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Detecting minor voltage drops on a 50V source

Discussion in 'Electronic Basics' started by nina.p20, Nov 5, 2005.

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  1. nina.p20

    nina.p20 Guest

    I'm trying to find a schematic of an external device (maybe a
    comparator ?) that may detect a minor drop from a 50V source. The
    source is exactly 50V, and I need an alert (Green LED = 50V, Red LED =
    less). I need to detect a 10uV or even less if possible drop for a very
    high precision device.
    If I'm powering a comparator or a schmitt trigger from a 9V battery,
    how can I measure higher voltages? Is it possible ?
    I'll be very thankful for aby help.
  2. Dan Hollands

    Dan Hollands Guest

    It is easy enough to use a resistor divider to measure the higher voltage
    but measuring a 10uV drop under real world conditions seems unrealistic.

    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
  3. nina.p20

    nina.p20 Guest

    Thanks for answering,
    I'm trying to detect a voltage drop on a Telephone line when a 10MOhm
    load is placed across (parallel) the line. I've measured the Voltage of

    line before and after the "load", with a Fluke 197 and found it was
    about 10uV. It interests me very much if using a precision high
    impedance comparator may detect this drop. Any other ideas will be
    welcome of course :)
    Best regards,
  4. JeffM

    JeffM Guest

    nina.p20 multiposted to sci.electronics.basics and
    Each time you post, you post INDIVIDUALLY to multiple groups.

    This is called Multi-posting and is frowned on.
    If you would put the name of EVERY group
    in which you would like your question to appear
    on the To: line (the Groups: line) the FIRST time you post,
    you won't have to post it a 2nd time.
    That is called Cross-posting.

    Everyone who reads the question will see ALL the responses
    and will know that the proper answer
    has already been given in another group
    (or a bogus solution has been offered and needs correcting).
  5. Guest

    I'm no expert on Usenetiquette but I was always under the impression
    that it's cross-posting that's frowned upon, and that it's better to
    post to each group individually.

    As to the OP's questions, detecting a 10uV difference would be
    difficult but not impossible. One of the high-precision, low-drift
    opamps running from a regulated power supply, with perhaps a
    temperature-controlled environment should be able to do it.

    And yes, as already stated by another poster, measuring a voltage
    higher than the power supply is very simple with a resistor divider.
    E.g., to measure 50V, use a 10:1 divider to step it down to 5V.
  6. John Fields

    John Fields Guest

  7. Bob Masta

    Bob Masta Guest

    I wonder if maybe a "trick" could be used for this application.
    The problem is that you need to measure this tiny percentage
    change in a large DC value. If you try to set up a comparator
    with precision resistors, you are going to have a heck of a time
    with stability issues.

    Here's the idea: Run the raw 50V through a super-long TC
    filter. A simple RC will be OK. This will be the reference voltage.
    Think of this as "zero". Now you just need to detect -10 uV, not
    49.9999 V. In order for this idea to work without
    a precision divider, you will need to "float" the comparator. That
    will be a bit more work, but probably worth the trouble.

    The other possibility is to capacitively couple to the line and
    just detect the transient when the load changes. You can
    amplify the heck out of that for detection, as long as you can
    be assured the line is otherwise clean.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  8. Wim Lewis

    Wim Lewis Guest

    There is disagreement on this topic. :)

    The original intent of crossposting was to handle situations like this,
    where one post (and all its replies) are on-topic for several
    groups. Crossposting has gotten a bad reputation because some
    people use it to incite flame wars: you can crosspost to two very
    different groups, and by default peoples' responses will be crossposted
    also, and especially if people don't *realize* that the thread is
    in multiple groups this can produce a lot of flamage really quickly.

    But, IMHO, crossposting *is* the right thing to do if your post is on
    topic for two groups whose cultures aren't terribly different, like
    say, three different groups in sci.electronics.*. Or comp.arch.embedded
    or comp.arch.fpga, to name two other groups whose subject matter overlaps
    with sci.electronics a bit. It's polite to mention in your message that
    it is crossposted, though, just in case the respondent doesn't read the
    headers and is writing a response that's only on topic for one of the groups.
    You can also use the Followup-To: header in your original post, if it
    seems appropriate.
  9. Wim Lewis () writes:

    But once again, if people routinely cross-post we might as well
    return to 1995, when there was only sci.electronics (and sci.electronics was split up because traffic
    was getting heavy, and split into what seemed to be a logical set
    of subsets.

    There may be times when cross-posting is appropriate. But much of
    the time, it's simply someone too lazy to figure out the most appropriate
    place. Or too impatient to see if the first newsgroup gets much of a
    response (which also may mean laziness, because they may not have taken
    the time to read the newsgroups they post to for a while before posting,
    so they have no real understanding of whether the post is appropriate).

    Just because a post is on topic to various newsgroups does not mean
    that cross-posting is the solution.

  10. John Fields

    John Fields Guest

    LOL, that's the _perfect_ reason to cross-post!

    Why do otherwise? If I post to several groups separately with a
    query which is on-topic to all of them, say: "Please, I need a
    schematic and the source code for a BASIC code-practice oscillator
    which I can run through my home entertainment system using a PIC and
    X-10, but I don't know how to design something like that." followed
    by the the inevitable "TIA", then I'll have to harvest my replies by
    schlepping from group to group, and everyone who replies to my query
    will have to do the same to make sure that they're not repeating
    what someone else has posted and to keep track of the thread.

    That's ridiculous.

    It would be vastly simpler for me and everyone else if I just
    cross-posted. It's not like there's a problem with bandwidth, and
    even it there was, that's the kind of thing that drives the
    technology forward. Usenet isn't supposed to dictate terms; it's
    our servant, and as long as we shelter it, feed it, and clothe it,
    it needs to do do what we tell it to.
  11. Jon Danniken

    Jon Danniken Guest

    No. When you crosspost *one* message to *a few* appropriate groups, a
    modern newsreader will notice this and mark the posting "read" across all
    crossposted groups. This means that if you have chosen not to display
    viewed messages, you only have to view the message (and the replies) once.

    When you multipost, on the other hand, the same message (and all the
    replies) is still there in *every* group you multiposted into, even after
    you have already read the article in another group. This is obviously a
    very unefficient and annoying way to read usenet, especially if you read
    groups with similar interests..

    Pretty much only problem with crossposting is when it crosses the boundaries
    of the groups, i.e., the incessant political prattling that goes on in
    strictly non-political groups, and also spamming, but these are not so much
    problems with crossposting so much as they are problems of basic respect (or
    lack thereof) of others in usenet.

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