Connect with us

Honeywell Pressure Sensor

Discussion in 'Electronic Basics' started by James_sgp, Jan 5, 2007.

Scroll to continue with content
  1. James_sgp

    James_sgp Guest

    I am using a Honeywell Pressure Sensor to measure water pressure, my
    question is that the terminals are +, Grd, Output +, Output -. I need
    a voltage on a sigle line to go into my micro controller; can i connect
    the Output- to Grd? And still be able to get the voltage from Output+?
     
  2. Chris

    Chris Guest

    Hi, James. I kind of wish you'd mentioned the Honeywell part number --
    that would have increased the chance of getting a correct answer.

    Many pressure sensors have a bridge-type output, where you measure the
    differential voltage across the bridge. Honeywell makes some of these,
    and it sounds likely that's what you've got. A bridge sensor works
    like this (view in fixed font or M$ Notepad):
    |
    | .--------o-----------.
    | | + | |
    | | .-. .-.
    | | R1| | R2| |
    | | | | | |
    | | '-' _ '-'
    | | |Out-/ \Out+|
    | +| o---( V )---o
    | ---V | \_/ |
    | - .-. .-.
    | | R3| | R4| |
    | | | | | |
    | | '-' '-'
    | | GND| |
    | '--------o-----------'
    |
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    The pressure sensor itself in this case is one or two of the resistors
    R. The other resistors in the bridge are set so, when there's no
    differential pressure, the sensor resistor balances the resistor on the
    other side of the bridge, and the voltmeter reads zero. But when a
    presure is applied, the sensor resistance changes, so there's a reading
    across the voltmeter. Simple, sensitive to small changes in pressure,
    safe.

    Connecting the Out- to GND should be OK if you're running the sensor
    off an isolated battery. But if you're using the uC power supply to
    power the sensor, it's not a good idea. Not that you''l break
    anything, but your resistive divider doesn't work. You're shorting
    across one of the resistors -- no balance, no measurement.

    If you want to amplify the difference between the two outputs without
    loading them down, you'll need to use an op amp configuration called an
    instrumentation amp. It's basically a difference amp preceeded by two
    voltage followers. A schematic of the circuit is in National
    Application Note 31 Op Amp Circuit Collection :

    http://www.national.com/an/AN/AN-31.pdf#page=8

    Look at the "Differential-Input Instrumentation Amplifier" on the top
    of p.14. You might be able to make something serviceable with 3/4 of
    an LM324 and four resistors (forget the balance pot in the diagram --
    that's not for an LM324).

    Since you didn't mention which sensor you were using, I guess you'll
    have to post back with the Honeywell part number to get an idea of the
    voltage gain you want & which resistors to choose for the IA, or we'll
    have to leave that as an exercise for the student.

    Good luck
    Chris
     
  3. Chris

    Chris Guest

    While you're at it, James, you might also want to protect the inputs of
    the op amp and your uC by adding a few resistors and diodes (1N4148 for
    D) like this:
    |
    | VCC
    | + .------.
    | | | |
    | D - | |\ |
    | ^ '--|-\ | ___ ___
    | ___ | ___ | >o-|___|-o-|___|--.
    | .----------------------|___|-o-|___|--|+/ | |
    | | VCC 100 | 1K |/ | VCC |
    | | + D - | + |
    | | | ^ | |\| |
    | | o-----------. | '--|-\ | Vout
    | | + | | VCC | | >--o--o---o
    | | .-. .-. + === .------. .--|+/| |
    | | | | | | | GND | | | |/=== .-.
    | | | | | | D - | |\ | | GND | |10K
    | | '-' _ '-' ^ '-|-\ | ___ | ___ | |
    | | |Out-/ \Out+| ___ | ___ | >-o----|___|-o-|___|--. '-'
    | '----o---( V )---o-|___|- -|___|--|+/ | |
    | | \_/ | 100 | 1K |/ | ===
    | .-. .-. D - ===GND
    | | | | | ^ GND
    | | | | | |
    | '-' '-' |
    | | | ===
    | o-----------' GND
    | |
    | ===
    GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    This should get you where you want to go. Please post again with the
    Honeywell sensor model number, and any other info if you have it (like
    a link to the Honeywell data sheet) for advice on choosing resistor
    values.

    Good luck
    Chris
     
  4. James_sgp

    James_sgp Guest

    Chris,

    Sorry for not including the part number, here it is: 24PCFFM1G
    Its a 0-100 PSI Gauge measuring PCB mount pressure sensor from
    Honeywell.
    I understand what you`ve mentioned. SO my question would be is how to
    configure it so I get one line with a voltage output going to my micro
    porcessor?
     
  5. Chris

    Chris Guest

    Hi, James. First off, as a Google Grouper (I'm one, too), you might
    not know it's considered good form to bottom post. On Google, click
    "show options", then "reply". Put your response to the post below the
    previous post -- that gives greater continuity, and makes it easier to
    answer the previous post with minimum scolling. Also, it helps for
    continuity -- most newsreaders don't act like Google Groups. Please
    check out Google Groups Help Topic "What's good 'netiquette' when
    posting to Usenet?"

    http://groups.google.com/support/bin/answer.py?answer=12348&topic=250

    Now I've scouted around the Honeywell website, and found something that
    may be pretty close -- the 24PCFFM6G:

    https://sensing.honeywell.com/index.cfm/ci_id/140301/la_id/1/pr_id/80994.htm

    Now assuming your sensor is pretty close, the docs suggest that you
    should provide a 2mA costant current source at a nominal voltage of 10V
    (but up to 12V will do) to help with temperature compensation. Now for
    the sake of simplicity we can just use a resistive voltage divider to
    get to 10V, by putting a 1K ohm resistor in series with the nominal 5K
    resistance of the bridge, like this:

    |
    | .--------.
    | | |
    | | .-.
    | | | |1K
    | | | |
    | | '-'
    | | |
    | | o-----------.
    | | + | |
    | | .-. .-.
    | | R1| | R2| |
    | | | | | |
    | | '-' _ '-'
    | | |Out-/ \Out+|
    | +| o---( V )---o
    | ---+12V | \_/ |
    | - .-. .-.
    | | R3| | R4| |
    | | | | | |
    | | '-' '-'
    | | GND| |
    | '--------o-----------'
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Now the data sheet suggests that the differential voltage at the output
    of the bridge will be 2.25mV per psi with a 100psi max rating. Since
    you want that to correspond to a max 5V input, you have to find a way
    to amplify that 225mV to5V. That means you need a gain of about 22.
    Again, referring to the "Differential Input Instrumentation Amplifier"
    schematic on p. 14 of National's AN-31, that means you should use 19K
    resistors for R2 and R3, and 220K resistors for R4 and R5, giving you a
    gain of 22. Make sure you match the two 10K resistors and the two 220K
    resistors with an ohmmeter as closely as possible. Again, the pot R1
    is not used with an LM324.

    All of this is assuming your sensor has the same specs as the one
    mentioned above. You should definitely try to obtain the data sheet
    (if necessary, call or email Honeywell for a link) and verify yourself.
    Also, if you're interested in temperature compensation, it gets more
    complicated -- you should post again if you need this capability.

    In the event of overpressure, it's possible the differential voltage
    could exceed the maximum input voltage of your uC. It might be good to
    put a 5.1V zener from the output of the third op amp of the IA to GND
    to limit the maximum output to 5V.

    I hope this is enough to get you started. If you need more help,
    please include in your post
    some information on what this project is about, as well as if this is
    just hacking around, a real industrial control project, or a class
    project. If the need requires, there are a number of additional
    features you can include which will enhance the project. And if this
    is a senior project, the above suggestions should be modified quite a
    bit for academic purposes.

    Good luck
    Chris
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-