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Diode reverse protection and current limiting.

Discussion in 'Electronic Design' started by Ignoramus7040, Nov 25, 2006.

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  1. Thanks to many people regarding your earlier advise on modifying my
    TIG welder. The sequel to that saga is that the welder works and I use
    it quite often.

    Now I have a smaller modification to ponder.

    After several unpleasant experiences with battery chargers (which did
    not work) I want to try something different. My typical use of a
    battery charger is to use it to replenish some battery that does a lot
    of engine cranking.

    I have a power supply that I am thinking should become my battery
    charger. It is a PP-1104B/G. If you google for PP-1104C/G you will
    find a description of a very similar power supply. It has a dual
    range: 13-19 volts at up to 100 amps, and 26-39 volts up to 50
    amps. It is a constant voltage power supply with no current limiting
    other than built in breakers. I added a few not very relevalt things
    to it (like handles for easier handling), but it has a few electrical
    quirks that I would like corrected.

    The first is that when it is turned off, it drains the battery (ie the
    current flows "backwards" as opposed to current flow when charging)
    and quickly consumes a lot of amps. It appears that adding a diode
    with a small heatsink should take care of the issue.

    My calculation is that at 100A, the diode with voltage drop of 0.7
    volts would produce 70 watts, which is trivial to dissipate. I have a
    number of suitable diodes and heatsinks. So, unless someone gives some
    great reason why adding a reverse protection diode is a bad idea, I
    will do it.

    My second question is whether there is an easy way to add a current
    limiting feature to this device, that is, make current limited to a
    value set by a potentiometer. That would make it valuable for, say,

    I read about current limiting today. The ones I read about were based
    on the principle that there is a transistor in series with the load,
    that basically acts as a variable resistor to limit current. That's
    wasteful, but I can live with it, but perhaps there are more clever
    schematics of a current limiting add-on. There is no space inside that
    power supply to add anything, so any such circuitry would be in an
    external device.

    Any suggestions on more inteligent current limiting add-ons will be
    gratefully appreciated.

  2. How is the voltage varied? Why don't the rectifiers prevent back current
  3. It is varied by turning a beefy knob that probably switches
    transformer windings.

    Second question is a good one, my guess is that there are capacitors
    and a big safety discharge resistor that play a role.

  4. From what I can make of the manual, yes, voltage taps. However there is a
    type of magnetic amplifier which helps keep the voltage constant under load.
    This is a fairly complicated gadget. I'd be inclined to add an external
    'box' to do what you want rather than tinker with it. A heat sinked diode
    should work fine re back flow. For constant current, maybe a big
    transistor - depends on the max current you want. You could use SCR's and a
    commutation circuit for more amps, but I'd look for an old GE SCR manual if
    you want to do that. Or. google for (scr commutating dc). (page 2) shows a typical
  5. That would be my own approach, as well.
    Yep. I just added a beefy 140A diode on a beefy 1/4" by 2" by 6"
    copper bar, right to the + terminal, it works great (as it
    should). Had to tap that bar to 3/8-24 NF thread. In testing of this,
    the voltage drop varied from 0.78 to about 0.9v or so depending on
    (light) load. Could not find my heatsink paste, I need to buy some or
    find what I have.

    So, I would say, my first problem of battery discharging through this
    power supply when it is turned off, has been SOLVED.
    I would say 50A max (so that I do not exceed 15 amp single phase power
    draw to not blow my house breakers), but it should be adjustable by a pot.

    I do have two big transistors, not sure if I could use them or not.
    Do you mean something as pictured in Figure 6? What would be the
    point, to turn current on and off like that? Or are you suggesting
    high frequency switching? I would rather regulate it, that is my
    first, unstudied thought, than switch on and off? I apologize if
    my comment is off base.
  6. The point is to handle large currents without large heat losses. SCRs were
    used for such tasks before large transistors or FETs were common. One use
    was as a speed control for a battery golf cart. Basically you fire the first
    SCR to turn the current on. At a certain point you fire another SCR across
    it with a capacitor in series. This shunts the first SCR which turns off and
    the capacitor then charges and turns the second SCR off. Then repeat.

    A more modern design might have transistors cycling on and off to control
    the current with lower heat losses than remaining on. Basically this is a
    form of switch mode power supply, perhaps without the transformer and
    Schottky diodes.

    As I say, it depends on how much current you want to use for electro
    plating. 50 A at 12 V drop (say) is mighty hot.
  7. Guest

    This is the kind of thing I just buy. I have a lot of gel cells I use
    for power in the field. I use
    They have other smart chargers at

    The problem with charging batteries is you can't sit there and watch
    the damn thing charge. Thus you need protection circuitry in the
    charger. I suppose you can spend you time designing such circuits.
    Generally I only build what doesn't exist off the shelf. I can tell you
    the Japlar Schauer I bought is a good product.

    Getting back to your situation. Lead acid batteries tend to self limit.
    I don't have my Gates Energy book handy, but I think the limit is
    generally around 25% of capacity. This means your 100A charger is set
    up for a 400AH battery. I am presuming your battery isn't 400AH, so you
    really should current limit the power supply just to be on the safe

    Seriously consider buying an off the shelf product.

  8. A 100 amp contactor in series with the output would be the simplest
    way to go. Wire the coil across the transformer primary to turn it on,
    when the supply is on.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  9. OK, I see now. 50 A at 12V is not that much -- it is just 500
    watts. Nothing that a 10 inch long heatsink could not handle. I have a
    heatsink with 2 transistors assembly, would be nice to try to make it
    into a variable voltage limited and current limited supply using them.

  10. Hm, thanks. That's interesting. If you vouch for them, that means a
    lot to me. The "smart chargers" that I tried were basically all very
    poorly programmed, had a lot of software bugs, and would not charge my
    deep cycle batteries correctly and would "hang" while continuing to
    charge etc. (that is, they would continuously display 88% charge
    whereas the battery was at 15V or something).
    My batteries are about 180 AH rated.

    Yes, so this comes back to current limiting discussion.
    So, let me ask you then, the charger that you are describing, charges
    all sorts of batteries well and does not hang or misbehave in any way?

    I have a 1.5v smart trickle charger/battery maintainer, these ones
    work VERY well, but they charge too slowly when I need to charge a
    battery quickly.

    Oddly enough, when I clicked on enlarge image, it showed a 24v
    charger, but I suppose it is their mistake and that's all.

  11. Mike, I put in a diode yesterday, it works just fine. I just need to
    find heatsink paste, I misplaced it. Otherwise it is great.

  12. John Fields

    John Fields Guest

    What you've described isn't a constant-current supply, but rather
    one where the _average_ current into the bath can be controlled by
    varying the duty cycle of the waveform into the tank. Still, for
    plating that should be adequate.

    The controller can take the form of either a fixed cycle time,
    variable-width pulse or a fixed-width pulse, variable cycle time

    In either case the current into the bath would have to be measured
    and then either the pulse width or the cycle time adjusted in order
    to get the right number of electrons into and out of the bath in the
    allotted time.

    I'd go the fixed pulse width, variable cycle time route myself
    because it would be easier to figure out how long to wait to pump
    another load of charge through the bath after I knew how much I'd
    just pumped through it than trying to figure out how much to pump
    through it on the fly.

    Google "pulse electroplating" (no quotes) for a lot of folks who
    have already done it.
  13. I believe this can be jumpered for 14 V or 28 V.
  14. Chris Jones

    Chris Jones Guest

    If you are thinking of any kind of switching regulator, and given that you
    have low voltage DC already, I would definitely avoid SCRs since they
    always drop quite a lot of voltage and are difficult to turn off. I would
    either use MOSFETs in a switching constant current source, or get a big
    heatsink with a fan, and build a linear regulator with either NPN bipolar
    transistors or MOSFETs. The switching regulator would be more intersting
    and more efficient, and an inductor to handle 50A DC could probably be
    scavenged / built up from some computer power supply parts.

  15. Chris, you know me. I am basically looking to see what would it entail
    to make the cheapest/easiest solution to making a more versatile power
    supply out of this PP-1104B/G. If the power converter is lossy, I
    think that in the end it is not a huge consideration.

    Is it true that a linear transistor based regulator is basically an
    extremely simple solution?

  16. Chris, would I be able to make a linear voltage and current regulator
    with these transistors

    I have two, conveniently mounted on a heatsink. Would be great to put
    them to some use. Ignore the for sale verbiage, whatever I sold was
    sold a long time ago. That heatsink also features a 150A current shunt
    and some SCRs, which I do not thinkI would need fora linear regulator.

  17. One would give you up to 30 A assuming dissipation was not a limit. You need
    to drop enough voltage to make the regulation work, but not so much as to
    create excessive dissipation.
  18. Actually one would give me 300A, not 30A. (I just double checked). My
    question is, are they a suitable type transistor for both voltage and
    current regulation with pots.

    Ie can use them to I make a device to convert incoming 14VDC with no
    current limit, to a power source with voltage up to some limit below
    14 VDC and with current up to some limit below 50A. (the limits being

  19. I was going by the speciication on the auction document..
    Seems doable.
  20. Guest

    15V is way too high. I don't know if that is dangerous, but it probably
    damaged the battery. The smart charger I bought uses a bi-color LED to
    indicate the charging.
    Well, that is a serious battery. I would contact the company at the
    link I provided and see if you can agree on an acceptance criteria for
    whatever charger you determine fits your need.
    I have charged a very narrow range of 12V batteries, 50AH to 70AH. Thus
    far, no problem. These are not deep discharge, but SLA.

    My issue with home brew battery charging is the fecal matter can really
    hit the fan if you screw up. Nothing like burning down the house to
    save $50 on a commercial charger. When I was designing chargers, I'd
    run the charger in an environmental oven, just in case something bad
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