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Voltage Scaling

Discussion in 'Electronic Basics' started by [email protected], Mar 22, 2008.

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  1. Guest

    Hi all,

    Can anyone help with the following,

    I need to scale a control voltage which ranges from 0VDC to 5VDC, to:

    1. 0V..15V (e.g. when Vcoltrol=2.5V, Vout should be 7.5V).

    and

    2. -3V to 3V (e.g. when Vcontrol=0V, Vout is -3V, when Vcontrol=2.5V,
    Vout is 0V.. essentially a linear scaling..)

    I have a power supply of +9V and nothing else (got suggestions to
    connect +/-18V to an opamp but obviously, i dont have these voltage
    ranges from the wall-wart, only +9V..)

    How would you go about doing this?

    Thanks,
    Gilad.
     
  2. Jamie

    Jamie Guest

    how about a simple OP-AMP using + input with a gain ratio of 3 network
    on the (-) input?
    and for the voltage supply issue.. You'll need a DC-DC converter to
    up the level.
    I don't know where your level of expertise is and how advanced you
    want to make this but, if you're looking for a weekend hack job. You
    could construct an inverter using a mini xformer like what radio shaft
    sells in the audio output line with a CT on it. You can drive the
    secondary side with something like a 555 timer chip and derive a +/-
    supply that is actually isolated if you wish on the other side.
    Input that into a post type regulator of 15 Volts and you have it.

    But Like I said, this would be a weekend worrier project or something
    that needs to be done in a hurry on a perf board or something etc..
     
  3. Chris

    Chris Guest

    Hi, Gilad. You aren't going to be doing anything with output voltages
    above or below your power supply rails unless you do something with
    your supply. By using a 555 and a handful of caps and diodes, you can
    generate a +17V and -8V supply like this (view in fixed font or M$
    Notepad):

    |
    | VCC VCC
    | + +
    | | |
    | .-. |
    | | |2K2 | N.C.
    | | | .---o----o---. VCC
    | '-' | 8 4 | +
    | | | | |
    | | | | D -
    | o----o7 | ^
    | | | | C | 2*Vcc - 1.4V
    | | | | ||+ | D (~+17V)
    | .-.15K| 3o----o----||---o--->|---o-------o
    | | | | | | || |
    | | | | LM555 | | +|
    | '-' .-o2 | | C ---
    | | | | | | ---
    | | | | | | |
    | o--o-o6 | | ===
    | | | | | GND
    | | | | |
    | | | 1 5 | |
    | --- '---o----o---' | C -Vcc+1.4V
    | --- | N.C. | +|| D (~-7.6V)
    | |.01uF | '----||---o---|<---o-------o
    | | | || | |
    | === === | |
    | GND GND D V C---
    | - ---
    | C = 100 to 220uF, 25V | +|
    | D = 1N4001 or 1N4148 | |
    | === ===
    | GND GND
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Now that you've got realistic supplies, get an LM324 (a quad op amp),
    and go to work:

    |
    | 0 to +5VDC in +17V
    | |\|
    | o-------------|+\ 0 to +15V out
    | | >--o-------------o
    | .---|-/ |
    | | |/| .-.
    | | -8V | |10K
    | | | |
    | | '-'
    | | |
    | '---------o
    | |
    | .-.
    | | |20K
    | | |
    | '-'
    | |
    | ===
    | GND
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    VCC
    + ___
    | .--|___|--.
    | | 12K |
    .-+2.5V|\ | |
    10K| |<---|+\ ___ | |\ |
    | | | >--o--|___|-o-|-\ |
    '-' .--|-/ | 10K | >----o-----o
    | | |/ | .-|+/ -3Vto +3V out
    | | | | |/
    === | | |
    GND '--------' |
    |
    |
    |
    0 to +5V in ___ |
    o--------------|___|---o
    10K |
    |
    .-.
    12K| |
    | |
    '-'
    |
    ===
    GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Assuming 1% resistors, the output of your 0-15VDC op amp should be
    accurate to within 2% +/-20mV, probably much better. The second
    circuit as shown is going to be mostly dependent on the stability of
    your 9V supply. If it's regulated, all the better. If not, you
    should consider getting an LM78L05 as a post regulator, generating a
    solid stable +5V, and running the tweaker pot off that, or use two 1%
    resistors instead of the pot to get your 2.5V from the 78L05.

    Here are a couple of references and data sheets to give you some
    background:

    http://www.national.com/ds.cgi/LM/LM555.pdf
    http://www.national.com/ds.cgi/LM/LM124.pdf
    http://www.ikalogic.com/shm_voltage_inv.php
    http://www.national.com/an/AN/AN-31.pdf

    Good luck with your project
    Chris
     
  4. Bob Monsen

    Bob Monsen Guest


    You can't really go from 0 to 15V with only a 9V power supply unless you
    play tricks. However, why do you need to go to 15V if you don't have a 15V
    power supply? What are you controlling?

    The tricks are using a dc-dc converter, or using one of the capacitative
    voltage doublers like an ICL7660, which can get you 16V from your 9V input.
    Then, use a rail-to-rail opamp configured to amplify 3x in the positive
    direction, using a circuit like this:

    20k
    ___
    .------|___|---.
    | |
    | |\ |
    o----|-\ |
    | | >------o------ out
    in -----------)----|+/
    | |/ 0-15
    0-5 |
    |
    .-.
    | |10k
    | |
    '-'
    |
    GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    For the -3V, you can use another ICL7660 to give you a -8V rail from your 9V
    input. However, I believe that getting the voltages to come out right using
    an opamp will require some kind of external reference, in addition to the
    ICL7660. However, there may be some kind of clever opamp circuit that can do
    this with just resistors, and no reference. I don't know.

    Regards,
    Bob Monsen
     
  5. You will need a negative supply to get the -3V output, and you will need at
    least 16VDC to get 15VDC output. So you need a voltage doubler and an
    inverter from your wall-wart. There are many off-the-shelf DC-DC converters
    that can do this, and there are countless circuits available if you want to
    roll your own. It all depends on your budget of money, time, and ability to
    build circuits. Let us know these details, and you can get the information
    you need.

    Paul
     
  6. Chris

    Chris Guest

    Hi, Jamie. You're right -- the diode is backwards. Good catch -- it
    should look like this:

    |
    | VCC VCC
    | + +
    | | |
    | .-. |
    | | |2K2 | N.C.
    | | | .---o----o---. VCC
    | '-' | 8 4 | +
    | | | | |
    | | | | D V
    | o----o7 | -
    | | | | C | 2*Vcc - 1.4V
    | | | | ||+ | D (~+17V)
    | .-.15K| 3o----o----||---o--->|---o-------o
    | | | | | | || |
    | | | | LM555 | | +|
    | '-' .-o2 | | C ---
    | | | | | | ---
    | | | | | | |
    | o--o-o6 | | ===
    | | | | | GND
    | | | | |
    | | | 1 5 | |
    | --- '---o----o---' | C -Vcc+1.4V
    | --- | N.C. | +|| D (~-7.6V)
    | |.01uF | '----||---o---|<---o-------o
    | | | || | |
    | === === | |
    | GND GND D V C---
    | - ---
    | C = 100 to 220uF, 25V | +|
    | D = 1N4001 or 1N4148 | |
    | === ===
    | GND GND
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Trust but verify.
    Chris
     
  7. Jamie

    Jamie Guest

    Something tells me the above circuit is flawed.
    Something about a diode seems to be out of place in the + output.
    Also the voltages don't look correct referenced from the GND.
    I just can't see how that is going to generate 17 Volts ?
    the (-) output side looks ok how ever.
    I guess if you were to use the (-) output as your common to the LM324
    , you could get a single output effect of ~ 17 volts maybe..
    Maybe I missed something, just an observation.

    http://webpages.charter.net/jamie_5"
     
  8. Jamie

    Jamie Guest

    Follow up.
    Yes,. it appears the diode is backwards in the + output multplier from
    the VCC+ source.
    I knew something didn't look correct, I came to me after I pushed the
    send button.
    It's a bitch getting old!
     
  9. Guest

    Thanks for the quick reply.. will definitely try this.

    For the curious:
    The project is to build something along the lines of the following
    clock (i'm using arduino diecimila):

    http://web.jfet.org/weston-clock/

    the board gives several variable 0-5V pwm outputs, but i can't find
    analog panel voltmeters for anything less than 0-15VDC (also i have a
    beautiful pre-WWII -3V~3V panel one which i really want to use, which
    explains the need one of the scaling circuits). If anybody knows where
    i can find 0-3V (or something along those lines) analog panels, that
    would save a lot of hassle..

    Also, usingDS1307 RTC the proper voltage is delivered to the 3 panels
    (for sec's minutes and hours display).
    In the spirit of open soft/hard-ware i'll post links to circuits and
    code for the whole project when it's done..

    Thanks again, gilad.
     
  10. Guest

    Paul, to avoid repetition please see my reply to chris's second
    message (three messages down..)
     
  11. Analog voltmeters usually have internal resistors to provide scaling. You
    should be able to open up the meter and change the resistor so the 0-15V
    meter becomes 0-3 VDC or 0-5 VDC which can be driven directly from the PIC
    PWM.

    If you can connect the (-3V)-0-(+3V) meter to a bridge type circuit, you
    can drive it without a negative supply rail. Or even directly between two
    PWM outputs of the PIC.

    It may also be possible to rotate the rear spring adjuster of the meter far
    enough to make the pointer rest at the leftmost mark on the scale. Long ago
    I worked as a meter technician and we would modify analog meters for
    suppressed zero, live zero, and center zero. But these were more modern
    meters, vintage 1975 or so, and not 1894 or 1940s. I'm not *that* old!

    You can also look for switchboard DC ammeters, which are generally 50 mV or
    100 mV full scale. They usually take a little more current to drive, but
    probably 10 mA or less, and you can just add a series resistor. Or look for
    0-1 mADC and add a scaling resistor (1000 ohms per volt).

    If you'd like to make an analog display with a full 360 degree display, you
    could use a "synchroscope", which measures the phase angle between two AC
    voltages. See http://en.wikipedia.org/wiki/Synchroscope

    Paul
     
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