Author Topic: Taking Proper Current Reading  (Read 1953 times)

Digital

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Taking Proper Current Reading
« on: October 10, 2019, 08:11:10 PM »
I am a bit confused on how to properly hook up a multimeter to take the current reading in the cell. I have a current limiter based on a lm317 that is capable of about 3.5ma to 21ma. How I found this current range was by connecting the positive of the power supply to the positive of the multimeter and the negative of the power supply to the negative of the meter. Apparently this is an incorrect way to measure the current in a circuit and Kephra shows the proper way in his notes, you connect the positive of the power supply to positive of the meter, negative of the power supply to the positive of the cell (silver anode) and the negative of the power supply to cathode (copper wire in my case).

I am confused because I am unsure which current reading to use when doing the PPM calculations, further to this confusion depending on where I connect the positive of the meter I have get a reading that can vary by +1ma and if the solution is stirring, it can also reduce the current reading by about +2ma. Also, when I take the reading in the proper manner, the current limiter maxes out depending on the depth of the cathode and can’t be adjusted higher unless the cathode is deeper in the solution. The odd part of this was that before when I was doing this and I lowered the cathode the voltage drop would be quiet significant, dropping from say 21v to 8v, now it doesn’t really move past 21v regardless of the depth of the cathode, but the current does increase. Although in the past runs I did, I heated up the water, which does increase conductivity. For the readings I mentioned I just used cold water, so this could be the case for the voltage not dropping as much.

With all that said, my question is which current reading should I use to do calculation with? And if it fluctuates and drops with stirring, do I use the current reading when the solution is resting or the one when it’s in motion?

Thanks

Offline Gene

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Re: Taking Proper Current Reading
« Reply #1 on: October 11, 2019, 03:06:58 AM »
No current source is dead nuts accurate unless you have a fine adjustment to tweak it dead on.  If its a current limiter though, it should be dead nuts stable regardless what current its designed to operate at or what you set it to if its adjustable.

Connect your positive power supply terminal to the limiter positive input.  Connect the limiter output (or negative) terminal to the positive meter probe (yep). Connect the negative meter probe to your power supply ground terminal. Turn it on and read the current.  This is what its set to and even connected to a cell it should be rock solid stable at this current (not splitting hairs, obviously).

You do need a minimum of about 3V headroom for an LM317 to function properly meaning your power supply needs to be 3V higher than the highest cell voltage you're shooting for at a minimum though I'd go at 6-9v higher for enough safe area (margin).

For an given electrode arrangement (depth of both which should be 1.5" apart), the lower the current, the lower the cell voltage and the higher the current, the higher the cell voltage. This is why you need to adjust the cathode because though stable at a given depth, the cell voltage may not be greater than 10V without changing the depth of the cathode.  By changing the depth of the cathode, given a fixed depth for the silver anode, you're changing the resistance of the cell - less submerged, higher resistance, more submerged less resistance. Given a fixed current, if you up the resistance, the voltage goes up and if you reduce the resistance, the voltage goes down.

The LM317 is only spec'd to limit to a minimum current of 10ma.  Depending on process spread for the chips, it may be anywhere between 0ma and 10ma but usually when they say 10ma, its rare to see it go below half this though it does happen.

When you stir, you lengthen the path the electrons have to traverse which increases cell resistance where this raises the cell voltage for a fixed current where you most likely would have to readjust the cathode depth to compensate to get the cell voltage in the range you want.  Your limiter, assuming you have enough headroom, should be rock solid stable until your cell voltage gets within 3V of your power supply voltage. If not, something is wrong.

After measuring the limiter current without the cell attached, set your cell up, add your electrolyte (1ml of 1 molar sodium carbonate per liter of water - 20 drops) and hook up your limiter which means your power supply positive terminal connects to the positive terminal of the limiter, the limiter negative terminal connects to the anode (silver) and the cathode of the cell connects to the power supply ground terminal.  Yes I meant exactly what I said.  Limiters are series devices - they're connected in series with the cell, not across it.

Measure the cell voltage and adjust the cathode until its over 10V (I usually shoot for 11-12V for a little safe area).

Now lift the negative limiter lead off the anode and connect it to the meter positive probe (yep), put the meter in current reading mode and hold the negative probe on the silver anode connection of the cell.  Read the current. THIS is the current you should use in Faraday's law of electrolysis to compute run time BUT if everything is working properly and your cell voltage isn't any closer than 3V to your power supply voltage, the current (not splitting hairs) should be the same as you measured with the limiter not connected to the cell.  If it isn't, something is wrong.


Digital

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Re: Taking Proper Current Reading
« Reply #2 on: October 11, 2019, 10:53:30 PM »
I just realized my first paragraph in the first post was a bit confusing, when I said "positive of the power supply" and "negative of the power supply" I meant positive of the current limiter and negative of the power supply.

The current limiter doesn’t have a negative output, only positive coming from the power supply. I don’t see how it could have a negative as the LM317 and the potentiometer only allows for voltage in. Lm317 Vin from the power source, Vout from the LM317, (still positive) to the potentiometer and resistors and from there to an alligator clip. So I am not sure how I can connect it to the meter as you described. The only way without connecting it to a cell is; positive from the current limiter --> positive probe of the meter and negative from the power supply --> negative of the meter. Maybe I am confused about the terminology here.

The power supply is 24 volts, so it does have that 3V headroom needed, which is why when I connect it directly to the current limiter and take a voltage reading it shows as 21-22 volts.

That was the odd thing with the last run I did, I was expecting to see the voltage change as I moved the depth of the cathode. But it didn’t have any really noticeable changes like it did before. Before I could barely put the cathode in the water and it would drastically drop the voltage. But this might be because the room temperature water has more resistance then water at 60-80c. Which, as a side note, is an odd property of water and conductivity, I thought generally as you cool electrical connections resistance is lowered. Which is why superconductors need to be supercooled with helium or nitrogen etc.

The particular LM317 I have can go as low as 2.5ma according to the spec sheet (if I recall correctly), but its more likely to be around 3ma. And it definitely can go below 10ma, because I tested that by taking a reading with the meter in the cell with it hooked up properly, the lowest I got was 4ma, briefly playing around with it.

The limiter is fairly stable as it doesn’t adjust much at all unless the properties of the cells solution change, temperature, stirring, heat etc. I’m just confused as to what current to use for the calculations.

The way you describe hooking up the limiter to the cell is how I’ve been doing it, output of the LM317 (which I thought was positive, when attached to the meter – limiter output --> positive meter probe – power supply negative --> meter negative probe, it shows a positive voltage). So attached to the cell it’s, power supply positive --> limiter input --> limiter output --> anode (silver) and negative of power supply --> cathode (copper).

So the problem I am seeing is, when the limiter is connected to the cell it gets a different reading as to when it’s connected to just the meter. For example, I can get a stable 15ma with it connected to just the meter, but when its connected to the cell and the meter is attached the way you described I get 6-7ma, which can then further drop 0.5-1ma with stirring. This has lead me to question if I was able to correctly take an initial current reading of the limiter or if I was reading the current in the cell correctly in the first place, which I wasn't.

Sorry for the long reply, I really do appreciate the time, help and well thought out responses you are giving me and have given me in the past.
Thank you

Offline Gene

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Re: Taking Proper Current Reading
« Reply #3 on: October 12, 2019, 09:17:19 AM »
http://www.ti.com/lit/ds/symlink/lm317.pdf

Look at table 6.3 - Recommended operating conditions - Output current.

Its stated as 0.01-1.25 amps.  0.01 amps is 10 milliamps.

Then look at table 6.5, minimum load current to maintain regulation. Its stated as 3.5typical, 10 max meaning the amount of current needed to keep the regulator regulating varies here. Typical for this is a bad number to use because its related to the process spread of the chips where the lowest they're guaranteeing always works is 10ma.

If you use the proper amount of electrolyte, as long as there is enough headroom, the current through the cell should be constant regardless the cell voltage.  If its not, you have some kind of problem.

You need a relatively thin cathode.  The thicker it is, the less depth you need in the cell because of the amount of surface area and the voltage adjustment becomes really touchy. I use a piece of 24 gauge wire from a piece of CAT-3 networking cable with the insulation stripped back a good 1.5".  This affords you plenty of fine control of cell voltage.  I originally used a piece of ROMEX electrical wire which I think was 14 gauge and the length of wire in the cell was barely glancing the surface to get the voltage up where we need it.  There was no fine control and worse yet, if even a smidge of water in the cell evaporated (which is a much bigger problem at higher temps), the cathode could come out of the water due to the lower level in the cell.  With 24 gauge, you'll easily be 1/2", maybe more into the water and if that much water evaporates, you have other problems.

As you heat the water in the cell, the conductivity does increase a little (lower resistance) but its minimal.  If you stir, the path the current takes is longer since its now arc'd and that increases cell voltage.

In all cases, as long as you have at least 3V of headroom for the limiter to do its job, the current through the cell should be constant (not splitting hairs).  If its not, you have some kind of issue with the limiter.

The current you use for the calculation is the actual current flowing through the cell because thats the current thats pulling silver ions into the water and the rate this happens at is what dictates run time, but whether you measure the current just across/through the limiter when not connected to the cell or by putting the meter in series with the limiter output and cell anode, the current, as long as there's enough voltage headroom for the limiter to remain functional should be the same.  If its not, there is some kind of issue.


Digital

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Re: Taking Proper Current Reading
« Reply #4 on: October 15, 2019, 07:10:25 AM »
So if I am understanding you correctly, for the limiter to maintain proper regulation the load on the cell needs to be at least 10ma or it'll become unstable. And If I understand you correctly about hooking up the multimeter to the cell and directly to the limiter, I was doing it correctly when I was hooking it up directly to the limiter but I wasn't doing it properly when I originally took a reading of the cells amperage. That's a good start and I can play with it more to trouble shoot it.

But I was running it at 15ma, so I don't see why I would have a different reading when the meter was hooked up to the cell in series, unless the limiter is damaged. It showed a reading of 15ma when hook directly across the terminals, but when it was in series with the cell it showed 6-7ma depending on the depth of the cathode or if the water was stirring. I should mention that the current was stable, apart from it jumping around 0.xx mA when stirring. It was just the drastic different in the current depending on how I took the measurement that's confused me.

I am using some copper wire as the cathode (from a previous recommendation by you) that came from some old telephone wire, I measured the gauge of it with a wire gauge tool and it is 24 gauge.

Thanks for the help again, I will need to trouble shoot it more when I have the time.

Offline Argentum

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Re: Taking Proper Current Reading
« Reply #5 on: October 15, 2019, 06:12:29 PM »
But I was running it at 15ma, so I don't see why I would have a different reading when the meter was hooked up to the cell in series, unless the limiter is damaged. It showed a reading of 15ma when hook directly across the terminals, but when it was in series with the cell it showed 6-7ma depending on the depth of the cathode or if the water was stirring.

Hooking the mA meter directly across the limiter is a dead short. There should always be some series resistance between the limiter and meter. A 1K ohm resistor is right for 15 V output at 15 mA.

Argentum

Offline Gene

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Re: Taking Proper Current Reading
« Reply #6 on: October 15, 2019, 06:37:20 PM »
Hooking the meter directly across the limiter works just fine.  Its called a LIMITER for a reason. If in a dead short it doesn't limit to the current its set to, its broken. This is the best test for a limiter.

Limiters limit current flow regardless the resistance of the load so long as the load voltage is lower than the power supply voltage. In this case, shoot for about 3V above cell voltage and it will continue to limit, though as I said in a prior post, for enough "safe area", realistically shoot for 2-3 times this because if the limiter ever stops limiting (meaning current has dropped below the setting), you have ZERO clue what PPM you made.

Putting the meter between the limiter and ground affords the lowest resistance (highest current load) where the limiter had better limit to the setting its set to or as I'd said, its broken. THIS is the purpose of a limiter - to prevent more current from flowing than its set to, usually used for protection purposes but here, we use it to set a current level for IS production because the cell resistance is low enough that the cell voltage remains low enough that we stay at least 3V below the power supply voltage where  the limiter always remains functional.

Even when you put the limiter in series with the cell where you've already added the electrolyte, for adjusting cell voltage to 10+V where there's still at least 3V of headroom for the limiter to function, from the limiter's perspective it IS looking into a dead short because the current that CAN flow would far exceed what its limiting setting is.

And then, given the LM317 limiter has a rather high minimum current to sustain regulation (5-10ma) testing it directly with the meter across its output and ground is the best case because it guarantees the minimum current flow to keep it functioning as you adjust it to the current you want.

Offline Argentum

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Re: Taking Proper Current Reading
« Reply #7 on: October 15, 2019, 06:47:02 PM »
Hooking the meter directly across the limiter works just fine.  Its called a LIMITER for a reason. If in a dead short it doesn't limit to the current its set to, its broken. This is the best test for a limiter.

Until the limiter goes into thermal shutdown... Or into fold-back mode due to the direct short... LOL. Say what you want, but an LM317 is really a voltage regulator that can be made into a current limiter. Although not a really good one.

Argentum

Offline Gene

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Re: Taking Proper Current Reading
« Reply #8 on: October 15, 2019, 08:53:38 PM »
If it goes into thermal shutdown you have a real issue. It would be quite difficult to heat up an LM317 in this application with a mere 15 ma or so flowing through it to the point the thermal shutdown kicks in.  IIRC, the shutdown temp is around 170C or something like that.

The LM317 does not do foldback limiting.

I don't know what you mean by the LM317 not being a really good regulator if thats what you're saying. Its as good as most these days though there are some with really low quiescent current mainly for mobile applications but a goodly number of those only work on input voltages up to 6V, some 15.  The LM317 comes in packages that can easily handle lots of current and IIRC, it can operate with input voltages as high as 37V or something around there.

As far as heat goes, the packages that the LM317 comes in can easily handle anything a Colloidal Silver limiter can throw at it.

Also, its NOT a dead short.  There is a current sense resistor in series with the limiter as part of the design.  If it were a dead short you'd get probably 1.5amps out of the thing until it heated up to thermal limit which it would absolutely eventually hit. This will not happen in a Colloidal Silver current limiter.

At even 20ma, the series resistor would be 62.5Ohms.  Does that sound like a dead short to you?

Offline Argentum

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Re: Taking Proper Current Reading
« Reply #9 on: October 16, 2019, 06:44:33 PM »
If it goes into thermal shutdown you have a real issue. It would be quite difficult to heat up an LM317 in this application with a mere 15 ma or so flowing through it to the point the thermal shutdown kicks in.  IIRC, the shutdown temp is around 170C or something like that.

The LM317 does not do foldback limiting.

Come on Gene, stay on topic. Which is placing a dead short across the output via an amp-meter. And the device does do foldback limiting, from the 2014 TI data sheet:

"8.3.2 Overload Block
Over-current and over-temperature shutdown protects the device against overload or damage from operating in excessive heat."

"8.4.4 Operation In Self Protection
When an overload occurs the device will shut down Darlington NPN output stage or reduce the output current to prevent device damage. The device will automatically reset from the overload. The output may be reduced or alternate between on and off until the overload is removed."

Quote
Also, its NOT a dead short.  There is a current sense resistor in series with the limiter as part of the design.... At even 20ma, the series resistor would be 62.5Ohms.

At the minimum supplied current of 1.5 amps, that resistor would be cooking at 140W. Which easily surpasses the design rating of the TO-220 package. Which is 3°C rise per watt.

Argentum

Offline Gene

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Re: Taking Proper Current Reading
« Reply #10 on: October 17, 2019, 03:18:27 AM »
You obviously don't understand how a current limiter operates.  Hooking the output of the limiter to ground is NOT a dead short!  Hooking the output of the regulator to ground, which you're NOT doing with a current limiter is. They're two completely different animals.

If grounding the limiter output (adjust pin side of the sense resistor) was a dead short, we wouldn't be talking about a current limiter circuit!

Current limiters maintain set current from a load voltage of zero, all the way up to the point the load voltage reaches VIn minus headroom requirement of the limiter.  Above this, currrent drops off because the limiter doesn't have enough voltage left to set its output voltage to the correct value to maintain the set current flow.

For the LM317 current limiter, the minimum headroom is actually 3V + 1.25V because the sense resistor is in series with the regulator output to the load and it always drops 1.25V if your cell voltage is less than Vin - 3V - 1.25V (minimum headroom met).  If its not, your current will start dropping off and then you don't have a clue what PPM you made. Don't go there (wink).

Current limiters are series devices that adjust their resistance (the regulator itself in this case for an LM317 limiter) such that the voltage across the sense element (the resistor between the regulator output pin and the adjust pin for the LM317) equals the sense voltage (1.25V in this case) where the circuit maintains this 1.25V differential across the sense resistor (regulator output is maintained at 1.25V ABOVE the adjust pin voltage).  For the LM317 used as a current limiter, there will Never be more than 1.25V across the sense resistor.

You choose a resistor based on R=1.25/<current desired in amps>, put that between the regulator output and the adjust pin (its NOT a ground pin on the LM317 because the LM317 is an adjustable regulator) where the adjust pin side of the resistor is the limiter output (not the regulator output).

The LM317 maintains a differential of 1.25V between its adjust pin and the regulator output (output is 1.25V higher).  There will never be more than 1.25V across the current setting resistor.

You calculate, using the aforementioned equation, what resistor value you need for the current you desire (whatever resistance at the desired current is necessary to drop 1.25V across that resistor), hook the circuit up properly and it works just fine.

So, if we wanted 20ma, R=1.25/0.02 = which is 62.5Ohms. There will NEVER be more than 1.25V across this resistor because the regulator will adjust things so its output pin is always 1.25V higher than its adjust pin. I=E/R = 1.25/62.5 = 20ma. Hmmm! If you ground the adjust pin (limiter output), there will STILL be 1.25V across the sense resistor and the current is limited to the setting so yeah, you can do this to measure the current set point.

If you pump in 30v and your cell is 10v, the regulator would have to dissipate 30-10-1.25 * output current or 19.75 * 0.02 for this example.  Thats 0.395 watts. A TO220 will maybe get barely warm, if even.  Connecting the output of the limiter to ground to test its setting would put 28.75V across it at 20ma or 0.575 watts, again, maybe not even making the TO220 warm.  In either case there'd be 1.25V across the sense resistor where there's 20ma flowing through it, which is a whopping 25mw (0.025watts). Even an 0603 sized surface mount resistor (30 mils wide by 60 mils tall) is rated at 100mw. 25mw is nothing.

Again, the circuit we're talking about is a CURRENT LIMITER circuit, not a voltage regulator circuit. They're two completely different animals.

If you set the current by connecting the multimeter in current mode between the limiter output and ground so you get what you want and then connect the limiter to your cell and the current is less, unless you have something hooked up wrong, either your limiter is broken or your cell voltage is too high to where the limiter required headroom (4.25v) is not being met (cell voltage higher than limiter input voltage minus 4.25v), unless maybe your limiter is operating partially in another dimension which may change how the laws of physics apply (wink).

Offline mfacen

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Re: Taking Proper Current Reading
« Reply #11 on: October 18, 2019, 06:06:25 AM »
  Gene is right.  The easy way to think about it:
  How does the current límiter does its job ? When the current goes over a set limit it lowers the voltage of its output via its feedback loop, the more the current rises the more the voltage drops. If you try to draw infinite current ( 0 resistance ) the voltage drops to zero, well almost, depends on the device.

Offline Argentum

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Re: Taking Proper Current Reading
« Reply #12 on: October 19, 2019, 04:12:15 PM »
  Gene is right.  The easy way to think about it:

It depends upon how you look at it. In this thread I pointed out areas where Gene is wrong. If you go through other threads that I started he also made incorrect statements.  Basically challenging what I had posted.

As for the current regulator testing, why test at a boundary condition that has nothing to do with it intended application?

I could go on but won't...

Argentum

Offline Gene

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Re: Taking Proper Current Reading
« Reply #13 on: October 21, 2019, 07:47:03 PM »
Lets stay on topic, shall we?

You need to read the datasheet more carefully - go look at the example of how to wire up an LM317 in current limiter mode because you obviously have no clue how to do this given you seem not to understand the current limiter topology for an LM317.

You can ONLY get 1.5 amps to flow in a current limiter configuration using the LM317 if you change the current setting resistor to about 0.8333Ohms. No one is going to even come close to this because the operating range we're interested in is between 2-3ma and maybe 25ma.

I NEVER said to ground the regulator output. That would just be plain dumb. I said to hook the meter across the current limiter (meaning current limiter output to ground). They're not even close to the same thing.

Thermal limit discussion is not on topic? I think you need to pay more attention to whats written and remember better what you wrote. YOU were the one who brought it up, not me!

With 20ma flowing through the regulator in current limit configuration, even if you put 30V into it and grounded the output, worst case, the regulator would dissipate 0.575 watts.  Once its in circuit with the cell set to 10V, that power dissipation drops to 0.375 watts. Thermal limit? NEVER!

As long as the load voltage stays below Vin-4.25V and the output current is greater than or equal to 10ma the current limiter will limit properly to the current its set to. And yes, all the way up from 0V (ground). If it doesn't its either not a current limiter or if it is, its broken. You may be able to limit to less than 10ma but this will vary from LM317 to LM317 given process spread though its not guaranteed.

Checking the current or setting it using just an ammeter across the limiter to ground works flawlessly and given what I said above, it matters little whether you do it this way or check it in circuit.

Its MUCH easier to set the current on the bench if its adjustable and a good practice to check the set current before you hook it up in circuit either way. Its also good practice to check the current once you do have it in circuit because if you're meeting the operating requirements of the limiter and its not limiting to what you set it to, the limiter is broken.

An LM317 makes a poor current limiter? I guess you didn't read the data sheet well enough. The LM317 was designed specifically to function wonderfully in both voltage regulator AND current limiter configurations. As long as you stay within the operating limits (IOut >= 10ma, load voltage < Vin-4.25v and keep the regulator below the acceptable power limit), its a wonderful current limiter.  I have no clue why you think otherwise.

If you don't like the operating requirements, you can go do your own homework and find another adjustable regulator to use that suits you better but thats getting harder and harder to do these days if you want one in a TO220 package. Most everything these days is surface mount.

The LM317 is readily available and has been since maybe the 1980's (yes, still in production today), its cheap, its a beast and it just plain works.

ALL current limiters are voltage regulators. The difference is that in current limiter topology, the voltage regulator regulates its output voltage so the voltage ACROSS the current sense element (resistor or potentiometer) is always equal to the voltage reference (1.25V in the case of the LM317), NOT between the regulator output and ground! Given this, picking the sense resistor value to get a particular fixed current is simply Ohms law.

Even the 2 transistor limiter is a voltage regulator. It limits current by always maintaining about 0.65V (base emitter junction voltage of the bipolar sense/control transistor) between the current sense resistor and ground (a.k.a across it) being this circuit is a current sink rather than source. Its not a very good current limiter because the base-emitter junction of the control transistor drifts in voltage a little over temperature and current through it.  An LM317 limiter is rock solid because the internal reference of the LM317 is a bandgap reference thats very temperature and voltage stable.

Offline Mer2112

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Re: Taking Proper Current Reading
« Reply #14 on: March 27, 2020, 11:18:57 AM »
Bringing back some good info, using the diagram below, could the current reading be taken in place of the cell? For example, connecting the positive of the meter to the anode terminal and the negative to the cathode terminal.