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Author: Subject: Power Supplies
IrC
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It should work fine, especially if they are all coming on at the same time. Since you have many risk a few and try it. As long as your AC input is fused reasonably whats the worry?
bio2
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When paralleling transformers the impedances should closely match and preferably they should be identical make and model for the best results.

This mainly applies to the AC secondaries in parallel and there are specific techniques for doing this after the rectifiers to prevent circulating currents.

Try to use those computer power supplies that are matched
(model, brand) and you will have much better results without losing a lot of power due to mismatched rectifiers etc. One technique is to place a power resistor after the rectifier on each unit say.22ohms or so. This provides some
compensation and is very simple.
The_Davster
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Why is it important that they both come on at the same time? Could current flowing into the PSU through the output cause 'bad things'(I seem to be overusing that phrase recently) The supplies will be on the main lines so the breaker for that room will be the regulator I guess(beats fuses, I went through way too many with my furnace). I have noticed with computer PSUs that inside they do have a fuse, although the filament looks pretty beefy, I think that only prevents major shorts as the PSU turns off when more amps are pulled from a line than what the line is rated for.

As I am using this for electrolysis I will not be doing this but for my own curiosity, can the PSUs outputs be hooked up in series(to get a higher voltage) or would that cause a short? I am wary of connecting outputs in series after an incident I had when I was little in which I thought if I conected the 'positive' and 'negative' parts of 2 separate wall outlets I could get 240V(I wanted to zap a pickle it higher voltage)...need less to say the wire I used(it was stranded Cu-wire exploded in a nice shower of sparks at the end when I stuck it in the socket...

Hmm...seeing as this is turning into some sort of super power supply I might as well add in an option to impress some AC on the DC, for those sensitive electrochemical oxidations

EDIT: Thanks Bio2, but I am afraid my source is pretty much the odds and ends, so matching brands, let alone models is not likely. I also understood very little of the terms in your post so I should likely do some electronics reading before I do this...
But a quick stupid question...what does a rectifier of the type found in the supply look like? I did a google image search and I do not see many similarities between the guts of my supply and the image result pictures.

[Edited on 24-1-2006 by rogue chemist]

woelen

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Putting voltage sources in parallel is not a really wise thing to do. If the power supplies have a small difference in voltage (e.g 4.8 volts and the other one 5.0 volts), then a current flows, which can become really high (the resistance probably only is in the order of 0.05 Ohm or even less).

Another effect is that when one of the supplies powers on, and the other doesn't, then huge currents may flow into the not-yet powered device. It depends on the electronics what happens precisely, but I bet that these things usually are not fun .

Making a series connection of power supplies is safe, provided that the power outputs are "floating", relative to earth. IIRC, a PC power supply indeed has floating output, its GND (0 Volt) is not connected to earth.

I made myself a fool-proof powersupply from a PC power supply. I use the concept of current control for electrolysis experiments. That gives much more consitent results (I tried with electrolysis of NaBr, making KBrO3, by precipitating the resulting bromate with KCl). In order to keep things chaep, I use fixed resistor networks, but it works like a charm. I also put all resistors in small glass vials, reducing the chance of accidental short circuits. A complete description is on my website:

http://woelen.scheikunde.net/science/chem/misc/psu.html

It is easy to build. Only the 12V output is needed, because I use current control. The PSU allows at least 10 A of current, but the resistor network limits the available current. Of course, with other resistors, you can obtain higher currents.

Anyway, at my situation, not the PSU or resistors are limiting the current, but my anodes. If I use more than 1 A of current, then my anodes pulverize very fast.

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bio2
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A rectifier is a diode and looks like the package it's in.
Usually a bridge with 4diodes in one plastic DIP. It's the
first thing after the transformer secondary and may be multiple barrel shaped type or TO220 in a switcher. Just look for the "D" on the board and then measure the forward voltage and try to match if you have others.

Very nice article and site Woelen.

You should consider using an LM317K ($3)as a constant current regulator. Requires one 25 ohm pot and a 8ohm fixed for max current so this gives 50mA to 1500mA. To produce higher currents the 317 can drive a large power transistor. There are many designs in the data sheet at National Semiconductor and the LM317 is very easy to find and apply. Lambda International Hazard Posts: 566 Registered: 15-4-2005 Location: Netherlands Member Is Offline Mood: Euforic Online Current limiting resistors on the output terminals Rogue chemist, by putting a ~0.1 Ohm resistor of ~20 watts (or a piece of maybe ~2 mm diameter coiled up Nichrome or Steel wire) in series with all PSU 5 Volt outputs (per PSU), you should be able to even out the differences sufficiently before connecting them parallel. I don't think the one PSU will short circuit into the others before they are turned on, for the rectifier is on the output stage with only a small capacitor, due to the high switching frequencies used. How the loop feedback sensing electronics will respond to the already turned on output, I don't know. Just make shore that these units stay closed, so that any exploding capacitor, diode etc. may not hit you in the eye. This shit flies as fast as bullets when high currents are involved. [Edited on 24-1-2006 by Lambda] tumadre Hazard to Others Posts: 169 Registered: 10-5-2005 Member Is Offline Mood: No Mood the following is how I connect PSUs in parallel. Get a diode and a resistor, sized for the full current and voltage drop needed to balence each psu's output voltage at full current, respectivly. Connect the anode to the 5 volt output, the cathode to the resistor and connect all the resistors together, and the grounds are connected. the result is that the PSUs can't "see" each other through the diodes you can find six 30 ampere diodes in a 60 ampere alternator, find one at a junk yard for a few dollars.  Quote: Originally posted by Lambda Just make shore that these units stay closed, so that any exploding capacitor, diode etc. may not hit you in the eye. This shit flies as fast as bullets when high currents are involved. [Edited on 24-1-2006 by Lambda] can't agree more, a friend of mine had a tantalem capacitor shoot though his hand woelen Super Administrator Posts: 7805 Registered: 20-8-2005 Location: Netherlands Member Is Offline Mood: interested This trick may work, but the power delivered by all PSU's will be very unevenly spread, and you loose approximately 0.6 volts plus the drop accross the resistor. So, your voltage at the output will be 4.2 .. 4.3 volts, not more. What this setup indeed does, however, is making power up safe. The PSU, which powers up first does not feel a big power surge from the other PSU's, the diodes of the other PSU's simply block. But then I have a question, why do you need such a high current? The art of wondering makes life worth living... Want to wonder? Look at https://woelen.homescience.net The_Davster A pnictogen Posts: 2861 Registered: 18-11-2003 Member Is Offline Mood: . Partly for the "I have a 50A power supply" bragging rights, and partly because in my antimony separation from solder experiments I have pulled more amps than the supply could handle on the 12V line(although I have changed designs since then). I guess its mainly a 'I have the resources so why not' type thing. And I perhaps wanted to draw an arc to make a bit of carbide, but after reading the above posts the number of supplies in series and parallel needed for that would be downright mad! So I do not think I will be doing that right away... Tumadre, you mentioned the diodes but did not say where they should go, but I am guessing in series with the output?. So after reading the above I will start with only 2 in parallel, use resistors of the appropriate value to bring all outputs to the same voltage, the diodes once I find out where they go, and put a switch on one of the output lines of one PSU so I have the option to only have one supply worth of current. Sounds good? I will vary the amps with number of cells, resistors, electrode distance and such. Luckily I have already built one powersupply from a single computer power supply in the past, so I am not completly clueless here. It has served me well in the past. Now any problems with impressing some AC in parallel on the DC output of a single power supply? Of course at the same voltage. And Does "impress" really mean to just have AC and DC going through the same wires at the same time? [Edited on 25-1-2006 by rogue chemist] IrC International Hazard Posts: 2710 Registered: 7-3-2005 Location: Eureka Member Is Offline Mood: Discovering "Why is it important that they both come on at the same time?" Because the schematic of the supplies is unknown so who knows if one slightly higher source will sink into another source. But with a few tenths of a volt difference who cares, why waste all the hassle worrying about isolation? You have a bunch for free see if they go or blow with simple parallel connections. No loss, and likely no flow between units assuming all the output sees is the internal diodes and filter capacitors anyway. A small few tenths of a volt difference between them isn't dick if there is no way for power to flow backwards into one, all you will see is one or more source more current than others. So what. Part of mad science is the theory of go or blow isn't it? What have you got to lose and look how many parts and wiring you will save. woelen Super Administrator Posts: 7805 Registered: 20-8-2005 Location: Netherlands Member Is Offline Mood: interested I do not agree with you IrC. What is the cost of a diode (<$0.50), and what is the cost of a complete PSU? I would not try it, unless you really don't care about the devices. Also, what would the result be on the long run? It may turn out OK one time, two times, etc. and then suddenly the whole thing blows up at the X-th time you switch on the stuff. I hate unreliable things, especially if there also is a risk of dangerous situations.

The diodes indeed must be in series with the output.

For each PSU, connect the anode of the diode to the +5 volt. Connect the resistor to the cathode of the diode. So, for N PSU's you need N diodes and N resistors.

Once, you have all PSU's with added diode and resistor, then connect all open ends of the resistors to each other, making a single large output.

[Edited on 25-1-06 by woelen]

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IrC
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I was looking at the fact that there already were diodes in the outputs and adding more merely reduces the output voltage available. If the voltage is more than high enough anyway then your added circuitry would failsafe the circuit. However so far these two I am trying are not showing any unusual warmth sitting here with no load, indicating indeed there is no appreciable current flow between them. If the possibility did exist then you are right the added circuitry would insure a problem free source. As I said before he got them for free so not much is lost trying a couple if his goal was to get as close to the 5 volts as possible. If I only needed 4 volts then probably I would add diodes for reliability also, but I am not sure the resistors are needed since the added diodes each have their own internal impedance in the "on" state. To each their own way.
12AX7
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If you have switchmode power supplies, they both actively regulate the 5V output (generally) so you have two feedback loops fighting. If you set them equal (there should be a potentiometer on the board inside with a dab of silicone on it), the current should share equally, to whatever extent the output impedance allows (a 0.2V droop across 30A range is all of 6.7 miliohm; variation in this figure will alter current distribution according to the Thevenin source impedances).

It is true that both supplies have diodes internally, so backflow isn't an issue. However, if a supply's output voltage is raised, the control loop chokes off the power flow, putting the entire load on the higher power supply. This is the concern.

Same applies to linear power supplies.

Paralleling anything constant-voltage (or equivalently, connecting constant currents in series) is a bad idea, and generally you need some external stimulus to keep things in order: master-slave configuration, master control loop, etc.

Tim

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The_Davster
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So I was at the place I get the PSU's, and I ended up with a 350W one, 35A on the +5V line. Looks like I may not have to risk the capacitors blasting through my hand. I think I'll just stick with a single PSU as I think 35A will work for most things. I really can't imagine ever needing more. I did learn some cool stuff from the replies above.

[Edited on 26-1-2006 by rogue chemist]

Lambda
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Rogue Chemist, just one final remark about these PSU's. If there is a sensing wire on the output connector and this is not hooked up, then the PSU will not stay on, for it will then encounter an error. This is a feedback sensor, and is used to check if the computer were they are to be used in has powered up properly.

If present, this wire must be connected in order for the PSU to stay on (power good sensor). Also, some PSU will not run without a load.

How to Convert a Computer ATX Power Supply to a Lab Power Supply:
http://wiki.ehow.com/Convert-a-Computer-ATX-Power-Supply-to-...
The_Davster
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Thanks Lambda

That link is actually quite similar to the page I followed when making my first power supply from a computer power supply: http://web2.murraystate.edu/andy.batts/ps/powersupply.htm
The only difference between the two pages seems to be the currents involved, and as such, the use of a power resistor instead of a sandbar type resistor. My current power supply has just a sandbar resistor pulling an amp, and it gets hot despite being next to fan. For a future power supply I will be sure to use a power resistor as those seem to be better intended for high current uses.

woelen

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Indeed, I also had to connect the sense line (brown) to the orange CPU voltage line. Not before the voltage across the orange line is within tolerance, the total PSU powers up. In my page I described this, as connecting the brown wire to an orange wire.

Besides that, you also need to ground the green wire. That is the soft on-switch. Failing to ground the green wire also prevents the PSU from powering up properly.

My PSU only needs a few mA of load, but I have heard of other people, that their PSU requires much more load. But these PSU's were older. The modern ones apparently do not need much load.

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IrC
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My solution to my mad science power supply for electrochem experiments was to take a Pyramid Gold series (13.8V/30A) power supply and gut it for everything except the transformer and bridges. Then I designed my own supply as I wanted zero to 45 volts at a maximum of 30 amps. To eliminate dissipation problems I altered the transformer-bridge configuration to allow only 25 volts on low and 50 volts on high. This way in the low range the pass transistors only have 25 volts on them and on high 50. Of course you have to remember to switch since on high the supply will still adjust from zero to 50 volts. I also mounted a thermistor on the bridge heat sink, transformer, and one on each pass transistor sink assembly (4 total). This signal feeds a circuit which will remove all power if overheat is detected, as well as an overvoltage circuit intended to kill the juice and sound a buzzer and light in the case of a shorted pass transistor. The design is my own and works so well it is used for many things around my lab.

I of course used the existing voltage and current meters and altered the volt meter circuit to be 0-25 or X2 which would be of course 0-50. The overvoltage is set to trip at 47 volts, and 0-45 is all I adjust for meaning no matter what in the event of pass transistor failure the supply is guaranteed to stop doing it's thing if it sees the full 50 volts no matter where it is being run at the time. Later I am going to add a fully adjustable constant current output independant of the normal output but the case is really crammed full now so I am having a hard time figuring out where to mount another circuit board in there. The 4 pass transistors are 16 amp 180 volt 2N3773's and the predrivers are the high voltage 2N3055HV's (16A/150V).

I forgot to mention but the low value resistors in the ground end of the supply provide a kill voltage to the power supply in the case of extreme high currents, a simple short circuit protection circuit. I had 5 to begin with but removed one to allow a lower current before tripping. The bleeder on the 50 volt rail is overkill at 60 watts but they were in the junk box so why not. In fact the whole supply was made from my junk box.

[Edited on 27-1-2006 by IrC]
Lambda
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IrC, you can also sense the output voltage with an OpAmp, which in turn drives a power relay via a transistor so that the Transformer secondary can be switched. A hysteresis has to be implemented into the OpAmp design to kill threshold relay vibrating. A mistake I often see in cheap OTC junk designs. More elegant, and a design I am working on now, is to regulate the transformer primary via Triac's or Thyristor's, to chop the input mains supply voltage sinus, which is regulated via the difference between the Transformer output voltage and regulated output voltage.

Just one tip, although depending on the manufacturer of the 2N3055's, the power dissipation may vary quite a bit. Generally speaking, the 2N3055 should not dissipate more than about 50 Watts, due to the ease of cooling you will then encounter. Cooling elements are often much to small or not sufficiently ventilated. In you tight-fit-enclosure, forced cooling will be very beneficial and save you a lot of hassle afterwards. Personally, I think there is nothing worse than the shutdown protection kicking in during my experiments because not that the current or power output was to high, but because the power supply just overheated. Imagine sitting there with molten Potassium or Sodium Chloride or Hydroxide during the synthesis of Potassium or Sodium and the juice dropping away !.
IrC
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"IrC, you can also sense the output voltage with an OpAmp, which in turn drives a power relay via a transistor so that the Transformer secondary can be switched. A hysteresis has to be implemented into the OpAmp design to kill threshold relay vibrating. A mistake I often see in cheap OTC junk designs."

This is exactly what the 741 IS doing for the overvolt trip, monitoring the rail. However if you study the circuit I implemented a latching circuit so hysteresis does not apply since the latching takes less than a few milliseconds. In this way the problem can be looked into before the reset button is pushed to unlatch the protection circuit. I designed this thing while watching the movie Serenity late at night with only one eye open. What amazes me is it works so well. I would like to see some of your AC control circuitry sometime as I am always into learning something new having at one time been a design engineer in a R&D lab involving electronics and chemistry. Another story for another day. My main goal here was to build a brick shithouse of a supply that would go up to 45 volts and down all the way to zero (not a few tenths above as sometimes I may be playing around with some experiment where I only want a few tenths of a volt), provide generous amps and regulate within a tenth of a volt.

Also, I wanted it done that weekend as I never have time for projects like this anymore. I will say I loaded it to 300 watts at 45 volts and switching the load in and out the output rail only varied by 0.05 volt, not even a tenth of a volt drop!

The 2N3055HV's are heavy duty versions of the 2N3055 and merely drive the pass transistors, which are 3773's. I like the primary control idea but I wanted this up and running soon and built soley from what was in my junkbox without waiting to order anything, and to implement your ideas would have taken me somewhat more time to design. Maybe I will sooner or later play around with your ideas but I need to add the proper thyristors or similar components to my stores, believe it or not I just ordered some last night for another project, and I always order extra to play so I think I will try your ideas but it would be nice if you had a schematic you could post to give us a starting point on the general idea. The fans (2) are powered through the 100 ohm 10 watt resistor right from the 24 volt supply, they are 12 volt 120 mm computer types.

Here is a pic of it, I added red and green high output LED's to the meters so it would look cool when runnning.

[Edited on 27-1-2006 by IrC]
Lambda
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IrC, please let me explain what I meant by implementing hysteresis into the OpAmp output voltage sensing design in order to drive a power Relay to minimize power consumption and thus heat dissipation of the Power Supply:

I will start at basics, although you already know this, it may be of interest for those who are not so familiar with these "old fashioned" analog Power Supply designs (I find them more reliable and robust than switching Power Supplies, "Murphy" may also have a word to say about this).

Lets start by assuming you want a Power Supply that can give 50 volts at 10 amps regulated output, and have a Transformer with two 25 volt secondary output leads that are more than sufficient to give the required output current. During a Metal Plating job to Chrome Plate a few nuts and bolts for use on your favorite Harley Davidson Motorcycle, only 5 volts is used with a current of 10 amps. What dose this then mean for the power consumption and heat dissipation of the Power Supply ?.

1 - The power (Watts = P) that the Chrome Plating bath requires will be P = I * V so that 10 amps * 5 volts = 50 Watts.
2 - The power that the Power Supply will consume during this operation will be roughly about P (more than) = I * V (more than, because of the Diode Bridge-Capacitor network) so that 10 amps * >50 volts = >500 Watts total power consumption to the output.

The efficiency will then be: 50 watts (output)/>500 watts (total) * 100% = <10% so that >500 watts (total) - 50 watts (output) = >450 Watts of power is wasted and has to be dissipated in the form of heat through the output Transistor regulator network. This means:

1 - A waste of precious energy.
2 - This heat will have to be dissipated, and the cooling plates will have to be made bigger (or made more efficient by means of forced cooling) and the use of more parallel output Transistors that can conduct this heat so that it may flow off. The design becomes more expensive, less efficient and a lot of heat dose not benefit long-life span of components.

How to overcome this problem:

I started of by mentioning that we have a Transformer with two secondary output leads of 25 volts, so if we only need 5 volts and 10 amps for the Chrome Plating procedure, then using only one 25 volt lead will mean: 10 amps * >25 volts = >250 watts power output. This puts the efficiency of the Power Supply at: 50 watts (output)/>250 watts (total) * 100% = <20% so that 250 watts (total) - 50 watts (output) = >200 watts of power is wasted. Less heat problems will be encountered here, the Power Supply efficiency is higher and thus less energy wasted. So by putting the secondary Transformer leads in series, one can chose between 25 volts and 50 volts if an output voltage of above 25 volts is required. This can be done in two ways: Manually or automatic.

The automatic way, the way of the lazy bum and the wise man:

The Potmeter that is used to regulate the output voltage has in this case two functions. First of all, the output of the moving slide is fead to the regulator electronics to function as reference voltage, the second function will be that to be used to switch the Transformer to the 50 volt conjunction. When this Potmeter has been turned to half of the slide, then an OpAmp senses this so that a Power Relay may switch the Transformer outputs to 50 volts. At a certain point the Relay will switch on or off. But just suppose you are using an output voltage at this switching point. This will mean that the Power Relay may turn on and off due to slight fluctuations (boarder line effect). By introducing a hysteresis in the sensing OpAmp, so that once it switches the Power Relay on (at about 25 volts output) then it will only turn the Power Relay off when an output of maybe 22 volts is required. This is what I meant by introducing a hysteresis in the sensing OpAmp. I have often seen OTC junk Power Supplies with a vibrating Relay at this critical point. The chances that a particular voltage is required that makes this possible may not be that big, but at the cost of a diode and a resistor of maybe \$0.10 in total, what the hell !.

I have a 24 volt at 40 amps Transformer with no in between secondary junctions which I want to use in a Laboratory Power Supply. So I am aiming at primary regulation to minimize power consumption at high currents. I do however, have a Power Supply that I have built that makes use of this principle. I got the design from the Dutch electronics magazine Elektuur (written in Dutch). Elektuur is published in many countries, and the English version is Elektor. I will try to find this design for you IrC. The problem is however, that these magazines don't publish with symmetrical articles, that have only been translated. Maybe they do nowadays, for in the past I have found articles in the German version that I did not know of being in the Dutch version. When I have found the English version of this article, then it will be uploaded to the Forum.

As for my design, I have just started. Roque Chemist got me hooked onto this "drug" with his 40 amp dream Power Supply desire. After I had read 40 amps in his post, I wandered of into my storage room, picked up this massively heavy Philips transformer, and thought to myself, I also want a 40 amp lolly in my lab. When the design has been completed, and all the monkey quirks have been worked out of the trees, then I will make it available for all to use and abuse.
IrC
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The dissipation problem is why I switch between 25 and 50 volts depending upon the desired range. I used to get Elector, they once paid me a for an article I wrote for them back in 1992. But I haven't written any for them since they vanished from US shelves. I really thought it was so much more advanced than the worthless build a led flasher promoting Popular and Radio Electronics magazines they puke out here. It used to be embarrasing to see that the US pubs were so far down the food chain in electronic sophistication as compared to the European magazines. Talking with the US representative of Elektor, I was told that Gersback pulled dirty tricks with the unions to keep Elector from being stocked on US shelves so soon it would only be available overseas. Paying for a yearly overseas subscription just got too expensive. Capitalist pigs at their best (Gernsback), and we here pay for it by not having good construction articles to read.

The article you mention may be Dutch but still you could post a schematic which is all I need to study anyway.
FPMAGEL
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I'am more plamsa science, than elctro(the lines become blerd), but, two 12v batterys in seiers, == 24v
two 12v batterys(car batterys) in parrel == 120amps hours

"Indeed, I also had to connect the sense line (brown) to the orange CPU voltage line. Not before the voltage across the orange line is within tolerance"
just a question is that earth or negtive?

"This is exactly what the 741 IS doing for the overvolt trip, monitoring the rail. However if you study the circuit I implemented a latching circuit so hysteresis does not apply since the latching takes less than a few milliseconds"
Just so i'am on the right track, is ther a pice of horse shoe of steel in there.

[Edited on 28-1-2006 by FPMAGEL]
Lambda
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IrC, apparently the article you requested is prior to 1995, because I could not find it on the "Elektor/Elektuur" web sites.

I do have the Photocopies, and maybe even the magazine in which this design has been described. They are all still in my storage boxes, the magazine will be the easiest to find if I have it, but to find it in my unsorted Photocopies (25.000 - 50.000 !!) may seem an impossible task at this moment.

Tomorrow (Sunday), I will go to the Library, and dig it up for you. When I find it, then I will download it from the Internet in the English (?) and Dutch language version.

Everyday Practical electronics from ~ 1999 to 2006 (Complete) (<500 MB).
Elektor from ~ 1989 to 2006 (Complete) + Data Manuals & Compilations (designs, audio etc.) (<10 GB !!).

About the Power Supply, I may also go for PWM (Pulse Width Modulation). I am still in debate with myself about this one though.
IrC
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