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Oscilloscope preamp

Discussion in 'Electronic Basics' started by RKovach, Dec 2, 2003.

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  1. RKovach

    RKovach Guest

    I have a P-C based scope that will only go down to .1 volt per division. I
    would like to use a clamp-on amp probe I have, but this probe puts out .001
    volt per amp, making it hard to see a good pattern for anything less than
    100 amps. I was told I could use a preamp to boost the signal. I would like
    to know if I could make one using a 741 op-amp, or would this distort the
    pattern. I would only be using this to check alternator output on cars, so I
    don't need to be concerned with high bandwidth.
  2. Gareth

    Gareth Guest

    A simple 741 (or just about any op amp) circuit should work fine.

  3. Here is a circuit you could use (view with courier font):

    | |
    .-. |
    | | |
    | |22k |
    '-' |
    | |
    1uF 10k | | 1MEG
    || ___ | | ___
    IN -||------|___|--------+----|-|___|------+ OUT
    || | | | |
    | | | |
    | | | |
    | | | |
    | | | |
    | | | |
    | | |\| 741 |
    | +--|-\ |
    | | >-----------+
    | |/|
    .-. |
    | | |
    | |22k |
    '-' |
    | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    It'll amplify the signal 100x. The amplification is set by the ratio of the
    1MEG/10k resistor, so you can change it if you need to. Note that if you
    change the resistors, the total resistance seen by the inputs of the opamp
    should be near each other to deal with leakage. In this case, they are both
    near 10k. Thus, scale it by changing the 1MEG resistor rather than the 10k
    resistor to keep the resistance seen by the -input around 10k. A 1MEG pot in
    series with a 10k resistor would be a good way to do this. All the way on
    gives you 101, all the way off gives you 1.

    The output will center around Vcc/2. Also, the output falls off to 1/2 (3db
    point) at about 17kHz with a 100k load, so keep the frequency you measure
    below that.

    Bob Monsen
  4. Rich Grise

    Rich Grise Guest

    Actually, you'd be better off just getting a new current probe.
    1 mV/Amp is way down there in the noise - if you amplify a 1mV
    signal to a useful level, you're also amplifying the noise and
    error(s). If you're dealing with only a few amps, a 0.1 ohm
    resistor in series with the circuit under test will give you
    100 mV/Amp, 0.01 ohm gives 10mV/A, etc.

    O a current probe might be pretty simple - carefully cut a
    toroid in half, polish the cut ends so they'll mate well, and
    wind 10 or 100 or 1000 turns on the one half, and clamp it on.

    If you do this, you _MUST_ provide a current path because the
    volts out of a current transformer will try to go high enough
    to force the current through the secondary.

    Good Luck!
  5. Hi Rich,
    I'd been trying to build a current probe for my analog scope to measure
    everything, but today I need it to test my alternator's output making sure it
    puts out 30A under load. Looks like you've explain it well to Mr. RKovach.
    But what does it mean to clamp it on? And if I do this, I _MUST_ provide a
    current path because the volts out of what current transformer will try to go
    high enough to force the current through which secondary? Sorry for asking,
    I'd never build a current probe before?

    "Rich Grise wrote in message
  6. Rich Grise

    Rich Grise Guest

    Well, I'll assume you know how a transformer works in general.
    There's the primary, the secondary, and the core, which is
    magnetic material. If you use a toroid - there's an example
    of a fairly large one here -
    The toroid on top,
    The windings are the secondary, the toroid core is the, well,
    core, and the primary consists of one "turn" of wire, which
    is the lead passed through the center hole of the toroid.
    (not shown on that page above) For the formulas, it acts
    exactly like one turn. So if you've got, say, 100 turns of
    wire for the secondary, and the one "turn" (it actually
    doesn't do any turning - if you did, and looped it through
    the hole twice, that'd be two turns.) for the primary. And
    the voltage ratio in a transformer is the turns ratio, and
    the current ratio is the inverse of the turns ratio. So
    with 30 A through a 100-turn-secondary current transformer
    would give you a current of 30/100 A, or 30 mA. Stick a 100
    ohm resistor across the secondary leads, and it will develop
    100 * .03 = 3 volts. A 1K ohm would develop 30 volts and so on.

    So in that case, you'd unbolt your alternator output lead,
    thread it through the toroid, and bolt it back into place.

    And you can probably find a suitable toroid at surplus
    somewhere. :)

    Good Luck!

    Here's just one sampling of |"current transformer" schematic|
    on google:
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