Constant Current Circuit using LM317 1200µA

[Electro_Smurf]

Dear fellow technicians,

As I wanted to make colloidal silver, just for fun, I tried to make a constant current supply. So my powersupply input is 31,5Vdc, and my desired limited output is aprox. 1000-2400µA.
In my test circuit I use a potmeter to limit the current, and a leak resister to get the minimum load over the LM317, as shown in the picture.
However, Iout keeps running away slowly, but undesirable. I'm a mecanic so I studied many hours to learn the basics and now finally had the courage to ask.
I read the LM317 specs, min 10ma load, Vadj-Vout=1,25V, Vin-Vout dif max 40V, but Iout keeps running up, whatever setting I use.
On the load side is the conductivity of the destilled water, starting low and rising because of the Ag-ions being produced. Resistance aprox 25-30k.
R2 is the leak resistor, creating the minimal load of aprox 10mA.

It would be a miracle to me if it would work, so please can someone help me out?
Thank you,
Richard, The Netherlands.

 

[Electro_Smurf]

I though of integrating the water resistor in the feedback loop..... would this do any good?

 

[crutschow]

my desired limited output is aprox. 1000-2400µA.

I read the LM317 specs, min 10ma load

Don't you see the conflict between those two statements?

 

[gophert]

I think we may have a member who is on the run for disobeying Ohm's Law.

 

[richngreen]

Not as bad as me, but that doesnt mean he doesnt know alot other than that.

 

[rjenkinsgb]

The LM317 is only suitable for relatively high currents, when used as a current source.

Just use a simple transistor or FET constant current circuit...
There are plenty of examples in the Wikipedia page:

Current source - Wikipedia

en.wikipedia.org

 

[Electro_Smurf]

Don't you see the conflict between those two statements?

Yes that's why adding extra load with R2

 

[Electro_Smurf]

I think we may have a member who is on the run for disobeying Ohm's Law.

Whahaha funny. Yes I wanted to learn something new. Will keep Ohm's Law in mind! Thanks

 

[Electro_Smurf]

The LM317 is only suitable for relatively high currents, when used as a current source.

Just use a simple transistor or FET constant current circuit...
There are plenty of examples in the Wikipedia page:

Current source - Wikipedia

en.wikipedia.org

Thanks for the direction, looks good / can work with that. Study-Purchasing-Create. Will report after some time how things worked out.

 

[crutschow]

Yes that's why adding extra load with R2

But then you no longer have a constant-current.

 

[gophert]

Yes that's why adding extra load with R2

R2 is not really a load on a constant current circuit until the circuit is completed and runs to ground. See black arrow. No current is flowing until you complete the circuit.

 

[Visitor]

A better approach would be a constant current (current regulating) diode. They are available in a number of current ranges between 0.1mA – 22.5mA. Use is simple, just place in series between the DC supply and load, and the diode will vary the voltage to maintain the rated current through the load.


P-22 – P-23 Current Regulating Diodes

 

[Electro_Smurf]

But then you no longer have a constant-current.

Okay then...have to study this to see if I get the idea. Will take some time az bizzy times @ work. Thanks

 

[Electro_Smurf]

[/QUOTE]

A better approach would be a constant current (current regulating) diode. They are available in a number of current ranges between 0.1mA – 22.5mA. Use is simple, just place in series between the DC supply and load, and the diode will vary the voltage to maintain the rated current through the load.

P-22 – P-23 Current Regulating Diodes

Wow if only that could work, yeah! Think I ran already across that following rjenkinsgb 's lead on wikipedia. But for sure I will studie this to! Thank you

 

[Electro_Smurf]

R2 is not really a load on a constant current circuit until the circuit is completed and runs to ground. See black arrow. No current is flowing until you complete the circuit.

View attachment 124565

Yes thank you. Happy I knew that one!

 

[atferrari]

See if the LM334 is good for you.

 

[Electro_Smurf]

From visitor: "A better approach would be a constant current (current regulating) diode. They are available in a number of current ranges between 0.1mA – 22.5mA. Use is simple, just place in series between the DC supply and load, and the diode will vary the voltage to maintain the rated current through the load."
Yihaaa, looks perfect this CRD's. GREAT ADVISE. Checked the specs and ordered some. Besides this idea I'm gonna use the LM234/334 also to be able to regulate

 

[Electro_Smurf]

See if the LM334 is good for you.

Checked the spec's and should work! GREAT ADVISE TO. Gratefull for all the help