50A or greater

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vectorup
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50A or greater

Post by vectorup » Sun Mar 09, 2003 10:07 am

I often have to service power supplies and assorted other electronics for people. I have seen several schematics of High Power power supplies for 12 to 15 volts, and 20A to 50A, and they all use a considerable amount of parts. Is there a way to make a high power 13.8 to 14.8 V supply that can utilize the full potentual of a monster traansformer for High Current, and regulated 13 to 14 v output, with only a few parts?

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Externet
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Re: 50A or greater

Post by Externet » Sun Mar 09, 2003 4:05 pm

Hi.
If the parts in a battery charger are few enough, connect it to a bulldozer battery and you will have way beyond 50 Amperes.<p>If you want to use your monster transformer, just get a monster bridge rectifier and make your own charger. But your transformer must have the RIGHT secondary voltage to do it.
That's 3 parts. The transformer, the rectifier, the battery. Less than that? A battery alone will give you over 50 Amperes for a while..<p>Miguel
- Abolish the deciBel ! -

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Re: 50A or greater

Post by chessman » Sun Mar 09, 2003 4:57 pm

You know, I'm not sure I follow the logic of a battery, transformer, and rectifier making a high-current power supply.<p>
You'll need a transformer, rectifier, and biiiiig beeeeeeeeeefy caps. That's my suggestion.<p>If you want true regulation, then I don't know.

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Joseph
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Re: 50A or greater

Post by Joseph » Mon Mar 10, 2003 1:00 am

You could come close to a few parts if you use a transformer, rectifier, filter cap, high current PNP transistor with a base resistor, NPN transistor with its base resistor tied to the filter cap, and a zener diode connected from its emitter to ground.<p>The output is taken from the collector of the PNP transistor and the feedback goes to the emitter of the NPN transistor through a resistor. I think that comes to 9 parts.<p>[ March 10, 2003: Message edited by: Joseph Meisenhelder ]</p>

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Re: 50A or greater

Post by Ron H » Mon Mar 10, 2003 9:33 am

Joseph, I can't follow your "schematic". Can you try again?<p>Ron

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Re: 50A or greater

Post by Will » Mon Mar 10, 2003 10:41 am

I believe all of the suggested solutions would work, in a fashion, none so far described are really practical because (1) In order to retain the high current output capability you need all of the max charging voltage for a twelve volt battery to go to 14 volts or more - consequently, when not in use or when only light loads connected the voltage will always be high i.e. around 14 volts. When a high current load is connected (The order of 50 amps) then the battery voltage may drop to 11.5 or less - dependent of course on the relative charateristics of both charger and battery. That answers the question of regulation. Of course you could connect a second load circuit for light loads and include in it a regulator. But then that may be missing the first objective of minimum parts.
2) With such a permanently connected high current source (The monster charger) the battery will , except when connected to high current loads, be continuously 'gassing' and producing a highly explosive mixture of Oxygen and Hydrogen (H20 plus elctrolysing current = 02 plus 2h2) If that were not considered then someone might die in a humongous explosion. Will
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Re: 50A or greater

Post by chessman » Mon Mar 10, 2003 11:17 am

Once again, where is the idea of a battery and battery charger coming in? Am I missing something? I thought the request was for a 50A power supply to be powered off mains...

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Re: 50A or greater

Post by gadgeteer » Mon Mar 10, 2003 12:47 pm

Use of a battery gets rid of massive filter caps; but it's not a good idea. Problems are "overcharge", "venting", "destruction of battery".<p>I think what Joseph was saying, was take a simple resistor-zener-diode chain, and amplify it with a common-collector. Which will output Vz + Vbe (in silicon, Vbe is 0.7 volts). But there are two problems: using such as 2n3055, the current is only 15 amps. You can "chain them" (say five in parallel) for 75 amps, but you hafta use little ballast resistors in each emitter (so that ONE transistor doesn't get ALL THE CURRENT, and then the SECOND gets it when the FIRST rransistor explodes, then the THIRD gets it when the SECOND explodes...) And the ballast resistor ads a voltage drop (which is why it protects the transistor---no two "Vbe" 's are alike, so the LOWEST Vbe would get all the current.) This is how you can build a 20 amp supply if you only have six-amp-diodes --- you can connect 5 in parallel, but you'd better throw in a couple tenths of an ohm of resistance in EACH diode branch...<p>(Or the LOWEST diode junction will get all the current, and then the NEXT lowest will get it when the FIRST one blows up, then the THIRD...)<p>"Current Hogging"...<p>The second problem, is DISSIPATION; power on a device is defined as "current times voltage drop". If a transistor is sourcing 10 amps, and a 20 volt supply provides 12 volts regulated, that's 80 watts of HEAT!!!<p>Suppose instead you could turn on your 20 volt supply, without any voltage drop. Then you SNAP off the power. In both cases, the power lost as heat is ZERO---first your voltage drop is zero, second your current is zero. Now snap the supply back on, and snap it off.<p>Suppose you take your "20-volt-squarewave" and fed it to some inductors and capacitors; what would you get? A fairly smooth DC, at some level less than 20 volts. now suppose you build a circuit to MEASURE the DC voltage (from your filters), and to CONTROL the "duty cycle" (time ON vs time OFF) of your controller; you adjust the squarewave until you get TWELVE VOLTS DC OUT. Would that work?<p>Of course it would---it's called a "Switching Supply". And it is very efficient (compared to linear). The faster your switch is (usually mosfets), and the lower the "ON" resistance, the cooler your unit will run.<p>Switchers put out lotsa noise, though---the faster a squarewave is, the more harmonics it contains. So you're balancing "evils"---power, vs noise.<p>Obviously a switcher takes lotsa electronics. (I built one using a 555 onve---its pin 5 "Control Input" can be used to control the duty cycle...)<p>There will be a test on this stuff next week...

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Re: 50A or greater

Post by josmith » Mon Mar 10, 2003 12:56 pm

Check out this like<p>http://www.sola-hevi-duty.com/products/ ... ng/cvs.htm<p>If you want dc just rectify the output. The voltage regulation is built into the transformer.<p>Of course the one big expensive component being elimitated for most power supplies is the transformer . Switching power supplies like the ones in most pc's supply large currents right off the line in a very small package.

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Re: 50A or greater

Post by chessman » Mon Mar 10, 2003 2:23 pm

This brings up another question I have.<p>How can you use a tiny transformer in a SMPS to provide 1000 watts of power at 120VAC from a DC source?

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Re: 50A or greater

Post by rshayes » Mon Mar 10, 2003 5:48 pm

Switching supplies delivering 12 V at 50 A are available off the shelf. Cost will probably be over $300. Some of these may be available in surplus for a fraction of that amount, but may be hard to find. I would not recommend attempting to design such a supply on your own. Slight errors in design usually have spectacular results.<p>Battery packs with a charger are a possibility. Regulation is not particulary good, but that is where the 12 to 14 V voltage range came from in the first place. Sealed Lead-Acid cells are available up to about a 25 Ampere-hour rating. These can be "floated" across a fixed voltage charger if the voltage is chosen correctly. This system has been used for years in cars, aircraft, and ships. It is a brute-force approach, and you need a very heavy switch and/or circuit breaker on the output side.<p>A variac can be used on the AC side of a transformer, rectifier, and filter to control the output voltage. Control can be manual, or the variac can be motor driven by a feedback loop. General Radio used to make multi-Kilowatt AC regulators using this scheme. Of course, they also made variacs.<p>Harrison Labs (later Hewlett Packard) made a line of laboratory supplies in this range that used phase-controlled SCR's on the secondary side of the transformer to control the output voltage. These still required a heavy filter on the output.<p>Sorenson (later Raytheon) made a series of power supplies using a variac feeding a rectifier-filter and followed by a linear regulator. The output voltage control was ganged to the variac so that the drop across the linear regulator was limited to a few volts. Some of these may be available as surplus.<p>IBM used linear power supplies in some of their mainframe computers in the 1960's. I remember one that delivered about 20 A at 12 V. It used five or six TO-36 ("doorknob") transistors in a series regulator circuit and weighed about 40 pounds. Some of these might be in a forgotten corner of a surplus warehouse.<p>One approach that I haven't seen would be to use a magnetic amplifier as a regulator in a transformer-rectifier system. This would be similar to the Harrison Labs approach with saturable reactors replacing the SCR's. The main advantage is that saturable reactors are very difficult to destroy compared to SCR's. The control circuits may also be simpler. This technique is occaisionally used to regulate auxillary outputs on switching supplies.<p>There are lots of possibilities, but most of them are heavy and the rest are expensive.

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Re: 50A or greater

Post by gadgeteer » Tue Mar 11, 2003 12:46 am

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Switching power supplies like the ones in most pc's supply large currents right off the line in a very small package. <hr></blockquote>They still have transformers---but because of the much higher oscillator frequency, the transformers are MUCH SMALLER. Of the "powdered iron" type. And the expected inductors on the output...<p>I built a test fixture powered by a surplus switcher---but the Fireberd registered too much noise on the telephone channels. I ripped out the wires and replaced them with coax, properly grounded---problem solved. The switcher was radiating RF, trashing the signals...

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Re: 50A or greater

Post by chessman » Tue Mar 11, 2003 8:30 am

Right, I realize they still have transformers, but I guess my question is as follows:<p>Does all the current flow directly through the transformer and out of the power supply?<p>For example, I took a look at a 12VDC to 120VAC 60Hz switcher for a car a while ago. I was suprised at how small the transformer was! Is it small because of the high frequency, or is it small because not all the current is going through it?<p>The inverter was 300W RMS and 600W surge, and I don't think I've seen a transformer that small (2" x 2" x 1") with a 2.5A secondary before.....

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Re: 50A or greater

Post by russlk » Tue Mar 11, 2003 2:26 pm

How this works (most likely) is that the switcher converts 12 VDC to 170 VDC at 2.5 amps. The switching frequency may be as high as 500 kHz or 1 mHz. A full bridge output converts to 120 VRMS at 60 Hz.

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Joseph
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Re: 50A or greater

Post by Joseph » Tue Mar 11, 2003 9:05 pm

Hello Ron, sorry I am slow in getting back. Maybe you could not follow it because I mistakenly had the zener diode connected to the emitter of the feedback transistor instead of connected from its base to ground.<p>But I have meanwhile thought of a much easier regulator to build for high currents. It is simpler, can output more current, and is adjustable from about 3V to 15V. Get to the schematic by the link:<p>http://groups.yahoo.com/group/switchmod ... ulator.GIF<p>Oh, by the way, since someone mentioned that high power switching power supplies are costly, the SMPS design at
http://groups.yahoo.com/group/switchmod ... jomeis.GIF
can be made to provide several hundred watts.<p>[ March 11, 2003: Message edited by: Joseph Meisenhelder ]</p>

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