Requesting help with a project - new to electronics.

Thread Starter

KOSR

Joined May 22, 2021
6
Popping my post cherry here.
Hello community!
Thank you for the platform, I hope I can understand a lot more about this practice.


I've been working on a very simple project, it's a constant current supply /current limiter, used with a DC 30v adapter, aim to be able to adjust the current from around 5mA to 20mA and hold it there.

Used to connect to two electrodes, it's for electrolysis.

I've followed a diagram, which I will attach.
Made another with a slightly different configuration, but should be the same circuit.

Using LM317LZ, 1kohm 25 turn trimmer pot, and a 49.9 ohm resistor, that's all.
Positive runs through this circuit and then to anode.
Negative power goes direct to the cathode.


..

Buuutt... It's not working like it should, or my reading are just totally off...

To read the current..
I attach.
Ps+ to meter +
Meter - to anode
Ps - to cathode

Switch to 20mA setting on meter and check.. getting far lower current than I should, and the trimmer pot adjustment isn't really doing anything..


Furthermore..

Adjusting the height of the cathode in the solution radically changed the current, which it shouldn't..

Reading current again..
If I attach the (meter negative) to the cathode instead of the anode as I was instructed, I did get a proper reading of a stable current that I could then adjust..

I must have wired something wrong. But I can't figure out how..

I'll attach a pic of my other configuration too..

I don't know what I'm doing wrong. It's such a simple circuit.

..

Some thoughts..
Maybe I'm wiring the trimmer pot in wrong, I have two legs joined together, input and middle, then the other to output. I read about doing that somewhere..

I'm keeping everything right together and mostly just using the legs to connect the components.

Other diagrams have the resistor before the pot, some after, not sure if it matters.
 

Attachments

huangme

Joined May 3, 2021
2
If I understand your issue you are not getting any resistance change from turning the pot? That probably indicates an incorrect setup on the pot. Read the resistance on your meter across the nodes until you see the resistance is changing on the output side.
 

DickCappels

Joined Aug 21, 2008
7,442
One thing that might cause the current to change (specifically to get lower) as you change the depth of immersion would be if the resistance between the electrodes gets too high, as may be the result of too small electrode area in the solution or low conductivity of the solution.

Your current source looks fine and has been used by many in all sorts of applications, so that is the last place I would look for problems beyond incorrect wiring.
 

Thread Starter

KOSR

Joined May 22, 2021
6
Incorrect wiring seems the most likely culprit. Every time I work out the circuit I come up with the same design though.
My understanding of these things is limited and I'm just beginning in this hobby.

Maybe just leaving the middle leg of the pot unconnected might help? I see some circuits have that. Or they have just two of thr 3 legs connected anyway.

Maybe having the resistor before the pot too? Would that change anything?
Shouldn't matter right...


The fact that it is reading when I measure from the cathode instead off the anode is super confusing for me.
(It's all pretty confusing for me really) but I'm getting there.

Both of your responses are helpful, thank you.
 

huangme

Joined May 3, 2021
2
Work only with the pot. If you connect your meter to two poles and read the resistance and it does not change, then those two poles are where you connect positive and negative sources. The third pole is your output. If you flip the positive and negative leads, you will see the output does the opposite when it turns. One configuration will increase resistance when turned clockwise. The other configuration will decrease resistance when turned clockwise. Play with it until you really have confidence since the pot is really the most complex component in your circuit. Good luck! You got this.
 

Tonyr1084

Joined Sep 24, 2015
5,990
Switch to 20mA setting on meter and check.. getting far lower current than I should, and the trimmer pot adjustment isn't really doing anything..
You can't force current through the solution. It will be whatever it is due to the chemical makeup and the depth of the electrodes.
Adjusting the height of the cathode in the solution radically changed the current, which it shouldn't..
Yes, it should. You're changing the total area of contact the electrodes are having with the solution.
If I attach the (meter negative) to the cathode instead of the anode as I was instructed, I did get a proper reading of a stable current
Yes. Current through the meter - the wiring is similar to wiring series batteries. One battery positive terminal is connected to the next negative terminal. If you put the battery in wrong the two voltages will cancel each other out. With the meter, depending on the model and design of the meter, negative current might not be readable for the meter.
One thing that might cause the current to change (specifically to get lower) as you change the depth of immersion would be if the resistance between the electrodes gets too high, as may be the result of too small electrode area in the solution or low conductivity of the solution.

Your current source looks fine and has been used by many in all sorts of applications, so that is the last place I would look for problems beyond incorrect wiring.
100% agree. If all is not performing as predicted then go back and check the wiring. It's often where I find many of my own mistakes. And again, as mentioned, the amount of electrode in the solution WILL affect the amount of current. It has to do with Total Contact Area. Smaller TCA means - well - less contact and higher resistance. And with higher resistance comes lower current.

Remember, you can't force current through a circuit. It will draw all it desires and no more. 10 volts and 10 ohms means 1 amp of current flowing. You can't force 2 amps through it - it won't happen. To increase the current you have to increase the voltage OR reduce the amperage RESISTANCE. Play with the numbers and you'll see what we mean.
 

DickCappels

Joined Aug 21, 2008
7,442
Remember, you can't force current through a circuit. It will draw all it desires and no more. 10 volts and 10 ohms means 1 amp of current flowing. You can't force 2 amps through it - it won't happen. To increase the current you have to increase the voltage OR reduce the amperage RESISTANCE. Play with the numbers and you'll see what we mean.
Not to put too fine a point on it, and this is a rephrase of something @Tonyr1084 said, your circuit does try to force current through the load but can only do so if the load resistance is low enough compared to the maximum output voltage of your current source. This voltage is often referred to as the amount of compliance voltage the current source is capable of.
 

Thread Starter

KOSR

Joined May 22, 2021
6
Very illuminating replies, thank you.

So when the limiter works it hold a constant current, regardless of the depth of electrodes, maybe because it's only limiting the current, not boosting it.. I don't know.

My meter does read negative current for sure.

Anode is pure silver coin, cathode copper wire, anode is about 10x the size of the cathode.

Distilled water with anhydrous sodium carbonate electrolyte is the solution.

Generally we keep the current constant and adjust the cathode depth to control the voltage between the electrodes.

I have personally used systems that worked, current never changed at all.


I will focus on the pot, it's the most likely culprit, everything else is pretty set.

I could, and probably do some incorrect readings too, but I've made two of these circuits now and each have the same result, used different meters to test too.

*I've seen it work*! So it's just my error somewhere.

There are for sure variables that I'm not aware of, mistakes to be made, learning to be done.. I've got a lot to learn so thanks for the input! I'm very grateful.

Can anyone confirm if my (childish) drawing of a circuit in my initial post looks ok, wiring would work?

I will get this right eventually.
It's a completely new medium to be working with for me.
 

atferrari

Joined Jan 6, 2004
4,329
Hola @KOSR

Could you draw a complete circuit including the power supply and the electrodes? Put some effort to get a readable schematic giving an ID to every component. Do no omit the cuvette in the drawing.

Please also show where do you apply the probes of your DMM.
 

KeithWalker

Joined Jul 10, 2017
1,789
To make your project work, the silver anode must be connected to the output of your current limiter (+) and the copper cathode must be connected to the supply negative.
the circuit you are using appears to be wired correctly. You will only get the full 5 to 20mA constant current flowing if both electrodes are large enough. This is most likely why you are having a problem.
Two dissimilar metals an alkaline solution will act like a battery. If you connect a voltmeter between the electrodes with the power disconnected, you will measure a voltage, with the silver electrode more positive than the copper. When you connect the supply, the voltage produced by the electrodes opposes the supply. That is why you get more current flowing when you reverse the supply as they are then both working in unison, but that would plate the copper on the silver which is not what you want..
I hope this gives you a little more understanding of what is happening in your experiment.
 
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Thread Starter

KOSR

Joined May 22, 2021
6
To make your project work, the silver anode must be connected to the output of your current limiter (+) and the copper cathode must be connected to the supply negative.
the circuit you are using appears to be wired correctly. You will only get the full 5 to 20mA constant current flowing if both electrodes are large enough. This is most likely why you are having a problem.
Two dissimilar metals an alkaline solution will act like a battery. If you connect a voltmeter between the electrodes with the power disconnected, you will measure a voltage, with the silver electrode more positive than the copper. When you connect the supply, the voltage produced by the electrodes opposes the supply. That is why you get more current flowing when you reverse the supply as they are then both working in unison, but that would plate the silver on the copper which is not what you want..
I hope this gives you a little more understanding of what is happening in your experiment.
Thank you very much, that's very insightful.

A little silver plating on the copper is expected.
But I didn't know that about the cathode size, I'll definitely experiment with some different gauge wires.

I'm really enjoying learning about all this, it's been quite challenging so far. Luckily some kind strangers have always been available to share some info.
 

Thread Starter

KOSR

Joined May 22, 2021
6
Hola @KOSR

Could you draw a complete circuit including the power supply and the electrodes? Put some effort to get a readable schematic giving an ID to every component. Do no omit the cuvette in the drawing.

Please also show where do you apply the probes of your DMM.
Will do this evening, thank you
 

Thread Starter

KOSR

Joined May 22, 2021
6
I have a newborn, so things take about 50x longer than usual..

But I did figure out my main problem, and it was a wiring issue, I had the transistor input/output switched around.. had to go through everything methodically and find any issues, it was kind of by luck that I eventually noticed that. Was about to start a new circuit.

Mistakes really are gems, now I know a LOT more about this from my enquiries.

Thank you.


I was playing with an app to try test my circuit.. here it is, just with a lamp instead of the electrodes/cell (which I don't know how to illustrate anyway. And it's an LM317LZ instead of the regular LM..

It's working, stable currents, still need to get my electrolyte and Cell dialed in , but that's not too tricky.
 

Attachments

KeithWalker

Joined Jul 10, 2017
1,789
It's good to see that you are getting some results. Be aware that the distance between the electrodes and the concentration of your electrolyte will also have an effect on the maximum current that can flow.
 
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