Current Limiting Diodes

[Trailboss]

I am helping my niece working on an electrolysis project to produce silver nano particles (colloidal silver).

A simple process applying a potential to the anode and cathode in a electrolyte solution.

The electrodes are pure silver rods (anode and cathode are silver), the solution is dilute sodium carbonate in distilled water.

The applied voltage should be 24 volts DC

Current should be limited to around 1 to less than 5 ma.

I was going to lend her my current regulated power supply, when I thought a simpler solution might be to just use a low power 24 volt DC walwart and put a 1 or 2ma current limiting diode in series with the electrodes.

RL would vary over time as the electrolyte becomes more conductive due to the silver ions being produced, but looking at the data sheets I think this simple solution would be viable?

any thoughts appreciated

[T3sl4co1l]

If you need current limiting, you won't have 24V.

Reduce the sodium carbonate concentration instead: this reduces conductivity of the solution.  Merely monitor current flow using a multimeter.

Tim

[ScottK]

Here's a simple constant current sink. At 1-5mA, you can use almost any little NPN transistor.



Regulation won't be as good as the three terminal regulator method mentioned by another poster, but it'll do for your niece's needs. You do need more than 24V to accommodate the headroom for the regulator, but that's true of any method you'd use to achieve constant current. If you don't have a pot, replace the combination of R3 and R4 with a single resistor that gives your desired current with 5.1V across it.

You can probably use three 9V alkaline batteries to power the experiment. Current will fall with battery voltage, but that may not be an issue over the duration of the experiment.

ETA: How do I get the attached image to show full size in the post? I'm new here!
ETA2: Thanks for the tip, Brumby.

[mtdoc]

I am helping my niece working on an electrolysis project to produce silver nano particles (colloidal silver).

Please tell me no one is planning to ingest this colloidal silver.

[george.b]

I am helping my niece working on an electrolysis project to produce silver nano particles (colloidal silver).

Please tell me no one is planning to ingest this colloidal silver.

I was about to ask what his niece wants the colloidal silver for. I, too, hope nobody's planning to ingest it.

[T3sl4co1l]

Meh, it's not really toxic or anything.  Not that it's particularly useful, either.

Doing it regularly, not a good idea for other reasons:
https://en.wikipedia.org/wiki/Argyria

Tim

[george.b]

Well, precisely. Who wants to end up looking like a smurf?

[JoeN]

Well, precisely. Who wants to end up looking like a smurf?

Could be useful if you are into cosplaying, Smurfs™...  Just sayin'...

[Trailboss]

Sounds interesting.  Do you have a reference to the process (electrolyte) and suggestions for obtaining the materials (metal, electrolyte)?
I have no present need / interest in it but it seems to ring a bell about something that I was thinking of for some reason or other in years past.


Yes she is working from a document for the process, but they just state using a 300ma or less 24 volt supply, wallwart I'm guessing. They go on to say you want to keep the voltage around 24v and the current as low as possible. So in my experience cleaning rusted parts using electrolysis electrolyte concentration and separation distance determine the current flow.

So, Upon doing a little more research on nano particles (I don't fully understand the physics) I found that keeping the current low is extremely important otherwise the nano particles become too large. In addition to the electrolyte the solution contains glucose and gelatin. The process is performed on a heated magnetic mixer.

Anyway you say "the applied voltage should be 24 VDC" and also that you want to limit the current to 1mA.  The problem is that if you have a solution with a given resistance (conductivity) which is affected by temperature, ion concentration, electrode spacing, etc. then the actual electrode voltages that produce 1mA will vary.
Certainly you may need "at least" 24V under some conditions (weakest solution, farthest gap electrodes, ...) but there's no point in trying to keep the electrode voltage constand and the current constant AFAICT.

Yes I get that.

Anyway to get a 1mA constant current with a 24V supply  and a variable load resistance could use some kind of series current regulator like a current regulator diode but typically your current regulator needs some "voltage headroom" to operate maybe a few volts so if your necessity is to have 24V available under the most resistive solution conditions (no current limiting needed by the external circuit) then you'd have to adjust your power supply to maybe 28V or something to allow the current regulation circuit some voltage to operate its own internal circuitry.  Then you could get something like 24V applied to a high resistance circuit like 100kOhms or something that would be drawing less than 1mA due to the solution resistance and as the solution conductivity became less resistive than 24kOhms (1mA at 24V potential) the current regulator would drop the voltage applied to the regulated electrode by dropping more voltage across the regulator itself until the net current was 1mA.

Yes, that's my "theoretical" dilemma, as the RL will vary based on the solution, distance of electrodes and as the solution picks up more ions, becoming more conductive I don't want the current to become to great.

So I guess I need to put this together and try it with current regulation diodes, and or build a little supply using a TL, with a current meter in the circuit at all times to see what is actually going to happen.

Looks like I need to put this together and see what works best!


Of course it is a good idea to also set the current limit of your PSU to something that will prevent problems if there was a short circuit.  I don't know if you can float the PSU output terminals or if they will be earth ground referenced but any case keep in mind possible short circuits and such.

For sure, non islolation is always a concern.


I doubt the current is that critical for the process, I suspect anywhere under a few milliamps down toward near zero would work and the only difference would be how fast the process proceeds due to the coulombs charge participating in the electrochemistry per hour / whatever.

Exactly my goal, to just keep it from running away.

You should get a more precise result (maybe 5% or whatever the device is rated for) from the TL431 than the current mirror due to the transistors not being matched and isothermal and such.

 
 

I am helping my niece working on an electrolysis project to produce silver nano particles (colloidal silver).

A simple process applying a potential to the anode and cathode in a electrolyte solution.

The electrodes are pure silver rods (anode and cathode are silver), the solution is dilute sodium carbonate in distilled water.

The applied voltage should be 24 volts DC

Current should be limited to around 1 to less than 5 ma.

I was going to lend her my current regulated power supply, when I thought a simpler solution might be to just use a low power 24 volt DC walwart and put a 1 or 2ma current limiting diode in series with the electrodes.

RL would vary over time as the electrolyte becomes more conductive due to the silver ions being produced, but looking at the data sheets I think this simple solution would be viable?

any thoughts appreciated

[Brumby]

ETA: How do I get the attached image to show full size in the post? I'm new here!

1. Prepare your post with the attachment.
2. Post it.
 ^-- This much you have already done --^
3. Click on the attachment thumbnail.  The full sized image will appear.
4. Right click on this full size image and click on "Copy image address" (Chrome) or the equivalent in your browser.  This will put the URL of the full sized image on the clipboard.
5. Edit your post and add the image tags where you want the image to appear and then paste the URL in the clipboard within the tags.
6. Click "Save" and you're done

... and it will look like this:

[Trailboss]

Meh, it's not really toxic or anything.  Not that it's particularly useful, either.

Low PPM and short term, has shown benefits in tackling drug resistant strains. Most burn wards use silver and has been used in MRSA cases too.



Doing it regularly, not a good idea for other reasons:
https://en.wikipedia.org/wiki/Argyria

Tim

[mtdoc]

Meh, it's not really toxic or anything.  Not that it's particularly useful, either.

Low PPM and short term, has shown benefits in tackling drug resistant strains. Most burn wards use silver and has been used in MRSA cases too.



Doing it regularly, not a good idea for other reasons:
https://en.wikipedia.org/wiki/Argyria

Tim

No. It's quackery.

The harm comes if you're using it  instead of real, proven treatments.

And the there's the whole smurf thing..

[Benta]

1...5 mA is actually the sweet spot for JFET-type current regulator diodes.
My favorite was always the J500 series from Vishay-Siliconix (2-pin TO-92), working voltage up to 50 V, and more important: operation below 1 V.
They're unfortunately obsolete. 

[T3sl4co1l]

<one giant wall of quote>

Do you mind editing that to unquote your responses?  It's hard to read.

I guess you used the [bold] markup, so you can see how this stuff works... just change the [bold] to end-quote and the end-bold to start-quote.

Tim