build the "SIMPLEST" PS circuit

Discussion in 'The Projects Forum' started by Coogrrr, Jul 16, 2008.

  1. Coogrrr

    Thread Starter New Member

    Jul 16, 2008
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    I am looking to make a desktop power supply and I have searched many threads here but came to this conclusion. There is not a schematic for what I am looking for.

    I think this could be a group effort over time and I am willing to do the work then post the final resulting circuit diagram and parts lists to go with.

    The goal is to build the simplest AC 120/230v to DC 0 - 50 volt adjustable and 0 - 50 amp adjustable power supply. Extras would be 5v and 12v outs, more than one output (might be too much), meters for V's and A's.

    Is it possible to rework a computer PSU these are switching PS's that have a transformer for 12v 5v 3.3v and -5v -12v already in them. Maybe swap some parts and add the adjustable circuitry.

    Do we make the AC adjustable prior to going into the transformer and thus we have lower DC ouput but build the DC side for a max voltage amperage?

    Can we do this as a community thread? Suggestions?



    BTW - long time geek first time "All About Circuits" poster! This forum is so much like the cnczone.com forum I call home that I now have a hard time splitting my time between that forum and this one!!! You all rock and thanks for the forum and brain share!

    Coog
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    While your concept is interesting, I'm afraid it wouldn't be very realistic to assume you could make anything approaching a 50v 50A PSU from a computer power supply. Even if it were 100% efficient, it would consume 2,500 Watts of power, and would require 21A at 120V to supply it.

    Most household outlets in the US are limited to 15A, 120V, or 1800 Watts. Attempting to use more than about half of that in one device could cause problems, such as not being able to power the device and the lights together in the same room. So, for practicality's sake, it should be limited to 900 Watts or less.

    If a 900W supply was designed at 92% efficiency for 50v, that would supply about 16.5A - and you could still keep the lights on :)

    As far as modifying a computer power supply - well, the outputs are standard voltages, but the circuits themselves vary significantly. A design would wind up being brand and model specific.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
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    As a compromise, one might be able to design an adjustable Vout supply which limits o/p current to 900/Vout, (max 50A) and can adjust current limit downward from that point.

    Not from a computer supply, of course.
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    If you are sure you mean 50Amps and not 50 milliamps the cheapest and simplest solution for amateur use would be to connect 2 lorry batteries (2 x 24v) or four car batteries (4 x 12) in series.

    If you want current and/or voltage control at this level I would look at connecting a contolled SCR bridge across this basic supply.

    But please confirm that you really want 50 amps first.
     
    Last edited: Jul 17, 2008
  5. John Luciani

    Active Member

    Apr 3, 2007
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    As was mentioned 50V at 50A is not practical. You also need to consider power factor
    which will make the problem worse (especially with a PC power supply). I wouldn't
    consider 2500W a "desktop" supply.

    If you want a single 50V 600W supply that you could adjust I would suggest
    something like a Vicor FlatPAC. http://www.vicr.com/products/configurable/pfcflatpac
    These have trim pins so you could adjust the output.

    You will want an adjustable current limit should as well since a FlatPAC has straight line
    current limiting.

    (* jcl *)
     
  6. Coogrrr

    Thread Starter New Member

    Jul 16, 2008
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    Yes I agree 50v at 50amps (not milliamps) not possible but working from standat 120vac 15amp input can we obtain 0-40vdc and 0-30amp v adjust. and current adjust? This is 1200 watts which is a bit less than a 1500w heater that I use in my home?

    Needs to be a Volt and Amp adjustable solution and yes not milliamps.

    Thx for the brain share that is already working out the logistics on this!

    Next step lets pick a transformer for this beast!

    Coog
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    If it's going to be a 1,200W linear supply, you won't need your house heater any more. OTOH, you'll need a 2,000W air conditioner just to lower the temp enough to enter the same room that the supply is in.
     
  8. Coogrrr

    Thread Starter New Member

    Jul 16, 2008
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    if it is used constantly at the maximum output yes I would agree but sometimes we need massive voltage and low amperage and sometimes we need low voltage high current so this allows us to tune it to match the need. If someone has a full 1200 watt need then they should get an extra AC I think too!

    I have applications where I need many different V and A DC source power and building a new power circuit each and everytime would cost much more in money and time than one good bench unit. Plus if we make it ourselves we dont spend 2000.00 dollars for 200.00 worth of parts.


    Thoughts?

    Coog



     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I'd like to see more thought given to the implications of what you are asking.
    And I don't see that you will manage to build it within a $200 budget.

    For instance what line regulation are you seeking?

    1% ripple current - not an unreasonable value for psus - would be 300 milliamps of ripple!

    Most heavy duty power supplies are designed for a limited range of voltage or even a single voltage. To get over the inherent inefficiencies of supplying high voltage at low current or low voltage at high current designers often use switched ranges, and fine control within each range.
     
  10. Coogrrr

    Thread Starter New Member

    Jul 16, 2008
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    If you go to this link
    http://www.testextra.com/extech/extech_382290_900_watt_switch_mode_dc_power_supply.htm

    you can see that for 799.00 Extech is selling a 0-30vdc and 0-30amp fully adjustable bench supply. If you shop around you can find anything in the 0-40vdc and 0-30amp areas and everything inbetween.

    I found this circuit that is 14vdc and 0-20amp adjustable
    http://www.mitedu.freeserve.co.uk/Circuits/Power/13v820a.htm

    and I know we could make that 20 or 30vdc if we added to the circuit.

    but here is the kicker...

    This link has the circuit for a large voltage or amperage supply
    http://www3.telus.net/chemelec/Projects/MC1466/MC1466.htm
    As quoted...
    "Note: This is a Heavy Duty, "Lab Grade Supply" and can handle Hundreds of Volts and/or Very Large Currents."
    AND:
    "I built a dual isolated version of this for my own use:
    Seperate Power Transformers for Each one
    They are Each rated at 35 Volts and 8 Amps, Continuous Output.
    I can Parallel them for 16 Amps.
    Or Series them for 70 Volts
    Or use them in series connection as a Plus and Minus Supply, with center as ground."


    Think we can make this last entry adjustable on the V and A with some nice POTS?

    James
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Marlin P. Jones & Associates usually has some pretty good deals on supplies.
    Here's a 0-50V 0-3A constant voltage/constant current supply for $99
    http://www.mpja.com/prodinfo.asp?number=14602+PS

    I have an old linear Lambda supply, puts out 0.25v-54v, Vreg only. Bought it at salvage a decade ago for cheap. It pretty well takes care of most of my needs, along with a converted ATX-ff psu from a defunct PC.

    Now if you really wanted a cheap higher-voltage supply that could crank out some amps, you might could serially chain the outputs from some converted PC PSU's. You'd have to float the grounds, though - and the Vreg error would be cumulative.
     
  12. Pich

    Active Member

    Mar 11, 2008
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    Got my interest Coogrrr, you will need to post design expectations
    What will it be used for?
    Laboratory grade or at the other end....battery charger?
    linear or switcher?
    What about control tolerances?
    In designing a power supply the voltage is usually not the problem the current IS.
     
  13. Coogrrr

    Thread Starter New Member

    Jul 16, 2008
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    Well my thought was to get this considerable amount of brain power here at AAC chatting back and forth until we finished our first design and looked upon our new baby with a smile.

    The baby would be a bench top unit with the MAX voltage and MAX amperage that we could affordably do from a 120vac 15amp start point. I think that we all develop power cicuits for our equipment and something like this device is something we all want if we dont already have one (myself included) and that it could cetainly go further than the retail market that gouges us goes.

    I think we could easily create a nice Lab style circuit that provides good control from 0 - ??? volts and 0 - ??? amps. These manufacturers certainly dont hold some super magical lease on design that cummatively we dont all have.

    The desire to make it so powerful is that when someone copies it they can sit around an Electrolyze things (high V low A) or they could run a motor for a bot (lower voltage high amps) and anything in between. So rather than build one supply 0-3amps and 0-40 volts and then a second supply 0-30amp and 0-12volts we could combine the 2 and end up with the best of both worlds. This assumes it could dim the lights and need a car radiator to cool when maxed output is utilized.

    But I think we can do this cooperatively and that we could make better than the best thing I can find for 800 bux and we can do it cheaper.



    Yes I am looking at 3 computer power supplies and the various outputs that they hooked in series could provide but the result is still not adjustable and the amps can run away if a project is capable of drawing as much as it wants. Some projects need to be tested with a set amount of amps to see if the circuitry will work within those tolerances (think Apollo mission here) many projects need tight tolerances and I think we could make a good circuit that would provide all of this.

    I want low ripple
    max control
    calibrated
    high current and voltage
    meters

    Optional
    multiple outputs (maybe a +5 and +12 static)
    some testing circuitry like OHM meter, diode, resistor,transistor testing (like a multimeter can maybe mate a meter into the project box)

    I think this would be much fun and the result is worth the effort for us all.

    Coog
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Well, I suggest that trying to build too much into it will be more of a "run amok" project.

    NICE functions like being able to select constant voltage or current mode, quantity, and limits - either via front panel or digitally - would be a big plus. That implies uC control for handling I/O; displays, monitoring, etc, possibly reporting error conditions or levels to external devices/computers.

    In the "good old days", one would use a GPIB bus for such things. That's pretty much overkill for a home workbench application. USB 2.0 seems to be all the rage nowadays, and there are a number of uC's that can speak USB. Might come in mighty handy, if you were to program up a semiconductor tester, etc. Remember the old curve tracers?
     
  15. thingmaker3

    Retired Moderator

    May 16, 2005
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    I humbly submit that the proper starting point for a project includes specifications somewhat more clearly defined. "???" is a rather ambiguous number.
     
  16. Pich

    Active Member

    Mar 11, 2008
    119
    4
    So we need to start somewere, I like the idea of using PC power supplys much more cost effective than buying a transformer. When connected in series each individual power supply can be tapped according to the output voltage need this will allow less heat disipation. The PC power supply will introduce a little more noise to the output (switching spike) but it should do the job. The circuit will need to be on a board. Large heat sinks and fan will be needed. I have a few circuit I can share to see if that is what we a looking for.
     
  17. thingmaker3

    Retired Moderator

    May 16, 2005
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    I still like the idea of sourcing more current at the lower voltages. Is there perchance any means of switching the switches? Rephrased: might there be any way to parallel the 'puter supplies for lower voltages and higher current, then series them for higher voltage with lower current limit?
     
  18. Pich

    Active Member

    Mar 11, 2008
    119
    4
    Yes both can be done automatically or toggle switch, I think the newer PC power supplys can output upwards of 30 amps at 12 volts The only issue will be the pass transistors handling the current, need lots of them in parallel.
    Here is a circuit I have put together this is only the control part of the power supply. It has excellent Voltage and current control and can be used as a current source.
    I've not got the pass transistors done yet, again it depends on the output current and complexity of circuit.

    Hopefully the drawing is legible
     
  19. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    If you want to use 4 PC supplies to get 48v, there's the catch of how to make the voltage adjustable (other than selecting 12/24/36/48v).

    50A seems to be really overkill. What kind of load can be connected that would draw such an amount of current?
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    The 250W ATX PSU I have sitting here puts out:
    +5V@25A
    +3.3@14A
    +12@8A
    -5@0.5A
    -12@0.8A

    I think you'll find that newer supplies will have more emphasis on 3.3v and lower. The trend seems to be going lower voltage. Circuits generate less electrical noise that way.

    As things are with my supply, I'd have to stack @ 5v to kingdom come to get decent current. Lessee, 250W supply, stack 10 to get 50v@20A, I'm up to 2,500W and only getting 20A out. Oh well... ;)
     
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