I'm trying to grow some metal crystals and I want to play around with different voltages and current level's. I have wall wort power supplies of 3.6, 5, 6, 7.5, 9, 10.8, 12v and higher, all with different current's but most of the lower v also have a current of 200-500ma.

Some of the tutorials I have read say to use 10-50ma in the cell when using something like a 2" long piece of 12g wire. I guess this has to do with current density (current & surface area of cathode).

I'd like to start with the 5v bc I have a lot of variations in current with these from 300ma up to 2.2a and about 8-10 ratings in between.

I can always make a couple cell's and run them in parallel to drop the current, but the problem is that this doesn't always work out the way it does on paper (the problems of real life application). Or I could run them in series to drop the voltage, which is easier than parallel IIRC.

But the best way would be if I could run some resistors in line (and caps if needed) I have a large selection of 1/4, 1/2 & 1w resistor.

So how would I drop 5v 500ma down to 30-50ma?

 

A very simple method of dropping the current is toput a light bulb in series with the load. BUT it must be an incandescent type light bulb. A small six volt bulb, like a #47, will drop the current from a 5 volt supply quite a bit, and give an indication of what it is doing.Various automotive bulbs will provide different current limits, and they can be cheap to get, sometimes. A variable resistor is another option, possibly 500 or 1000 ohms, wire wound. A new one will be expensive but a used one can be very cheap. They are often found in the older tube type television sets. The much newer solid state sets will have the higher powered resistors but they will be the fixed types, not adjustable.

 

A very simple method of dropping the current is toput a light bulb in series with the load. BUT it must be an incandescent type light bulb. A small six volt bulb, like a #47, will drop the current from a 5 volt supply quite a bit, and give an indication of what it is doing.Various automotive bulbs will provide different current limits, and they can be cheap to get, sometimes. A variable resistor is another option, possibly 500 or 1000 ohms, wire wound. A new one will be expensive but a used one can be very cheap. They are often found in the older tube type television sets. The much newer solid state sets will have the higher powered resistors but they will be the fixed types, not adjustable.

Thanks. I have about 5,000 new 1/2 watt resistors in all sizes & about 1,000 1watt resistors of same values. I do have a bunch of 1/2w 1000 ohm POTS's as well.
I just need to know what equation I use to figure out what drops or limits the current. I've looked at some ohm's law equations but they always ask for R1 and R2 (with a given input voltage of 5 in this case) but how do you solve for 2 unknowns like that. they could be 5 and 10, or 55 and 110, or 1500 and 3000. You have to know one of the R values to figure out the value of another, and I never know where to even start.

I would think this would be a simple issue for anyone who knows electronics, especially with actual resistors with SET VALUES, instead of some "lightbulb" of unknown wattage, unknown resistance, just find one the one that works.... Yeah. I'm not knocking you MrBill, i've had similar responses, but I thought it could be figured out with ohms law but IDK how to find a starting resistor value.

Wouldn't you admit that it would be easier to use a resistor(s) with known resistance and wattage rating than some random automotive lightbulb? I can't see to many professors suggesting that, and again, not a knock on you, I've seen that suggested many times, and maybe that is good if peope don't have resistors, but I have more than I could possibly need for this project and would like to use them.

 

I should start by saying I know essentially nothing about growing crystals with electricity.

Nevertheless, I think you'll find that there isn't a good formula for choosing what resistor you need because (l presume) the resistance of the solution and any crystals that form are changing over time. If that resistance is variable, then the only way to control the current would be to also vary the external resistance as well.

If the changes are slow enough, and if you're monitoring the situation, you could use an adjustable resistor, as someone suggested above, and just make small adjustments as needed throughout the growing period.

Another approach might be to use a current regulator. It will adjust automatically to maintain the current you choose. There are a variety of ways to make a current regulated circuit, but the easiest is probably to use an adjustable regulator configured for current control, like the LM317:

Using 120 ohms for R1 gets you 10mA. 27 ohms gets you 44mA. Etc.

 

The LM317 circuit in post #4 alway tries to maintain 1.2V between the ADJUST and OUTPUT pins, thus the output current equals 1.2V/R1.

Note that the LM317 requires the input voltage be a least 3V higher than the output load voltage for proper operation.

 

Also its better to use the 120Ohm, 27Ohm is a bit little.

 

There's no real need for an lm 317 circuit , you do not need an accurate current ... below 10mA you get bigger crystals , but they take months to grow (copper metal crystals with CuSO4 electrolyte) ...

Your power supply is not ideal , it consumes a lot of electricity if running for months , and if like, mine is noisy(fan)...

Any old rechargeable batteries are best , discarded old phone cells or an old car battery that will not start an engine and so has been discarded is perfect ... forget about voltage ,the cell just sees the same approximate voltage (about 2 V) if you use a 12 V source you just have to put more resisters to get the voltage to the electrolysis cell down to 2V . Lithum battery (3.7) will be more efficient but at such low currents it doesn't really matter . Just get a 20mA max. ammeter and put enough resistors in the circuit to get the low current you want .

Above 20mA creates clumps of lots of small crystals that break apart ... low current and patience is the key.

 

There's no real need for an lm 317 circuit , you do not need an accurate current ... below 10mA you get bigger crystals , but they take weeks or months to grow (copper metal crystals with CuSO4 electrolyte) ...

Your power supply is not ideal , it consumes a lot of electricity if running for months , and if like, mine is noisy(fan)...

Any old rechargeable batteries are best , discarded old phone cells or an old car battery that will not start an engine and so has been discarded is perfect ... forget about voltage , just get a 20mA ammeter and put enough resistors in the circuit to get the low current you want .

Above 20mA creates clumps of lots of small crystals that break apart ... low current and patience is the key.

If he does not need an accurate current or voltage, only a pot or resistor should be enoug. But there is always a need to limit the current, I do not know if these crystals can drain a large current or get damaged by that.

 

If he does not need an accurate current or voltage, only a pot or resistor should be enoug. But there is always a need to limit the current, I do not know if these crystals can drain a large current or get damaged by that.

This is very simple , the current will not run away or change rapidly ... and can be anywhere from 100 to 1 mA ... the lower the current the bigger a single crystal can be ...high currents lots of tiny ones in a clump ... the mass of copper deposited is totally dependent on the current

100mA will deposit copper 100 times faster than 1mA ... voltage is not relevant don't wast your time measuring it .

A coulomb is one amp passing for 1 sec ..or 1mA passing for 1000secs ...

every coulomb will grow your crystal by about 0.00033 gms (for Copper)

If your current is 10mA your crystal/s will grow by 0.29 g each day

 

Of course, setting the exact current to pass through an unknown resistance that has a tendency to change as the process goes on is a bit complicated with anything other than a current regulated supply. My suggestion was based on what somebody who has limited resources, such as a large collection of wall-wart power supplies might have.
The challenges are dual: First, the resistance of the crystal growing setup are initially unknown, and next, they change continually. So a nice dual mode power supply would work very well. Digital readouts make setting the current and voltage limits very convenient. But most folks don't have such a nice supply as that.
The suggestions that I offered will work but they will also take a bit of experimenting because of the two unknown variables that are changing continually.

 

Of course, setting the exact current to pass through an unknown resistance that has a tendency to change as the process goes on is a bit complicated with anything other than a current regulated supply. My suggestion was based on what somebody who has limited resources, such as a large collection of wall-wart power supplies might have.
.

Of course !!.... on reconsideration the easiest and best solution for a power supply is a cell phone charger ...most people will have one of these in their junk box ... they output 5V , and can handle 100mA , just put the required resisters with the ammeter and electrolysis cell all in series , change current level ,by changing the resistance in the circuit.
It wouldn't even be a problem if the current drifted between 10 and 5 mA ... in practice it never will ,,, it may perhaps drift a few %

To get a current of 20mA , voltage supply (5V) minus the push back from the electrolysis cell (2V) ... that's 3 V ....V=IR ...... R = 3/0.020 = 150 Ohms

So if using a cell phone charger you need resistance in the order of 150 Ohms ...size a tenth of a watt is adequate

 

To get a current of 20mA , voltage supply (5V) minus the push back from the electrolysis cell (2V) ... that's 3 V ....V=IR ...... R = 3/0.020 = 150 Ohms

That's interesting. I'd assumed the cell would behave more like a resistance, but you're describing it more like a mostly-steady voltage drop, like in diode calculations. I learned something new today. Thanks!

 

That's interesting. I'd assumed the cell would behave more like a resistance, but you're describing it more like a mostly-steady voltage drop, like in diode calculations. I learned something new today. Thanks!

This is only a description, all things that consume electricity behave like resistance.

 

It you want to approximate constant current by using a resistor, the higher the power supply voltage and hence the higher the series resistor, the better. This simply helps to make the fixed resistance the dominant term in the current. Taken to a silly extreme for illustration, if you started with a million volts and used a 100 million ohm resistor, you'd get 10 mA. That 10 mA would change by only a tiny amount if you added a 50 volt source (of either polarity) and 10k ohms into the circuit. (An ideal current source has infinite impedance, as the converse of an ideal voltage source that has zero impedance.)

More realistically, using say 12 V instead of 5 V would give much more consistent current if the were variation in the uncontrolled part of the circuit resistance, especially when the electrolysiis cell potential must be subtracted from the power supply voltage (e.g. if the cell potential were 2 V, a 5 V supply would leave 3 V across the total circuit resistance, whereas a 12 V supply would leave 10 V; there will be some resistance in the electrolyte). It does come at the cost of lower efficiency, of course. A 12 V supply at 10 mA is 120 mW, amounting to about 0.95 kilowatt-hours per year of operation, ignoring the inefficiencies of the power supply itself.

 

This is only a description, all things that consume electricity behave like resistance.

No.

The "electrolysis cell" referred to here is an example of something that "consumes electricity" that does not behave like a resistance. At one electrode of the cell there is an oxidation reaction and at the other there is a reduction reaction. The potential (voltage) required for those reactions does not change with the magnitude of current, so the behavior is not at all like a resistance. There will be some parts of the circuit that are resistive in nature, such as the electrolyte itself. All rechargeable batteries behave in the same way.

"Also its better to use the 120Ohm, 27Ohm is a bit little."

No.

In the circuit shown, as described, that resistor sets the current because the 317 regulates the voltage across it to 1.25 V. If you regulate the voltage across any fixed resistance in any series circuit you regulate the current through the entire circuit.

 

No.

The "electrolysis cell" referred to here is an example of something that "consumes electricity" that does not behave like a resistance. At one electrode of the cell there is an oxidation reaction and at the other there is a reduction reaction. The potential (voltage) required for those reactions does not change with the magnitude of current, so the behavior is not at all like a resistance. There will be some parts of the circuit that are resistive in nature, such as the electrolyte itself. All rechargeable batteries behave in the same way.

"Also its better to use the 120Ohm, 27Ohm is a bit little."

No.

In the circuit shown, as described, that resistor sets the current because the 317 regulates the voltage across it to 1.25 V. If you regulate the voltage across any fixed resistance in any series circuit you regulate the current through the entire circuit.

The meaning behind my post is this: its not good for an LM317 circuit to use 27Ohm and to find a 27Ohm resistor might be hard.
Second all things that consume electricity behave like resistance. You might choose to describe it from a voltage point of view, but there will always be the resistance point of view. Its a matter of description desire. And yes in this case where we have an LM317 regulator and a voltage drop , it will be the resistive part that we care for. Unfortunately you post a constant voltage drop of "2VDC" and a constant current of "20mADC" but you say its not resistive!?

 

The meaning behind my post is this: its not good for an LM317 circuit to use 27Ohm and to find a 27Ohm resistor might be hard.
Second all things that consume electricity behave like resistance. You might choose to describe it from a voltage point of view, but there will always be the resistance point of view. Its a matter of description desire. And yes in this case where we have an LM317 regulator and a voltage drop , it will be the resistive part that we care for. Unfortunately you post a constant voltage drop of "2VDC" and a constant current of "20mADC" but you say its not resistive!?

27 ohms is a standard E12 value and common as dirt.

As for the rest of what you said, it is clear that you just don't understand any of it.

 

27 ohms is a standard E12 value and common as dirt.

As for the rest of what you said, it is clear that you just don't understand any of it.

I do understand what do you mean, but I did not post that.

27 Ohm may be standart, but when you go in the shop they might have it and they might not. Also "1.25VDC" / "27R" = "0.046A" instead of the "20mA" we want.

 

Very cheap ($17.99)
Digital High Precision Signal Generator, Current LED Display, Stable Signal Source with 0.01mA Adjustable Potentiometer, 4-20mA, DC 12-30V Constant Current Source .
You can feed it from your 12V or 24V wall wart:

 

The initial post suggested a possible range of 10-50mA:

Some of the tutorials I have read say to use 10-50ma in the cell

So I gave basic examples with standard value resistors that would cover most of that range (based on the 1.2V reference value used in the datasheet I was looking at):

Using 120 ohms for R1 gets you 10mA. 27 ohms gets you 44mA. Etc.

If you want 20mA, that's fine, choose a different resistor accordingly.

Also "1.25VDC" / "27R" = "0.046A" instead of the "20mA" we want.

And this may all be moot, because the people in this thread who actually have experience doing this stuff seem to think constant current sources are overkill, and that an appropriate voltage with some resistors works great. I defer to those with more experience in the field.