Need to build a adjustable dc current circuit

Discussion in 'The Projects Forum' started by deere_x475guy, Aug 16, 2010.

  1. deere_x475guy

    Thread Starter New Member

    Aug 16, 2010
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    0
    Hello everyone,
    First off I am a total novice with electronic circuits. I did some googleing and came up with a data sheet for the LM 338 regulator. On page 12 of this data sheet I believe I have found the circuit that may work for me. The mains power will come from this power supply:

    http://onlineshop.murrelektronik.co...turning=10&queryp=85088&search.x=0&search.y=0

    The link above is to a power supply that is rated at 24-28 volts and 20 amps.

    I am anodizing small parts to compliment my home shop machining hobby and currently am using a variac and a full bridge rectifier. I monitor my amperage constantly and adjust the variac voltage till my amps are inline with what the calculation say they need to be maintained at. The variac is almost 80 lbs and a real killer to move. A constant current power supply is what is normally used to do the anodizing. The range is amperage I am adjusting for is anywhere from .5 amps up to 3.85 amps. The volts just float where they need to be. Usually anywhere from 10.5 volts to 16 volts.

    I guess I have a couple of questions. With my usage in mind and the supplied volts that I am going to feed and the volts I am going to use do you think this circuit will work? I have sourced all the components locally here in the Lansing Michigan area. I also have no clue what to do with the part of the curcuit that is coming out of the LM 117 so I could use some help that...in addition I think I read in one of the other posts on this board that I could use a LM 317 in place of the LM 117. I have read through a bunch of post this evening and some of the touch on this curcuit but I would like to see if I can narrow things down a bit in this thread.

    Oh...I would like to add that I am hoping this curcuit will allow me to adjust the amps to what ever number the calculation calls for and it will stay there for the hour or 90 minutes needed.

    TIA all...

    Bob
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Suppose you have a load condition of 3.85A at 16V. Assume the selected input power supply terminal voltage is 24V. Using a linear control method (LM317 reg say) the regulator will have to dissipate 8x3.85= ~31 Watts which will require a substantial heatsink. The worst case condition might correspond to a load of 3.85A at 10.5V (?) and an input DC of 28V - in which case you would have to dissipate ~67 Watts across the series regulator. That's clearly a lot of heat. So the regulator heat sinking requirements may be significant.

    You could consider a more efficient switching regulator solution but the DC current ripple may need to be controlled to a value that doesn't impact the anodizing process - if that's potentially a problem. A switching regulator solution is more complex than the linear approach but the undesirable heat losses would be far less significant.

    Whatever current control method you use (linear or switch mode) it will no doubt be more practical than the massive variac you are currently using.
     
  3. deere_x475guy

    Thread Starter New Member

    Aug 16, 2010
    4
    0
    Thanks t-n-k,
    Clearly, if I am going to pursue this circuit I am going to have to find a powersupply with less voltage output. Given the circumstances that I have described for my Anodizing do you think I could use perhaps a PC power supply to power this circuit? Also, as I mentioned in my first post, this needs to be a constant current that will hold to the value I adjust it to using the pot. If it will not hold this current then I need to abandon this circuit and find one that will.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    A transistor current regulator can handle that range of current, and you can get power supplies relatively cheap. There is a 24VDC 6.5A going for $15.

    The actual transistor section is really simple, something similar to what drives LEDs. Some LEDs can take as much as 3A or more.

    Some examples from a local internet electronics seller...

    This one:
    http://www.bgmicro.com/PWR1278.aspx

    http://www.bgmicro.com/pwr1283.aspx

    http://www.bgmicro.com/12v7aregulatedsupply.aspx

    Here is an electronic load concept that is similar in some ways.

    [​IMG]

    If you're interested in pursuing this I can sketch up something closer to your needs.
     
  5. deere_x475guy

    Thread Starter New Member

    Aug 16, 2010
    4
    0
    Hi Bill, thanks for taking the time to reply. I already have the power supply that I linked to in my first post. Will your circuit work with this? Also I don't really understand your circuit. Will it allow me to adjust the current between .5A and 5A and have it hold that setting for the time the part is being anodized. This process takes anywhere from 1 to 3 hours.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hi Bob,
    I grew up in E. Lansing. Used to do some canoeing in the Eaton Rapids area.

    While you could accomplish the current limiting with a linear regulator, you will be dissipating a lot of power in the regulator itself.

    I've read that you want anywhere from 0.5A to 3.85A to 5A constant current. Can you nail down the max current a bit closer, and what your maximum desired ripple current is?

    There are actually some fairly easy circuits that you can build using readily available switching regulators, an inductor or two, and a few supporting parts (resistors, caps, etc.) It will be somewhat more complex than a linear regulator circuit, but it will be vastly more efficient than a linear regulator, and the ripple current can be kept quite low if desired.
     
  7. deere_x475guy

    Thread Starter New Member

    Aug 16, 2010
    4
    0
    SgtWookie small world isn't it. My wife and I are from the West Coast....of Michigan...:D(Allegan area)...We moved here with a job transfer about 13 years ago. I really like the lifestyle in ER...very laid back and out of the city.

    Typically the anodizing process that I am doing now has been using .5A to 3.85A. However I guess it could go as high as 5A in the future. As far as current ripple I guess I have to admit up front that I am not sure what you mean....I think you are talking about the how flat I would like the wave form to be...sorry I need to pick up on the lingo...I don't think with anodizing that the high and low is of to much importance. I am not married to the circuit I posted about. I discovered it while searching the internet for what I needed so I am open to your recommendations. Like I have posted the main criteria is that the current stays constant for the whole time the part is being anodized and I would like to use the power supply that I posted the link to if possible. Oh..and yes the simpler the circuit..the better...just a newbie here.
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Wouldn't it be easier to use a "light dimmer" type circuit on the input of what you've already got? They make them to drive inductive loads such as fan motors.

    This is where the fun enters, however it shouldn't be hard to incorporate some sort of opto-triac in the "light dimmer" control circuitry to provide an isolated feedback path of the current being drawn.
     
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