Constant Current control cct. help please

Thread Starter

PhillT

Joined Feb 26, 2010
2
Although I am capable of building a project from a circuit diagram, I have not got the skill or education to design anything more than simplistic circuits, so I am here to ask for assistance from those with the education & skills.

My project is to build a circuit that will result in a constant output current of 3.8ma supply to a varying resistive load. I have a 36 volt dc power supply unit (from an old printer) as the power source.

The resistive load consists of two electrodes in an electrolyte which begins as Distilled Water, so very high resistance, which over time will reduce resistance as ions and particles increase in the electrolyte, but I want to maintain 3.8ma (well, as near as possible anyway) as the resistance of the load drops. This is vital to ensure the correct particle size and ratio of particles to ions in the solution. (Colloidal Silver)

Any help in this regard will be greatly appreciated.
 

robert.e.lee

Joined Aug 21, 2011
13
After doing some research on the antibacterial properties of colloidal silver, I decided to make some.

Yet I haven't been able to find any information yet about to to keep a constant current while the resistance is getting lower and lower as time goes on.
If you or some other knowledgeable person could post what I can do to keep the current from a AC to DC wall adapter constant it would be greatly appreciated.
 

wayneh

Joined Sep 9, 2010
17,496
I hope we're talking about topical application, not internal? Otherwise, I'd say this is too unsafe (eating the silver, not the circuitry) to discuss here.
 

kubeek

Joined Sep 20, 2005
5,794
Nevertheless, constant current source is as simple as a LM317 and two resistors. One resistor like this and another between out and gnd to maintain the 10mA minimal loading current, if your set current is lower than that.

Tell us your input voltage and wanted current and I will help with the values.
 

SgtWookie

Joined Jul 17, 2007
22,230
Well, an LM317 needs 10mA to provide guaranteed regulation, but an LM317L only requires 5mA. You might be able to get by with just 3.8mA current and one 329 Ohm resistor connected from OUT to ADJ to get fairly decent regulation. However, if the water has really high resistance, you won't be able to apply enough voltage to any LM317 to get the current up to 3.8mA.

You can get ~329.3 Ohms by wiring a 470 Ohm and a 1.1k Ohm resistor in parallel.
 

THE_RB

Joined Feb 11, 2008
5,438
I tried a standard LM317 (TO220) recently and it regulated 1.25v ok with a 1k resistor, which was 1.25mA.

So you should be able to use LM317 for 3.8mA with no problems using a 329 ohm (330 ohm) resistor.

The "minimum 10mA" spec in the LM317 datasheet is only required to ensure proper regulation for the full range of operation of the device including transients etc.
 
sorry for the late reply, ohk so the AC->DC adapter I'm going to use (I'll pick it up today) is 30v, I think something like 530mA. Apperently I only need a few mA to get the electrolysis started like 1 -10, so this should be just right for this project.
BTW I have next to no experience in electronics so if you could explain things in as common terms as you can I should have no problem. Thank you all for your time and help, I could never do this without it.

Also I am going to test the current and voltage from a small solar cell that I have, I might try rigging up a setup with one or more of those as the power source, I'm just not sure if they can push enough power for it. I'll post the readings when I get them.
 

SgtWookie

Joined Jul 17, 2007
22,230
Most likely, the big problem you will face initially is not having enough voltage to obtain the desired current flow through the water. If the water is distilled and there are no impurities (minerals, metals, etc) dissolved in the water, it will be an insulator. You would have to have a fairly high voltage in order to get any current at all flowing through it.

I = E/R, or in words, current through a resistance, is equal the voltage across the resistance. This is a part of Ohm's Law.
 
Ohk so I just got the adapter its 30vdc .4A. I'm pretty sure that's too much current, but I may just put the electrodes (coins) farther apart.

So to my understanding, the rate at which the silver particles come off the electrodes increases rapidly as the resistance drops. So what I am wondering is if I can make the rate stable vs. exponential.
 

robert.e.lee

Joined Aug 21, 2011
13
I got the LM317T they didn't have the L version, I don't know what the difference is. And if I=E/R then how will a 250 ohm resistor provide 5mA. .005mA=\=30V/250ohm. I'm sure that I'm missing somthing. Thanks for you time btw.
 

SgtWookie

Joined Jul 17, 2007
22,230
The LM317 family of regulators has a reference voltage, called Vref in the datasheet. It's the difference between the OUT and ADJ terminals. The regulator attempts to keep that difference at 1.25v (nominally) by sourcing current from the OUT terminal.

1.25v / 250 Ohms = 5mA.

Vref can vary anywhere from 1.2v to 1.3v and still be within manufacturer's specifications. However, this is the easiest, lowest-component-count way to create a reasonably constant current source.
 
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