrolysis, but JPG is correct that you can really only control the voltage (without a much more sophisticated setup). So you have "wall warts" at three different voltages. A good start.

Keep in mind that the wall warts are constant-voltage sources. If rated at 500mA, that means only that they can supply up to 500mA, without overheating and/or shutting down. It does not mean that they will supply 500 mA. The current you will get depends on the resistance of the load, as JPG said.

As to whether 500mA is enough juice to perform the desired electrolysis, I have not a clue. But there is one more factor that you can easily control: the size of the workpiece. I have a strong hunch that if you cut the workpiece in half, you will need only half the current to achieve the same rust-removal rate. So size the workpieces to match the current (and power) that you have available.

To determine just what size workpiece this might be, I'd start with the 12V wall wart. Set up a test unit, monitoring the current with your multimeter. You will need to set it up for DC current measurement, which usually means selecting the right multimeter mode and moving the red test lead to a different socket. Then connect the meter in series between the wall wart and the electrolysis tank.

Play with the size of the workpiece until the current draw is in the 300 to 400 mA range. That will give you a bit of safety margin, so that you don't overload the wall wart. Then use that size workpiece for all three experiments. The other tests should draw less current, because the supply voltage is less.

If your multimeter cannot measure DC current, do as JPG suggested and wire a resistor in series with each wall wart. These serve as current-sense resistors, and you can measure the DC voltage across them to determine the current. I'd suggest using 1 Ohm resistors, for two reasons. First, they will give you an easily measurable voltage level (with a decent meter), but not drop the voltage so much as to affect your experiment. Second, the voltage measured by the multimeter will be exactly the same as the amperage (current) through the resistor -- no calculations or conversions required.

Of course, it remains to be seen what the rust-removal rate will be. But hey, this is science, right!

[BuckeyeDennis]

A couple more thoughts:

1. Make sure that any 1 Ohm current-sense resistors are rated for at least 1/4 Watt. A 1/2 or 1 Watt resistor would be preferrable (so that they won't get hot enough to burn you).

2. Make sure that your wall warts all have DC outputs. A lot of them have AC outputs.

[dusty]

A couple more thoughts:

1. Make sure that any 1 Ohm current-sense resistors are rated for at least 1/4 Watt. A 1/2 or 1 Watt resistor would be preferrable (so that they won't get hot enough to burn you).

2. Make sure that your wall warts all have DC outputs. A lot of them have AC outputs.

A 1ohm resistor in series with a 12volt supply will draw 12amps of current or said another way will dissipate 12watts of power.

E=IR while P=IE Where E = volts dc, P= power in watts, I=current in amps and R=resistance in ohms.

Example: I=E/R = 12v/1ohm = 12 amps

What am I missing?