current control again!

[Roff]

Plumber, I'm re-posting this in case you missed it.

Plumber, I'm going to ask what will probably sound like a stupid question. You may have even already answered it, and I missed it.
When you had the CLD, the ammeter, and the electrolyte in series, did you still expect it to require 6 hours to reach 1mA?

 

[plumber]

Plumber, did you still expect it to require 6 hours to reach 1mA?

Hi, roff. both drawings are in series. The bottom drawing took 6 hours as if I had no cld. The top drawing was kind of a trick question and here is why. Fearing I had fried my brand new cld, I copied a picture that had a resistor with it's leads wrapped around the testing probes and power in and power out. The picture did not make sense to me. I did this to get any kind of response that would help my next direction. My meter bottomed out back down to the starting current of 224u...this is what confused me. The electrolyte had already achieved 1.5mA. (I had pulled the plug on the last process done with out any thing in-line) So, I let it run this way for several hours, it reached 1.24 mA and stayed there...seemingly working. I didn't believe the upper drawing really worked because of agglomeration in the electrolyte (a plating out of metal on the stirring rod) So I posed my twisted question. For this I apologize, and I no longer care why my meter went back down and then slowly back up and stopped at 1.24 mA. The plate out told me it wasn't working. So, actually I didn't know what to expect in either scenario. In my defense, I did try the humble straight approach on how to months ago, but got no where. plumber

 

[mbarazeen]

ke5frf
1) dis you consider his source is not a pure DC? its an unregulated one?
2) did you consider he doesnt want to control it by timer, until it reaches 1mA? he just wanted the solution to have the control at the begining until its conductivity allows a current of 1mA then to limit it within 1.5mA.
3) did you consider when he go for different solutions, i mean density or other change in it, then he will end up in different result when he implement as you said?
4) 24kOhms when it comes in series? any consideration of the resistivity of the solution? you mean it will have zero voltage across? to have that ultimate current of 1.25mA?

so some electronic device can serve better for him.
also i am not sure wheather he can experiment this much to make such a timer and make it for many stages

 

[Roff]

Plumber,
I'm sorry, but I read your reply 3 or 4 times, and I am still not sure what happened with the attached circuit, which is the only one of the two original ones that should work (which I think you know).
It should start out at low current (you said 22-24µA), and build up slowly, at essentially the same rate as with no series resistance. With a 1mA CLD, after 6 hours or so, the current should limit at 1mA. Is this what you expected, or did you expect to get 1mA essentially instantaneously?

 

[ke5frf]

"ke5frf
1) dis you consider his source is not a pure DC? its an unregulated one?"

..........I'm not sure that matters. Can't see why it would.

"2) did you consider he doesnt want to control it by timer, until it reaches 1mA? he just wanted the solution to have the control at the begining until its conductivity allows a current of 1mA then to limit it within 1.5mA."

I don't think he cares how it is done, timer or not. He is interested in his final product, and making it to specification in as quick a time as possible. He also wants simple, reliable, repeatable, easy to understand.

"3) did you consider when he go for different solutions, i mean density or other change in it, then he will end up in different result when he implement as you said?"

.........I considered that I am pretty sure I know what he is doing with this electrolysis project, the method and the final product, and he will be starting out with distilled water of the same purity each and every time.

"24kOhms when it comes in series? any consideration of the resistivity of the solution? you mean it will have zero voltage across? to have that ultimate current of 1.25mA?"

The overall resistance of the circuit (circuit-R plus electrolyte-R) will be around 24kOhm when it is drawing 1.5 mA. I am not concerned with the exact values, this is why I suggested potentiometers, because he can set them easily with a few trial and errors. I should think doing it 3 times he'll have it right.

Simple fact remains, he will need around 24kOhm of overall resistance to achieve his limiting. It will probably be a little less than this (potentiometer value) because the electrolyte will doubtfully ever have zero voltage. But the potentiometer offers the flexibility to adjust as needed.

"so some electronic device can serve better for him.
also i am not sure wheather he can experiment this much to make such a timer and make it for many stages"

Yes, an electronic circuit might make it better for him, but this will require trial and error and most likely ordering parts mailorder. He can get the parts I suggest at a local hardware store anywhere, perhaps even Walmart. Inexpensively.

And most importantly, it will work.

I do not think he is doing this for mass production or for an exacting experiment for a school project.

 

[ke5frf]

Plumber,
I'm sorry, but I read your reply 3 or 4 times, and I am still not sure what happened with the attached circuit, which is the only one of the two original ones that should work (which I think you know).
It should start out at low current (you said 22-24µA), and build up slowly, at essentially the same rate as with no series resistance. With a 1mA CLD, after 6 hours or so, the current should limit at 1mA. Is this what you expected, or did you expect to get 1mA essentially instantaneously?

Roff, I think he already answered this with the comment here:


the ultimate goal is to let the current rise slowly to 1mA on it's own with only the electrolyte as resistance. Then, the real goal is to hold the process under 1.5mA for another 4-6 hours

 

[ke5frf]

Going back to this comment:

4) 24kOhms when it comes in series? any consideration of the resistivity of the solution? you mean it will have zero voltage across? to have that ultimate current of 1.25mA?
s

No, you do not understand, obviously. The electrolyte would not have zero volts across it until it is almost purely metal. But when the electrolyte gets to the point of drawing 1.5 mA, the circuit must have the value of 24kOhms to maintain it. When the series resistance is added, it will drop the current back down, and it will take some time for the electrolyte to increase conductivity back to the point where the series circuit is 1.5 mA. No matter what, whether he has a CLD, a transistor with feedack, or whatever, a series resistance of a certain value will have to be maintained in order to hold at 1.5 mA. This will be somewhere between 0-25,000 Ohms, no? A potentiometer allows the flexability to adjust to the exact value he needs.

 

[Roff]

Ke5frf, I understand what you are proposing, but it's a Rube Goldberg approach to the solution, when a simple current source will do the same thing with more precision and hands-off operation.
If it were my problem, I would continue trying a current source, and beat my head against the wall until it worked, because I know it should.

 

[mbarazeen]

ok i got it, you meant all together it should be 24k, minimum to have the current bellow 1.5mA always, you are correct.

he may select the idea he feels its easy to him and then can have a try.

 

[kchriste]

The OP should replace his "water bath" with a 50K POT & 1K series combo and verify that the constant current diode works as expected.

 

[plumber]

Plumber,
It should start out at low current (you said 22-24µA), and build up slowly, at essentially the same rate as with no series resistance. With a 1mA CLD, after 6 hours or so, the current should limit at 1mA.

In my limited way of thinking ...yes, after six hours or as the current climbs towards the diode's 1mA rating it should...I wish it would... start limiting the current. But it didn't. And still wont. At one point of fiddling I shorted the electrode leads and my meter read 1.24mA when it was wired in the bottom config. However, I wasn't keeping notes and don't remember what I actually did. Don't know if it matters either. Are you saying it should have worked? Should I order another? Did I fry it? Yes, I have been beating my head against a wall, but I think it is probably way beyond me to construct and understand a pure circuit current control device. Unless it is as simple as power in to cld, power out and into ammeter, back out to anode. But it didn't do anything. I can follow the pot and timer approach though. plumber

 

[Roff]

You stated that the CLD was from Mouser, and was made by National. I don't think National makes CLDs, and Mouser doesn't sell National parts. A part number would be really helpful. Maybe we can find out why it didn't work, because it should have.

 

[plumber]

You stated that the CLD was from Mouser, and was made by National.

My bad. It was Central Semi. Part# 1n5299.

Mouser# 610-1n5299 1.2mA current regulating diode.

 

[ke5frf]

Ke5frf, I understand what you are proposing, but it's a Rube Goldberg approach to the solution, when a simple current source will do the same thing with more precision and hands-off operation.
If it were my problem, I would continue trying a current source, and beat my head against the wall until it worked, because I know it should.

I totally agree that it is a clap-trap way of doing it, this is so at least with the modified suggestion I made with two timers and two potentiometers. The first suggestion, with one timer paralleled with the potentiometer, not so much...and I think it would work just as well. Only drawback is that when the timer circuit opens, the single potentiometer will have to be adjusted in such way to bring the current down significantly, and it will take just that much longer to get it to his target range.

But all of this got me to thinking, why isn't his CLD working? It should, if everything is set up right, and if I understand it correctly.

But then, I thought about the first stage of the process, when the distilled water is practically infinite in resistance. How different is the distilled water solution from any other insulator at this point? Is the resistance enough to keep any kind of current path from forming? Will the diode permit ANY current to flow without even the slightest path to ground? Perhaps its is like an AC motor that has to have a run capacitor to establish the momentum to make it run. I have a pretty good knowledge base, but I can't answer that question.

If I were there, I would set up the diode like it is supposed to be, and then read my voltage at the electrodes. If the diode is conducting at all, it should read 35 volts. If not, Perhaps his voltage is stopping at the anode? And because of this, the electrolysis process won't even kick start. But then, I considered the impedance of a voltmeter. Would it fool me if I read the voltage at the electrodes?...because even though the meter is high impedance, it may be low enough impedance to establish a path to ground and forward bias the diode.

So, between this thought experiment and the timer idea, I came up with an experiment I believe he should try.

He should set up his diode, ammeter, power supply, and electrodes in the series configuration as it should be, with fresh distilled water. But to start, he should use an alligator jumper to bypass the diode. The electrolysis should immediately begin. After the time he expects the current to reach 1 mA, he should disconnect his alligator jumper. At this stage the electrolyte solution should have a resistivity of 24kOhms on its own accord. If the diode works as I understand it, it should now be forward biased and conducting. it will see the "load resistance" of the electrolyte and adjust itself to begin limiting the current. As the process continues and the electrolyte becomes more conductive, the CLD SHOULD increase its resistance to make up the difference. In other words, if we help it get there in the first place, the diode should do its job.

If the experiment goes as I suggest it will, then it will be a simple matter of wiring up the diode in parallel with a timer switch, and setting the timer for the next test for the duration we experienced when the current reached 1 mA.

As I suggested before, all things being equal and assuming the purity of the distilled water is the same each time, the electrolysis process should have predictable time constraints, so a timer would be an effective way to kick start the procedure until the resistivity of the electrolyte is sufficient for the CLD to do its thing. He may even find the timer doesn't have to be set for long at all, as the conductivity of the electrolyte might be sufficient within an hour or less.

So, to sum it up, perhaps my original idea was Rube Goldbergish, but hopefully we can bear fruit with a simple modification to blend both methods together.

 

[ke5frf]

Aha, I think I have the solution to why our CLD isn't working.

Go to this JFET current regulator tutotrial at AllAboutCircuits.com:


A test rig is described that basically replicates the CLD with physical components, resistors etc. The supply voltage is different in this example, but the lesson is the same.

TP5, at the end of a 10 kΩ resistor, is provided for introducing a large change in load resistance. Connecting the black test probe of your ammeter to that test point gives a combined load resistance of 14.5 kΩ, which will be too much resistance for the transistor to maintain maximum regulated current through

when you touch the black probe to TP5, the current will fall dramatically. Why? Because at this level of load resistance, there is insufficient voltage drop across the transistor to maintain regulation. In other words, the transistor will be saturated as it attempts to provide more current than the circuit resistance will allow.

I may be misreading this, but I believe the load resistance of the electrolyte is too great initially for this to work.

As I'm reading this explanation, if the electrolyte resistance is very, very high, practically all of the voltage is being dropped across the electrolyte, leaving virtually none to drop across the JFET terminals. But the device does have the internal resistance to contend with.

Ah, another issue! In the example circuit, the CLD is DOWNSTREAM of the load resistance, examining it as a conventional current circuit. Seems this might interfere with the feedback biasing that the internal resistance creates. According to this page's example, maybe we need to wire the CLD after the electrolyte, on the negative side of the solution. Of course, the anode would still be facing the closer to the positive and the cathode would still be closer to negative to maintain forward bias.

The answer to this lies in one of these two observations.

 

[Roff]

But all of this got me to thinking, why isn't his CLD working? It should, if everything is set up right, and if I understand it correctly.

But then, I thought about the first stage of the process, when the distilled water is practically infinite in resistance. How different is the distilled water solution from any other insulator at this point? Is the resistance enough to keep any kind of current path from forming? Will the diode permit ANY current to flow without even the slightest path to ground?

I went through the same thought process yesterday. Then I looked at the V-I curve of a CLD. They just look like a low-value resistor when they have ≈0V across them. I ran sims on a couple of the Central Semi CLDs.
Unfortunately, they don't have a spice model for 1N5299 (1.2mA), but they do have one for CCL1000 (1.1mA). I'm willing to bet they are the same device, with a different sort parameter. In a series circuit with a 34V battery, the CCL1000, and a 1Meg resistor, the resistance of the CCL1000 was about 800Ω (current was 33.973uA). This should not prevent the electrolysis process from starting, and a jumper across the CLD would make essentially no difference. Still, it won't hurt anything to try it.
I suspect that, at some point, the CLD was reverse biased. In reverse, it looks like a standard forward-biased silicon diode, i.e, it will burn out immediately in the absence of current limiting. So - if it was installed backwards, and the electrolyte was highly conductive or shorted out, the CLD would fail open.
If it were me, I would buy a few more of them and try it again.

 

[Roff]

Ah, another issue! In the example circuit, the CLD is DOWNSTREAM of the load resistance, examining it as a conventional current circuit. Seems this might interfere with the feedback biasing that the internal resistance creates. According to this page's example, maybe we need to wire the CLD after the electrolyte, on the negative side of the solution. Of course, the anode would still be facing the closer to the positive and the cathode would still be closer to negative to maintain forward bias.

It's a series circuit. All components are 2-terminal devices. It doesn't matter in what order they are connected.

 

[ke5frf]

I went through the same thought process yesterday. Then I looked at the V-I curve of a CLD. They just look like a low-value resistor when they have ≈0V across them. I ran sims on a couple of the Central Semi CLDs.
Unfortunately, they don't have a spice model for 1N5299 (1.2mA), but they do have one for CCL1000 (1.1mA). I'm willing to bet they are the same device, with a different sort parameter. In a series circuit with a 34V battery, the CCL1000, and a 1Meg resistor, the resistance of the CCL1000 was about 800Ω (current was 33.973uA). This should not prevent the electrolysis process from starting, and a jumper across the CLD would make essentially no difference. Still, it won't hurt anything to try it.
I suspect that, at some point, the CLD was reverse biased. In reverse, it looks like a standard forward-biased silicon diode, i.e, it will burn out immediately in the absence of current limiting. So - if it was installed backwards, and the electrolyte was highly conductive or shorted out, the CLD would fail open.
If it were me, I would buy a few more of them and try it again.

Yes, perhaps. A failed component would cause all sorts of grief wouldn't it?
I'm not sure how a continuity check would work for one of these, since they are not a true diode.

What about my observation about the biasing issue potentially caused by the electrolyte being AFTER the CLD?

I think it is worth a try, before he buys new diodes, to hook the diode up downstream of the solution and see if it makes a difference. Stranger things have worked, I'm sure.

 

[Roff]

Yes, perhaps. A failed component would cause all sorts of grief wouldn't it?
I'm not sure how a continuity check would work for one of these, since they are not a true diode.

What about my observation about the biasing issue potentially caused by the electrolyte being AFTER the CLD?

I think it is worth a try, before he buys new diodes, to hook the diode up downstream of the solution and see if it makes a difference. Stranger things have worked, I'm sure.

Read my last post.

 

[ke5frf]

It's a series circuit. All components are 2-terminal devices. It doesn't matter in what order they are connected.

Ah, but the CLD, as I understand it, is actually a 3 terminal device internally though, correct? I've never dealt with one, so maybe you could explain why the gate terminal of the JFET is not a factor?