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Schematic for 0-2 Amps voltage regulated output

Discussion in 'Electronic Basics' started by Mickey, Mar 30, 2005.

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  1. Mickey

    Mickey Guest

    Does anyone have schematic for Output that is regulated with voltage (small
    up to 2 V) and
    otput is 0 - 2 Amps according to the input voltage?

    Mickey
     
  2. So you need a current source that goes from 0 to 2 amps, and is
    controlled by an input voltage of 0 to 2 volts, right?

    The typical way to do this is with an opamp. The current through a
    transistor is measured using a resistor, and that measurement is fed
    back to control the output.

    If you measure the output using a 1 ohm resistor, then the current
    through it has a 1 to 1 relation with the voltage across it. So, 0A to
    2A will create a voltage of 0V to 2V across the resistor.

    If you get an opamp, you can use it to control the output given the input.

    Now, it sounds like you want to have a ground referenced current source,
    meaning that your load will be between the source and ground. To do
    that, you need some way to measure the current coming out of the
    transistor (which is at some higher voltage) and compare that to your 0
    to 2 volt control voltage (which is referenced to ground). Thus, you
    need a way to 'level shift' the control voltage appropriately.
    Thankfully, an opamp can do this for you.

    Here is a schematic: (please view with a fixed-space font. Othewise, the
    lines don't line up)

    Vin=10V
    o---o------------------o----------o------.
    | | | |
    | | | |
    .-. | | .-.
    | | 100k | | | | 1 ohm
    | | | | | | 0.1%
    '-' | | '-'
    | 100k | | |
    | ___ | .----)------o
    | .-|___|--)--. | | |
    | | | | | | |
    | 100k | | | | |\| |
    CTRL | ___ | |\| | '--|-\ |<
    ---)--|___|--o------|-\ | | >---| PNP
    0-2V | | >-o-----|+/ |\ up to 2W
    o----------------|+/ |/| |
    | |/| | |
    | | | |
    .-. | | '----------o Out 0-2A
    | | 100k | |
    | | | |
    '-' | |
    | | |
    | | |
    o----o------------------o----------o-----------------o Out GND
    GND
    (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

    For this to work, the opamps will need to be 'rail-to-rail' input and
    output opamps. You can get these in ICs that contain two (dual packages).

    The first opamp takes your 0-2V input, and maps it to 20-18V. The second
    opamp takes that, and tries to make the voltage across the resistor
    equal to it by controlling the voltage on the PNP transistor's base.

    The voltage 'compliance' is defined as the voltage range the thing can
    produce. This will produce a voltage output between 0 and perhaps 7.8V.
    If you need more voltage, you can increase the Vin to something larger.
    It'll always have a voltage drop of about 2.2V at full current.

    Also, there is no limit on the current at 2A. Thus, your voltage source
    should limit the current it'll supply to 2A.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  3. Correction, from 10 to 8 V. Also, make sure the 1 ohm sense resistor is
    rated for at least 2.5W.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  4. Chris

    Chris Guest

    Hi, Mickey. The simplest way to have a voltage-controlled current
    source requires that your load can be floating. Given that, you can do
    the whole thing, including perfboard, with Radio Shack parts like this
    (view in fixed font or M$ Notepad):

    ' VCC
    ' +
    ' |Iout = 0-2A
    ' |---> +
    ' '------o
    ' VCC
    ' + .------o
    'Vin | | -
    '0-2V ___ |\| |
    ' o-|___|-o----o-------|+\ ___ |/
    ' 750 | | | >---o-|___|--| TIP120
    ' .-. |.1uF.--|-/ | 220 |>
    ' 220| | --- | |/| --- |
    ' | | --- | | --- |
    ' '-' | | | |.01uF |
    ' | | | === === |
    ' === === | GND GND |
    ' GND GND '--------------------o
    ' |
    ' .-.
    ' 0.22 ohm | |
    ' 5 Watt | |
    ' '-'
    ' |
    ' ===
    ' GND
    '
    (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

    Here's the drill. The 750 and 220 ohm resistors divide down your
    0-2VDC signal to about 0-0.44V (plus or minus -- use 1% resistors for
    better precision). The op amp (use 1/4 of an LM324 if you're doing the
    Radio Shack thing) is set up as a voltage follower with the darlington
    transistor in the feedback loop. The .01uF cap is in the feedback loop
    to help prevent oscillations. Your 0-0.44V signal is impressed across
    the 0.22 ohm power resistor, which translates to 0-2A through the
    resistor.

    This should work well with assembly on a perfboard. The power
    dissipation across the transistor is dependent on the power supply you
    choose. The above will work with a commonly available 5VDC regulated
    power supply. With a 5V supply, your transistor could dissipate as
    much as 9.5 watts for a short circuit load at 2 amps, so be sure to get
    the big TO-220 heat sink.

    Have fun, and good luck
    Chris
     
  5. Mickey

    Mickey Guest

    Thanks all.

    Best regards, Mickey
     
  6. Chris wrote...
    Fun with oscillation Mickey may well have with the 10nF capacitive
    load on the opamp, and good luck he'll need fixing it! But luck is
    here - I've fixed the cap connection in the drawing below. :>)

    ' + VCC
    ' | Iout = 0-2A
    ' | ---> +
    ' '------o
    ' VCC
    ' + .------o
    ' Vin | | -
    ' 0-2V ___ |\| |
    ' o-|___|-o----o-------|+\ ___ |/
    ' 750 | | | >--o--|___|--| TIP120
    ' .-. |.1uF.--|-/ | 220 |>
    ' 220| | --- | |/| --- |
    ' | | --- | | --- |
    ' '-' | | === | .01uF |
    ' | === | GND | ___ |
    ' | GND '--------o--|___|----o
    ' | 4.7k |
    ' | .-.
    ' | 0.22 ohm | |
    ' | 5 Watt | |
    ' | '-'
    ' |______________________________|
    ' |
    ' ===
    ' GND

    I've also shown the mandatory 4-terminal Kelvin connection that
    should be used with low-value resistors at high currents.
     
  7. Chris

    Chris Guest

    Thanks, Mr. Hill. I saved your earlier ASCII art, but couldn't find
    it. While batting it out from memory, I munged it (ouch! -- not paying
    enough attention, I guess).

    Here's from Mr. Hill's earlier post on s.e.d.:

    .. Rload=8ohm Lload
    .. 12V |---------/\/\/\------88888-------.
    .. |
    .. 5V |
    .. --- .1uF |
    .. |---| |---. |
    .. | | +------.
    .. | /// | |
    .. 10k |\| | |
    ..---/\/\/\--+------|+\ 220 C |
    .. | | >--+--/\/\/\--- B _|_/
    .. | .--|-/ | E /_\
    .. --- | |/| --- | |
    .. .1uF --- | | --- 0.01uF +------'
    .. | | /// | |
    .. | | | 10k |
    .. | '--------+---/\/\/\-----+
    .. | |
    .. | \
    .. | / 0.2 ohm
    .. | \ 5W
    .. | /
    .. | |
    .. '---------------------------+
    .. |
    .. ///

    (Zener added for inductive load)

    Thanks again.
    Chris
     
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