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Author: Subject: PC PSU to laboratory PSU
dann2
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[*] posted on 22-1-2008 at 08:06


Hello,

Thanks for that Bio2.
It took a bit of hammering home.

Dann2
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Xenoid
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[*] posted on 1-2-2008 at 17:33
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Here is the ATX PSU I have modified with current control, using the simple circuit from the site originally linked to by Aqua_Fortis_100% and subsequently reproduced by 12AX7 a few posts back. This simple little modification is great if you are working with electrochemical cells or doing electroplating. It allows one to dispense with "dropping resistors" to get the appropriate current. Note, this is not a constant current circuit, and current will vary somewhat as cell parameters change, it is however easily readjusted using the potentiometer control.

I used an ATX purchased from a recycling centre for $3, the only additional expenses were for the two heatsinks for the dual 2N3055s, these cost me $7 each. All the other components I had on hand in my electronic parts collection.

With dual (parallel) 2N3055s I can control up to 20 amps into a suitable load. I specifically built this with (per)chlorate cells in mind and I found that replacing the 100 ohm resistor with a 680 ohm gave me a good control range (the 1K resistor was dispensed with, see note below). Image 1. shows the modified ATX delivering 15 amps (20 amp meter on top of the case) into two of my carbon gouging rod resistors wired in series, as an approximate simulation of a chlorate cell. The multimeter is indicating a voltage drop of 3.66 volts across these resistors, indicating a total resistance of .24 ohms. In doing so the 2N3055s are dissipating 20.1 watts ((5 - 3.66) x 15) and are slightly warm. With the heatsinks shown, about 40 watts is the maximum I would want to dissipate on a continuous basis.
The yellow terminal is connected directly to the 3.3 volt output of the ATX, it was easy to do, but I will probably never use it!

The two 2N3055s on their respective heatsinks are mounted on the sides of the ATX cover, and the base, collector and emitter connections are joined in a terminal block (Image 2). This allows the lid to be completely removed by unscrewing the connecting wires. I did not use .05 or .1 ohm resistors in the emitter legs of the 2N3055s because with the two sets of transistors I have tried in this circuit they were not needed. The current seems to be split quite evenly between each transistor.

The internal arrangement is shown in Image 3. The red line points to the rear of the 5K potentiometer. The yellow line points to the TIP41 mounted on the black heatsink, which is screwed on to the front of the case. In this position it catches good air flow through the front vents. The 680 ohm resistor is mounted on a small piece of strip-board screwed to the bottom of the heatsink. Just to be safe, a piece of plastic (blue in image) is inserted to separate the high voltage components.

Note: In my earlier ramblings on this subject, I mentioned that the circuit worked well. 12AX7 suggested that the 1K resistor was not required and would effect the control circuit (whacky control curve). This only became apparant to me when I built the final version for mounting in the ATX. My earlier test circuit was built as a "rats nest" on the bench and I grabbed the first 1K resistor I found, when I found my final version was not behaving properly, I had another look at the circuit, did a few calculations and then realised I had used a new 1K resistor. The first 1K resistor had in fact been a 10K and had an orange multiplier colour band that looked red. So the circuit worked fine with the 10K resistor, it works better if you leave it out all together!

ATXmods.jpg - 81kB
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dann2
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[*] posted on 2-2-2008 at 16:28


Hello Xenoid,

Wondering what is the max. voltage out.
Will it be high enough to drive current into a Perchlorate cell?

Cheers,
Dann2
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Xenoid
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[*] posted on 2-2-2008 at 20:35


@ Dann2

Well there are perchlorate cells and perchlorate cells, mine have operated in the 4 - 4.5 volt range at currents up to 5 or 6 amps. I have never actually run a high current perchlorate cell.

Unfortunately you will take a big performance hit having those old 2N3055s, even when fully turned on they treble the internal supply resistance from about .02 - .03 ohms to .06 - .09 ohms according to my measurements. This limits the current available. Really, when I built this I was more interested in controlling in the 0 - 5 amp range for testing anodes. I'm sure the circuit can be fine tuned a little more, the 680 ohm resistor could be lowered and alternative transistors used, its just that I had those on hand.

I have some comparative figures for the modified variable supply compared to a "plain" 5 volt supply, using different load resistances. The voltage is measured across the load resistance.

.27 ohms
Variable (max. setting) (3.70 volts - 14.1 amps)
Plain (4.44 volts - 17.1 amps)

.4 ohms
Variable (max. setting) (4.16 volts - 10.6 amps)
Plain (4.63 volts - 12.0 amps)

.5 ohms
Variable (max. setting) (4.35 volts - 8.6 amps)
Plain (4.71 volts - 9.3 amps)

.8 ohms
Variable (max. setting) (4.67 volts - 5.6 amps)
Plain (4.87 volts - 6.0 amps)

So depending on the type of perchlorate cell you have it is unlikely you would be able to control more than about 0 - 10 amps into the cell. If you want more than this use an unmodified supply with a very low resistance in series.

Actually, I am thinking I might replace the 3.3 volt output terminal with a 24 volt / 20 amp switch and so be able to switch out the transistors for when I don't need adjustable current.
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[*] posted on 2-2-2008 at 22:35


I don’t usually disagree with Xenoid’s posts, but to call an emitter follower circuit a “current control” violently offends my engineering sense. The cell controls the current! It is at the mercy of a low impedance supply generously provided by dissipating volts in a transistor instead of a resistor. There is no current control at all – you are at the mercy of cell temperature, electrode level, electrolytic composition, and the phase of the moon.

If you want current control, use a circuit like those proposed by 12AX7 and Rosco Bodine. I’d rather use a junction transistor (with collector output) for this but I am convinced MOSFETs can be made to work, with some care. (I suspect Rosco might agree after we chatted about these matters on U2U).
******
I wasn’t going to post this, which I wrote some time ago, but it is partially germane to the topic and may help a tyro or two:

Re: PSU’s in general, electrolysis and Lamps as current limiters.

I hope this might be of some interest to the non-expert unversed in electronics. It started out as a quick note about using lamps as ballast and somehow grew… Rather than start yet another thread in a crowded field, I decided to tack it on to this one.

Some of you may have been intimidated by the electronic content of this thread and suppose that the whole business requires complex circuits that you don’t understand, and that the crude old method of using your battery charger as a source of DC is completely passé. Don’t be. You can use one to do many electrolyses but they it not work quite as nicely as the stabilized supplies promoted by Rosco or 12AX7.

Computer PSU’s give you fixed controlled voltages: 12v, 5v, and 3.3v normally- unless modified as suggested in the thread. You can control current drawn by electrode spacing (wasting the rest of the power in ohmic dissipation in the cell and heating it), with a series resistor, or varying the immersion depth of an electrode in the electrolyte. But really what you want is a prescribed current density most of the time. This determines the cell voltage*.

If all you have is an old auto battery charger the available current is usually limited to about 5-6 amps or so and it probably has 6/12v capability. (The charger usually has a current limiter as protection. You disable this at your own peril. You may blow the diode rectifiers or overheat the transformer). The voltage waveform is that of a full wave rectified sine wave, not ideal for electrolysis. The current density varies wildly over a cycle. Never slap a charger across a cell and hope! The current must be limited somehow.

First, you need to smooth it by using a shunt capacitor/series inductor/shunt capacitor filter using at least 1000 uF capacitors (rating at least 15v; eg Radioshack) and at least 20-30 millihenries inductance. Getting (buying) the inductor will not be easy. It should not have more than 1 ohm series resistance. Wind one on an fat iron core using adequately thick wire, leaving an air gap of 10-20 mils in the magnetic path. This will reduce ripple to adequate levels. (I’m not going to give any formulas but this core must not saturate under maximum DC load current – technically the air gap increases the reluctance of the path and ensures the flux does not saturate the core. The real design is complex and only able to be understood by an EE or a practical physicist).

A good alternative is to use a 12v lead acid battery across the charger to smooth it. This acts like a giant capacitor, and also allows currents vastly in excess of the charger’s 5 amps to be used by a cell (while the battery remains charged). The capacitor causes the peak current from the rectifier to be high, but the same is true of the battery, and chargers take this into account.

To get a given current density you must know the immersed area of the cathode or anode, at whichever the desired product is produced. The other electrode is often the same or larger in area. Only a cylindrical set up allows this to be estimated accurately. Parallel plates do conduct on both sides even if the other electrode is facing only one. The density will vary some over a flat plate but an fair average can be easily estimated.

The internal resistance of a cell is difficult to compute. The expected voltage drop (from SEP or experimental evidence) needs to be estimated. The rest of the voltage from the PSU must be dissipated in a series resistor R, computable by Ohms law. Without current control, the current drawn is then the I=V/R, where V = (nearly) the average PSU voltage (under load) minus the cell voltage, and I the desired current. If the cell voltage varies, the current varies*.

As an example, suppose the charger or PSU produces a nominal 12 V. The drop in the cell is estimated to be, say, 3V and the desired current 3 amps. The total cell ‘resistance’ is then 1 ohm. We need a total of 4 ohms to produce 3A. So the series resistor should be an extra 3 ohms, but we have to include our smoothing inductor and source resistance, so we’ll need a bit less than that. You need an amp meter to adjust it.

R ought, then, to be a resistor which can be varied from say, a maximum of 5 ohms down. If you have a rheostat of this size and rating, fine; otherwise you could use a length of suitable nichrome or other resistance wire and a slider, as I do. Or, for higher currents, a stout carbon rod with a slider works nicely.

For the same example, what happens if the cell voltage increases to 4v? The current in the 3 ohm resistor becomes (12-4)/3 =2.67 amps – not a huge change. If cell volts drop to 2v, we get 3.33 amps. The series resistor limits the current change considerably compared with what you would get by slapping, say, a 3.3v PSU source straight across the cell..

So where does the light bulb come in? It can act as a current variable resistor. As the filament heats up, resistance increases, very approximately as the absolute temp in deg K. (R(T) ~ = R(300)*T/300).When taking no current at room temp it has minimum resistance; at full rated voltage, resistance is maximum (it’ll melt if you go much higher!). This change is about 9 times from zero to rated volts for a tungsten filament.

Making the (gross!) assumption that the filament obeys Stephan’s law of radiation and its resistance follows temp. as shown, and that all the heat goes into the filament and is radiated, I derive that the I/V law is about I=V^0.6 or V=I^1.67 (a dedicated experimental scientist could find out what it actually is for a given lamp. A higher exponent can be expected for the actual I V equation. Heat conduction and convection in the gas inside the bulb mean that my figure is only a gross approximation).

Let’s see what we could do with a light bulb as the series resistor R. The idea was suggested by, I believe, Twospoons in this or some thread recently. It is a method I have often used in the past in spite of being an electronics engineer by trade. I never felt the need to build an exotic stable current controlled source for this application.

Suppose we took an auto lamp rated at 12v (for a reasonable life on our 12v charger). Suppose we could find one with a 9v drop at 3 amps (unlikely!) – it would then fit our requirement. At a full 12v it would consume I = 3*(12/9)^0.6 = 3.57 amps (43 watts – a 12v, 40w lamp is close).

At a cell voltage of 3v, then, I=3 amp. At a cell voltage of 4v, I = 3*((12-4)/9)^0.6 = 2.8 amp, and at 2v the figure is about 3.2 amps. Not a bad control. Much better than just a series R alone, but not as good as a settable current source..

Of course you can use any voltage stabilized PSU the same way. For massive cells taking 20-40 amps like 12AX7’s chlorate cell you’ll probably need to build your own crude transformer/rectifier set-up – or find one as suggested in the thread above. I doubt you’ll find a suitable lamp for that.

For voltage setting, if you happen to have a variac (expensive) you can vary the input to your crude PSU to vary the output (not a computer type or a stabilized lab supply, though). I’ve never tried it, but a suitably rated light dimmer might work, (SCR’s generally don’t like inductive loads). Or you can use a transistor circuit like Aqua_fortis showed above to adjust the output voltage (but not stabilize it to a fixed level). A similar circuit with collector output can be used for current setting.

*{Note: The electrolytic cell voltage depends upon many factors, current density, temperature etc, anode and cathode material and preparation, separation of anode and cathode, whether or not a diaphragm is used and so on. To understand what determines the relationship between voltage and current density and the relationship to SEP you need to research. Look under ‘Tafel’s Law’ or ‘Tafel equation’. A good starting resource is

http://electrochem.cwru.edu/ed/encycl/art-t01-tafel.htm}

Der Alte
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Xenoid
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[*] posted on 2-2-2008 at 23:38


@ Der Alte

Clearly you are not reading my posts correctly .... :mad:

NOWHERE have I claimed this is a CONSTANT current control! In fact I have gone out of my way to stress that this circuit is little more than a variable resistance on several occasions, I won't quote them all, but in my earlier post, note the fourth sentence.

Quote:

Note, this is not a constant current circuit, and current will vary somewhat as cell parameters change, it is however easily readjusted using the potentiometer control.


I am fully aware of its limitations, but it is a useful circuit for varying the current through a cell. Constant current is not even NEEDED in a (per)chlorate cell. I have just finished a run with my 10 litre cell which used just a variac, halogen lighting transformer and a couple of bridge rectifiers. Unfiltered DC and I barely had to adjust the variac over the entire 21 days!

I am not at all sure why the term "current control” violently offends your engineering sense, it doesn't offend mine. I set the control to minimum, attach a chlorate cell, turn the control to the desired current, "voila" "current control. What you are referring to, is constant current or current limiting. I have never, ever, claimed that of this circuit!

In fact it works so well I'm going to build another, I may experiment with different components however.
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[*] posted on 3-2-2008 at 20:47
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Hello,

Actually I don't defend Xenoid too often!! but he is operating on a circuit (as he has said) that has been described as being really like a handy variable resistor.



Cheers for input.

Dann2


* coppycatting, but just could NOT turn down that opportunity..............:D
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DerAlte
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[*] posted on 4-2-2008 at 18:58


@Xenoid,

Sorry a few ill chosen words caused offence! I have every respect for your experimental technique and achievements so well displayed in these pages.

And "violent"offence was putting it a bit strongly. I merely meant to indicate that this circuit was not a stabilized current source but actually a low imedance voltage source. You did not explicitly claim this I agree, but I guess I got the impression you had. I am certain you are very congnisant of its limitations, and they aren't that severe.

Also since current and voltage are roughly represented by the equation V(cell) = A + B*logI, (Tafel's Law), actually the voltage across the cell is not a particularly sensitive function of the current density- unless passivation or electode erosion takes place. Hence even a low impedance voltage source is often quite satisfactory in a well behaved cell.

The current density (current) once set by a variable voltage is likely to remain reasonably constant at least while reactants are not fully consumed. And the transistor voltage dropper is much neater than a fat old resistor. I am not denigrating it at all. As Dann2 says, it a very convient way of dropping volts.

A fixed current supply can be made almost as simply, with junction transistors, should one desire.

The rambling post above was actually intended to give a boost for the light bulb as a half-arsed current stabilizer cum voltage dropper, superior to a fixed resistor, and to point out to the unitiated that the humble battery charger or simple rectified supply were still good for elecrolysis. It sort of grew under its own momentum.

Regards,

Der Alte
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[*] posted on 5-2-2008 at 10:44


I already gave an extensive analysis of this circuit; further analysis and arguing is wasting extra posts (and you know how *I* hate wasting posts).

Tim




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[*] posted on 2-5-2008 at 08:30


I'm kinda stuck. After removing the PSU from my old pc I have sorted out the wires. Green needs to be soldered to a black, and a 10ohm 10W resistor needs to be soldered between a red (+5V) and a black. All well and good. However the problem arises in finding "terminals" that can handle the current. The best I've found so far are 6A rated 4mm test sockets and matching 4mm banana plugs. But is this sufficient enough? I cant find anything more appropriate :( The PSU is rated 235W, and has a maximum current output of 22A (at +5V, 14A at +3.3V, and 8A at +12V). There is also no 3.3V sense wire (although one of the orange (3.3v) wires is thinner than the others and is connected to a different part of the circuit... guessing thats it?). A wire that I have no idea what to do with is a white on, stated on the label as being P.G. Signal. I figured it was nothing important as it has not been mentioned in any of the methods I've found online so far.

Any help would be much appreciated :P thanks in advance
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[*] posted on 2-5-2008 at 09:17


1. Then don't use them for more than 6A each.
2. Ignore the warnings and ease off on load if they start getting hot.
3. Use twisted connections. My lazy, informal power supply is wired this way.
4. Get better terminals. 1/4" quick-connect lugs are good for 30A or so. Binding posts and screw terminals come in various sizes. Bolted lugs are available well into the kiloampere range for power distribution; check with your local electrical supplier.

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DJF90
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[*] posted on 2-5-2008 at 09:26


I intent to purchase the required components from maplin (www.maplin.co.uk). If you could select the appropriate parts then that would be very helpful, and much appreciated :P


Quote:

Then don't use them for more than 6A each.


How is this possible when the whole idea of using a PSU is for the high current properties (i.e. electrolysis)? Please elaborate.

I intend to make my PSU much like the one on Woelens webpage, so maybe he could provide the specs for the components he used?

[Edited on 2-5-2008 by DJF90]
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[*] posted on 2-5-2008 at 13:51


6 Amps seems very conservative for a binding post, I have used these at 15 Amps with no problems and I am sure they could handle 20 Amps. As a rough guide, a copper conductor of 1 mm^2 cross sectional area can carry a nominal current of 10 amps. And about 2.3 mm^2 is required for 20 Amps.
A 4mm diameter binding post has a cross sectional area of say about 7 mm^2 (allowing for the depth of the thread grooves). If you are still worried, make sure the internal connections are soldered on, and dispense with the banana plugs and wrap your wires around the post or put through the hole if it has one.

As an alternative you can use bolts, nuts and washers from a hardware store. This is what I used on my 48 amp halogen lighting transformer (see earlier in this thread). Brass would be best, but zinc plated steel is OK, make sure they are mounted on a sturdy piece of insulating plastic of course.

The PG wire can be disconnected. The 10 ohm / 10 watt resistor is to provide an initial load to enable the supply to start up. It seems a little excessive, try higher value resistors (and consequently lower wattage). Some of the "newer" supplies need very little load to start, my last supply did not require any additional load resistance at all.
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[*] posted on 2-5-2008 at 13:58


Quote:
Originally posted by DJF90
Quote:

Then don't use them for more than 6A each.

How is this possible when the whole idea of using a PSU is for the high current properties (i.e. electrolysis)? Please elaborate.


Each, as in, use several in parallel.

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[*] posted on 2-5-2008 at 14:32


Thanks Xenoid. I think I'm gonna go ahead and order the plugs/sockets, heatshrink and a resistor. What sort of current should I expect on the +5V line with a 10ohm resistor in place? I'm unsure about how much load the PSU will actually need to power up, its from an old pentium II pc.

@12AX7; Using two(or more) binding posts per voltage (in parallel) is not a bad idea but that would mean doubling or possibly tripling the cost of the components (as i would need more sockets and plugs). I think I will just see how things go and if overheating occurs then maybe your suggestion will come into play.
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[*] posted on 2-5-2008 at 15:14


Quote:
Originally posted by DJF90
What sort of current should I expect on the +5V line with a 10ohm resistor in place? I'm unsure about how much load the PSU will actually need to power up, its from an old pentium II pc.


Ohm's Law: E=IR or I=E/R therefore I (current) = 5 volts / 10 ohms = 0.5 amps
Wattage = EI = 5 x .5 = 2.5 watts, a 5 watt resistor is probably OK.

There is no problem using that resistor, it's just that high wattage resistors are fairly expensive, you may get away with using a higher resistance 1 watt resistor or even no resistor at all. With 20+ amps available from the 5 volt line, loosing .5 amps to the load resistor is no great loss!

If the supply is starting OK with the 10 ohm resistor, try replacing it with say a ~50 ohm or higher. A 50 ohm resistor will only draw 100 mA and only .5 watts will be dissipated.
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[*] posted on 2-5-2008 at 15:37


Thanks alot. I have an A2 physics exam soon and I know all this, for some reason I thought that the I=0.5 amps was what would be output current. Guess I didnt learn it well enough :(
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[*] posted on 2-5-2008 at 16:07


Yeah! The resistor is a "dummy load" to draw a little current and insure the supply starts up. It means you can switch your supply on without having "your" load connected. If you connect "your" load (eg chlorate cell) first, the supply will start up without the resistor anyway! It's just a convenience feature really!

Don't go connecting your cell or whatever in SERIES with THIS resistor.

However you may need a "dropping resistor" of some sort in series with your cell to limit current (see discussions earlier in this thread).

Good luck - :D
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[*] posted on 2-5-2008 at 16:16


Thanks :) I read about the dropping resistors on woelens site, very helpful resource that... So the resistor just needs to be in the PSU connected between one black wire and one red wire correct (black is gnd, red is +5V). And then the other black wires are connected to the gnd terminal, and the other colours are also connected and wired to their own terminals right? the reisistor itself is only 40p. Thats about 70c IIRC so its not all that expensive. I'm just hoping that the plugs and terminals hold up, thats the only place where I can see this project falling on its face :(
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[*] posted on 2-5-2008 at 16:38


Here's how I did it on one of my old supplies!

Disconnect all the existing wires, use a thick red wire for the +5V and a similar wire (blue) for the -ve (earth). Connect the startup resistor across the output terminals (in this case I used a 33 ohm 5 watt resistor - overkill). Those binding posts will easily handle 20 amps.

Note: I'm only using the +5 volts, not bothering with the +3.3, +12 or -12 volts etc.

[Edited on 2-5-2008 by Xenoid]

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[*] posted on 23-10-2008 at 18:16


I noticed that the last message on this particular thread was over a year ago, so I may be replying to something that live in a void somewhere.

I have been searching the web to locate a lab power supply, stumbled across the million conversions involving the ATX Power Supply, but not really what I wanted... I was looking for something a bit more functional, more control, and by chance, I came across this site. I am directly replying to the schematic that Rosco posted on the first page (a goldmine I say). Rosco posted 3 revisions of this current control / limiter, voltage control to turn your typcial ATX power supply into a fully regulated supply, which is exactly what I've been searching for.

So... my question is, and after reading the entire thread many times over, not fully understanding all that has been fought over, has it been decided that this last .pdf posted on the first page of this thread, that it should be functional with no issues? I am wanting to build this. I am eager to put an order in for the parts and get the project going...

Questions:

-Has anyone built it?

-Is there a PCB layout available? (planning on using perf board, just curious if there was a layout available)

-If I wanted to regulate from a different power source other than an ATX power supply, what is the max current (guessing by the title 50a) and voltage?

-I am also wanting to build another circuit with only the current control / limiter only with only (1) 12v power source... i guess it would be too much to ask for an altered schematic of just that?

-Last, the components listed, are there other parts equivalent to whats listed that would be more readily available? I've found the parts with not much problem online, but locally, the parts stocked seem to be limited... mostly NTE stuff.

Thanks,
Michael
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[*] posted on 23-10-2008 at 19:28


Quote:
Originally posted by saki2fifty
So... my question is, and after reading the entire thread many times over, not fully understanding all that has been fought over, has it been decided that this last .pdf posted on the first page of this thread, that it should be functional with no issues?


Uhhh?

The entire thesis of my point is that it will not work. This will;



You will probably need a small capacitor from each op-amp's output to its -in to compensate the response, otherwise it'll just oscillate, and that ain't no good. Easiest to analyze with an oscilloscope, but simply measuring AC voltage on the output (after checking that your meter doesn't misread DC as AC, because the cheap ones will!) will tell you if it's bonking out.

Alternatives include hacking the PSU's voltage regulation, which works to some extent. There should be a trimmer potentiometer on the board, probably gobbed up with glue, that sets the voltage. Un-gob it and you can adjust it over a small range. You can follow nearby tracks to see if there are resistors limiting its range (there should be); altering these allows much more adjustability. DO NOT set any voltage greater than the rating of the capacitors on that rail (I believe they are usually 6.3 or 10V for the 5V rail, and 16 or 25V for the 12V rail).

Tim




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[*] posted on 24-10-2008 at 00:28


At the point where I left off working on this there were still
concerns about the stability of the output stage. The module
which seemed most likely to work was based on using the LTC1152 as the mosfet driver. This would have been used in the place of the output stage shown in the complete schematic, but I haven't had time to go back and edit the ECAD schematic to reflect that change.

It would be interesting for someone having the time to prototype and evaluate the stability of the LTC1152 driven Mosfet and its damped feedback loop which I modeled,
and compare it to Tim's solution, see what is stable.

I don't have time to work on this. And since I already have
three huge working power supplies , a 50A, a 120A, and a 150A , there are other things I don't have that I will be building when I do have time.

I would say that the "engineering" on either of these is
unusual to say the least :D and there are uncertainties
which will probably lead to debugging / revising the actual
designs . No guarantees on this stuff from me , and if
you get them from Tim ......he's lying :P

As for substitution of equivalents, there will be little joy there.
The specs are pretty sensitive on everything I surveyed as
candidate parts with few substitutions even possible.

[Edited on 24-10-2008 by Rosco Bodine]
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[*] posted on 24-10-2008 at 06:43


Well, i'd be more than interested in putting in the time and resources in building both circuits, testing, logging my work and construction, etc. It would be great however if I could get a complete and modified schematic and possibly the cad file itself?

If on Tim's solution there were exact part numbers of the IC's of choice, as well as all parts (I trust your judgment over mine), much like what Roscoe has, I can start on ordering the parts, building and testing.

I'm not wanting to hack the power supply but rather build what you 2 guys have argued over :)... im excited.

Thanks,
Michael
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[*] posted on 24-10-2008 at 09:11


Quote:
Originally posted by saki2fifty
Well, i'd be more than interested in putting in the time and resources in building both circuits, testing, logging my work and construction, etc.

Great. I was hoping that maybe these postulated circuits would intrigue interest for someone having the time to actually lay 'em out on a board and see what they will do.
I know the DC logic is valid on the layout I proposed, but
what curves will be thrown by noise and AC components
which could adversely affect stability is the unknown for
either my design or Tims. I know I could "make it work"
purely by slowing down the response enough through various revision schemes, including using vactrol signal
buffering in the feedback loop . And mosfets are not essential for the output elements either, big NPN's could be used with different drive schemes. The DC logic for the
control loop being valid, goes a long way towards the solution....giving the front end controls, and then you
have a starting point for control of whatever ends up being the output stages that work.
Quote:

It would be great however if I could get a complete and modified schematic and possibly the cad file itself?

The cct file I have is for an old drawing program called DesignWorks which is a freebie. I haven't transferred the entire model in total to a SPICE simulation for the whole shebang, but just run sims on "blocks" , mainly the output module. That LTC1152 would substitute for the mosfet driver U5 and U6 in the corresponding parallel elements shown in that last pdf on page 1. The dual LT1218's are an active filter which provides the needed damping in the local feedback loop from the voltage across the current sensing shunt. That dual op-amp arrangement replaces U7 or U8 as shown on the last pdf page 1. I haven't redrawn the schematic to reflect the changes because I have to rescale the entire drawing to fit in the changes .....I ran out of room for making the changes without rescaling the whole damn drawing. It will take hours I don't have to redo the whole thing to fit the edits.
Quote:

If on Tim's solution there were exact part numbers of the IC's of choice, as well as all parts (I trust your judgment over mine), much like what Roscoe has, I can start on ordering the parts, building and testing.

You might be able to get free samples for parts from the
manufacturers if you are building a novel experimental circuit.
There was possibly a better choice of output Mosfet available from STM and they are the *only* company I found which has really looked at linear region operation of Mosfets.
Quote:

I'm not wanting to hack the power supply but rather build what you 2 guys have argued over :)... im excited.

Thanks,
Michael


It is actually a whole lot easier to hack an existing supply,
than to try to devise a more universal kind of solution .....
it's like the difference between minutes of hacking what's there, and months of abstract analysis and theory of what could be put there to do something analogous as an add-on.

The general concept of an adapter for an ATX might be implemented in a different way more easily in a conventional positive regulator configuration, and just go on and have
a local discrete 16 volt or so power supply for separately
powering the control elements, eliminating any noise
interaction from trying to power the control elements from
the same ATX supply as is being controlled. The scheme
of either Tim's design or mine is defintely not illustrative
of the "finest principles of quality engineering" but is more
a kind of what is the minimal thing which might work ......
rather than a more elaborate and conventional configuration
which could be better guaranteed to work....but at greater expense and number of parts. So you would correctly
view these configurations as some minimalistic afro-engineering of a "self powered adapter kind of circuit"
rather than either Tim's or mine "best work" which would involve a more sophisticated scheme. The parts expense
adds up fast as the quality of the thing is increased.
So if economy was no object, there are *definitely* better ways to do this.
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