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constant current source problems

Discussion in 'Electronic Design' started by john, Oct 23, 2006.

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  1. john

    john Guest

    Hi,

    Please go the following link and then figure 16 page # 13.
    http://focus.ti.com/lit/ds/symlink/ina2133.pdf

    I can not find a way to post the schematic on the web so i am
    explaining that I what did I do with the circuit. The INA133 is getting
    the input from a voltage ouput DAC ( span is +/- 2.5 volts) . DAC is
    connected to pin# 3 of the INA133 and pin#2 in grounded. A capacitor of
    47uf and is connected between the non-inverting pin of the OPA131 and a
    relay1 ( which connects the output of the capacitor to two different
    loads ). The capacitor provides AC coupling. The relay1 connects the
    current source to one laod at one time. The power supply of the circuit
    is +/- 16 volts. Inorder to discharge the capacitor, I connected a
    relay2 in series with a resistor at the input of the capacitor or at
    the node where the OPA131 and the capacitor connects together. So, when
    the capacitor gets charged I just turn on the relay2 and connect the
    node to zero volts.

    Problems:
    1. My application needs to switch between the loads, So when I turn on
    the circuit, the circuit is connected to the load A, I get AC signal
    across my load with no DC offset. But when I switch to load B then I
    get DC offset with AC signal across the second load. I do not know why,
    may be the current source does not like to be floating and it hits the
    maximum power supplies during the switching time from one load to
    another load? Is it true or is there some other explanation.

    2. Am I discharging the capacitor in a rigth way. I a resistor
    necessary to disharge it or I can just short it with the ground. The
    capacitor charges upto 14 volts.

    Thanks
    John
     
  2. Tim Wescott

    Tim Wescott Guest

    If I'm interpreting your description correctly, the only way that a load
    can get current is through the capacitor -- correct? If that is so,
    then there are only two possibilities for getting a DC current into a
    load: One, you're switching things around fast enough that an induced
    bias on the capacitor gets dumped into load B, and two, your capacitor
    isn't working like a capacitor.

    You can test the first theory by just holding the output on load B for a
    while, and see if the bias dies out.

    The second theory would easily prove correct if your capacitor is
    electrolytic and you're running it backwards. Electrolytics are polar
    devices, and act like crappy diodes when you use them backwards -- could
    this be your difficulty?
    Define "right". I would connect the resistor across the cap, rather
    than grounding one end of the thing, and I would give it sufficient time
    to discharge (5x RC at least, more if you need a great deal of
    precision). Shorting the cap directly is manly, but hard on both cap
    and relay.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  3. john

    john Guest

    Hi,
    Thanks very much for ur reply

    "Define "right". I would connect the resistor across the cap, rather
    than grounding one end of the thing, and I would give it sufficient
    time
    to discharge (5x RC at least, more if you need a great deal of
    precision). Shorting the cap directly is manly, but hard on both cap
    and relay".

    If I connect the resistor across the capacitor then the current ( DC)
    can flow into the load. My application can not afford it.
    I am not using the electrolytic capacitor. I am using the polarity less
    capacitor. If I discharge the capacitor and during the disharging I
    just switch the load, I think then the load B will not get any thing.
    What do u advice?

    regards
    John
     
  4. john

    john Guest

    Hello,

    1. What if I just turn off and on the +/- 16 volt supply of the INA2133
    by using two relays simulataneously. Now the DAC is ON ( but generating
    only zero at the output ). Will this scheme hurt the INA2133 OR OPA131
    in any way? If not then I think this will solve the problem. Turn OFF
    the power supply, swith the relay to load B and then turn ON the power
    supply again?

    2. If the constant current source have no load then how does it behave.
    Does it output hits the rail voltage?

    Please advice
    John
     
  5. Forget it. Does not exist two relays having the same response times so
    depending on sensitivity of your cirquit to supply unballance
    (momentary!) you are for nice head scratching.
    Do everything KISS.

    Stanislaw
     
  6. john

    john Guest

    Hi,

    anybody else can advice me.

    John
     
  7. John Fields

    John Fields Guest

    ---
    Yes. It has to.

    Consider: If the source is supposed to pump current into a load by
    increasing the voltage across the load until the desired current is
    in the load then, if the load's resistance is so high that the
    source's voltage can't push that current through the load, the
    source's output voltage will go to the rail(s) in its attempt to
    supply the required current into the load.
     
  8. john

    john Guest

    Hi ,

    Thanks for the reply. Would you please advice me on the point#1 that if
    I trun OFF or ON the power supply. Will it hurt the INA 133 in any way?

    Thanks
    John
     
  9. john

    john Guest

     
  10. John Fields

    John Fields Guest

     
  11. john

    john Guest

    Hi,

    Anybody who knows!

    John
     
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