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Building a ~15V 70A VCCS

Discussion in 'Electronic Design' started by Andy Neillans, Oct 21, 2005.

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  1. Hi all,

    I'm looking at building a Voltage Controlled Current Source; approx 15V
    output, with up to 70A available - I need to be able to control the
    current.

    I've found this schematic (http://www.neillans.co.uk/VCCS1.pdf), and if
    I were to uprate all the components (e.g. from 1/4W resistors to 1W or
    higher), and replaced the IRFZ48Vs with the IRFZ48Ns which have a
    higher current load, would it work?
    I tried to build + simulate this circuit in EWB, but it didn't work out
    - but that could be down to my VHDL :)

    I've been working with electronics etc for years, but never ventured
    far from digital solutions, although I have knocked up power supplies
    in the past, they have never been VCCS.

    Please Help :)

    Andy
     
  2. Phil Allison

    Phil Allison Guest

    "Andy Neillans"

    ** Ummmm.


    15 volts at 70 amps = 1050 watts of dissipation in the mosfet bank, worst
    case.

    I reckon that might take 35 of them, 2 amps apiece on a whopper heatsink.

    The PSU will be a treat as well.



    .......... Phil
     
  3. Correct; planning on building it up as multiple large heatsinks, which
    are actually copper water cooled.

    The PSU will usually only run at around 12 - 15V @ 40 - 50A (so peek of
    750W), although I need the extra capacity to be available. I'm also
    planning on expanding the system as time goes on with additional units
    to increase the current availability.

    The PSU's are to be used for electroplating aluminium at the company I
    work for, and they will be expected to run for around 8 hours a day
    under the above load (another reason why I'm over-specing).

    Andy
     
  4. Ken Smith

    Ken Smith Guest

    Since you didn't say you needed more than one nor how good it really has
    to be:

    How about just buying an off line switcher power supply that has a
    remotely programmed current limit and running it at the current limit all
    the time? I'd look for a product like:

    http://www.xantrex.com/web/id/79/p/1/pt/20/product.asp
     
  5. Rich Grise

    Rich Grise Guest

    You should look around for a "carbon pile". It's just a big huge
    honking power resistor that you adjust by cranking down on a
    pressure screw - one of those should have no problem handling
    only 70 amps. As long as your supply is stable, you should be
    able to set it and forget it.

    Plus, I'd have to have this corroborated by somebody who knows
    their elbow from a SMPS - but would PWM control work for plating?

    Thanks,
    Rich
     
  6. Hi all,

    Thanks for the advice so far - its appreciated :)
    The schematic is getting there - just one last question.

    I've come across a lot of advice to put a resistor and capacitor in
    parallel to the tansformers primary coils in order to help regulate the
    fluctuations between the mulitple transformers - however, how do you
    calculate the required values?

    I'm going to be using 4 x 500VA tordial transformers, with 240V primary
    windings, with two secondary windings of 25V @ 10A. The secondaries
    will be connected in parallel to provide the required amperage, and
    there will be two 56000mF electrolytic capacitors to smooth the output
    connected in parallel. The primaries will be connected in parallel to
    the same 240V A/C source (suitably fused etc).

    Any advice on the method to calculate the required resistors + caps on
    the primary windings ?

    Andy
     
  7. Phil Allison

    Phil Allison Guest

    "Andy Neillans"
    ** Name the source of that advice.

    I could do with a good belly laugh.


    ** Better fuse each tranny separately.


    ** Someone is having a "lend" of you.

    The scary bit is that you are so gullible you believe it.

    Transformer theory a bit shaky is it ??



    .......... Phil
     
  8. Hi Phil,
    I thought it was a bit insane too, but hey, I've never parallelled
    transformers, so ....
    Already am :)
    Very - see above :) Thought I'd post, just to make sure though, as I
    was planning on leaving them out. Just didn't want to possibly risk
    damaging the tordials.

    Cheers for replying tho :)

    Andy
     
  9. John Larkin

    John Larkin Guest

    Does plating need filtered DC? You might consider a big transformer
    driving a packaged SCR bridge rectifier puck thingoe, with a primary
    circuit breaker and a huge output series resistor as the ultimate
    protection/current limit. That would be simple, low loss, and
    practically indestructable.

    The multiple-fet thing could turn out to be an immense headache for
    the relatively inexperienced... envision coffee cups full of exploded
    fets and plating lines down for days at a time.

    Best yet, buy a commercial plating supply.

    John
     
  10. Don Foreman

    Don Foreman Guest

    I've read that chrome works best with low ripple, though many
    commercial plating supplys employ no filtering at all.
    Pricey! Absurdly so, for what they contain.
     
  11. John Larkin

    John Larkin Guest


    How much would a plating supply like that cost?

    It's really hard to design your own gear and come out ahead of a
    commercial product. If it's expensive, there's usually a reason.

    I wouldn't build my own DVM, oscilloscope, TV, or bench power
    supply... It's just not worth my time, usually by a huge factor.

    We see a lot of posts here by people, usually beginners, who think
    they can build one-piece electronics cheaper than they can buy it. I
    think that's seldom true.

    John
     
  12. Does plating need filtered DC?
    We plate a lot of different finishes, including chrome but also basic
    dyed anodisation - the best results always come from a fairly ripple
    free supply - any ripple changes the thickness of the layer, and
    although we are talking microns, it can make a massive impact on the
    end result.
    I did get a quote for a unit for this. And it was over a thousand
    pounds. With a 5 month lead time.
    The cost could be bearable, but the lead time certainly is not. And we
    wouldn't have as much control over the system - it could end up that we
    add additional production lines, tanks etc, so need more PSU's - rather
    build them in house instead of coughing up large sums and waiting an
    eternity.

    Andy
     
  13. Don Foreman

    Don Foreman Guest

    See
    http://www.caswellplating.com/kits/rect.htm

    Start at $1686 because the slope front jobs have no filtering at all;
    they're just xfmrs, variacs and rectifiers.

    None of these are voltage controlled AFAIK. They're all adjusted by
    turning a knob.

    Here is a suitable transformer for $110:
    http://tinyurl.com/8o8ur

    Add rectifiers for maybe another $100 (being generous)
    http://tinyurl.com/cdhtq

    The brute-force water-cooled MOSFET regulator would do the rest.
    IRF1404 is rated at 162 amps at 40 volts, $3.80 ea from Digi-Key. .
    Thermal considerations would require using several of these in
    parallel so figure 30 to 50 bux worth of MOSFET's. We're now up to
    $250 -- time spent screwing around being free, of course. Control
    cctry would be trivial, perhaps another $25. The OP already
    contemplates the humongous heatsink.

    Oh, I forgot filter caps. Threephase would be a lot better here
    because then you could skip the filter caps though you'd need more
    (but smaller) transformers and more rectifiers.

    Figure at least 100,000 uF of capacitor here. 68,000 uF at 25 WVDC
    are $21.43 ea at Digi-Key. I'd go with several 22,000 uF 40 WVDC
    caps at $18.77 ea. to handle the ripple current better. 10 of 'em
    (220,000 uF total) are about $175. Total now is still <$500.

    Brute force approach, you'll need a hawg of a current sense resistor,
    lots of ways to do that.

    This unit would be voltage-controllable from 0 to 70 amps, it's
    output ripple would be milliamps and it would be immune to short
    circuits.
     
  14. John Larkin

    John Larkin Guest

    That's a TO-220 rated at 202 amps DC, 333 watts dissipation. If the OP
    (or anybody else) believes these numbers, he's in for some serious
    pain.

    Note the derating factor of 2.2 watts/degC. Then note the thermal
    resistances. Then read the footnotes. Why does IR *do* this? Do they
    want their fets to explode?

    TO-247's, directly bolted to a water-cooled sink with silicone grease,
    no insulator, not too close together, might prudently be used at 100
    wats each or so.

    No packaging, no heatsink/plumbing, no switches/fuses/breakers, no
    loss-of-water cutout, no meters, no labor cost.
    Just apply time and money.
    It would be a serious project. SCRs would be simpler and a lot safer.
    There are cheap trigger boards around, packaged SCR bridges, stuff
    like that. 3-phase would be nice... no filters.

    John
     
  15. Don Foreman

    Don Foreman Guest

    Right! That's why I recommended using 10 of them.
    I stipulated that heatsink-related issues were "given" per OP's post,
    and that cost of labor was not included. I think that's the OP's
    intent: to use available in-house labor to reduce cashflow cost.
    Fooey! Please say more about how SCR's would be safer -- or simpler
    for that matter. I don't think so. Please show sources and prices
    for the cheap trigger boards and packaged SCR bridges that you
    mention -- and how these might be assembled to meet the OP's request.

    Even with 3 phase excitation there's no way they'll strip ripple like
    a series-pass bank of MOSFET's would. That flat isn't on with SCR's.

    This is not a "serious project" if the OP can do the watercooled
    heatsink he said he could. Please show how doing it with SCR's
    would be simpler, even with significantly inferior performance.
     
  16. Don Foreman

    Don Foreman Guest

    Andy, where are you located? I'll be in London soon, would a visit
    be easy and possible? I'm a retired EE and not looking for work,
    slug lazy and proud of it -- but I'd like to see your plating op
    because I'm interested in plating. I do a bit of that, just small
    stuff in my garage, no chrome. I'll be in London to visit with
    new granddaughter -- but I can only take so much baby stuff before I
    start getting antsy. I'd love an excuse to hop on the tube or a
    train for a day if it's not too heroic an adventure.
     
  17. John Larkin

    John Larkin Guest

    You want a design? I'll need a purchase order. If you can build these
    for $500 each, and the market price is 3x as much, you should go into
    the business.


    John
     
  18. Chris Jones

    Chris Jones Guest

    I've made a couple of VCCS type plating power supplies capable of reverse
    pulse waveforms (or other waveforms if you wanted...), but none as big as
    what you're suggesting. The first one I did used a TDA2030 audio op-amp
    (very cheap opamp, puts out at least an amp but has large DC offset, and
    not unity gain stable). The circuit ran off split supplies, probably about
    +/-10V. I connected the supply pins of the op-amp to the bases of 2N3055
    and 2N2955 transistors to boost the output current, so when the op-amp
    would start to struggle with the output current, it gets a bit of 'help'
    from the power transistor. I think it would do about 10 Amps comfortably
    but I can't remember and didn't need to push it too far anyway because the
    object being plated was small. (The emitter of the PNP 2N2955 goes to the
    positive supply, the collector goes to the op-amp output, the positive
    supply pin of the TDA2030 goes to the base of the 2N2955, and there is a
    resistor maybe 10 Ohms from the base to emitter. Same for the NPN 2N3055,
    but the emitter goes to the negative rail and the base goes to the negative
    supply pin of the TDA2030. The output of my boosted power op-amp went
    (through a small resistor or inductor to help stability I think...) to one
    electrode of the plating tank, and the other electrode of the plating tank
    went through a current sensing resistor to the common of the power supply.
    I say common rather than ground, because I think I actually floated the
    power supply itself to allow one of the electrodes to be grounded if so
    desired. Of course the current sense resistor develops a voltage which is
    fed back to the inverting input of the TDA2030, and the non-inverting input
    is driven with the waveform which you want the current to follow, that
    waveform being with respect to the common of the power supply, (not
    necessarily ground). I think I used a 555 timer and a 4066 chip and
    several pots, to make the reverse pulse waveform. I think I needed some
    fancy things like Zobel networks to make it all stable, most of which I
    copied out of the TDA2030 datasheet (which contains only audio circuits),
    and tweaked by trial and error, with various test loads and looking for
    ringing on a scope.

    By the way, the TDA2030A and 2040 are probably better, I think one of them
    includes protection diodes to the supplies which are absent from the 2030,
    and the current rating might be better.

    The second plating supply was much fancier, it had 16 channels to drive 16
    independent plating outputs, and the current sources were proper ones which
    could drive a real ground referenced load - I used a current sensing
    resistor in series with the output pin of the power op-amp. I think each
    channel would drive 1Amp and it was all software controlled using DACs and
    a PIC microcontroller connected to a PC through a serial cable. If you're
    really interested I could try to find the old schematics, but it's in
    protel format so it'd take me a while to fix up a PC to convert it to
    anything else.

    Chris
     
  19. First off, just want to thank everyone for their input; been unwell, so
    haven't been online a lot recently (hence my dissappearance).


    Chris;

    Many thanks for sharing your experiences :)
    If you would be willing to share, I'd certainly like to take a look! I
    work a lot with the Atmel AVR Risc uC's at work, and have been drafting
    including one in the PSU - specifically to digitise the regulation and
    allow monitoring and control via our network (already have TCP/IP
    Stacks for the AVR, as well as code that makes use of the built in ADC
    - both from existing projects).


    Dan;
    Based a little bit further up the country these days (although until 8
    - 9 months ago, I lived just south of London!). We are in the Scottish
    Borders, about an hour south of Edinburgh. If you want to come up, let
    me know - am sure I could arrange for a visit! You never know, I might
    even have the new supply up and running..

    Andy
     
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