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Building Circuits & Capacitor Direction

  1. Jul 15, 2012 #1
    I have a 12 volt transformer (120v in) that I'm trying to use with low current devices (LM386 amp). My question is when I use my meter to read amperage or voltage I'm fully loading the circuit so I get like 2 amps across my resistors and capacitors. I don't need anywhere near this amount, instead I want maybe .350 amps. Currently I'm using a full-bridge rectifier.

    So a couple questions:

    If I put this DC system into a circuit that 'draws' .350 amps (the LM386) will it still try to 'push' the 2 amps? Presumably not, which leads me to the next question.

    If I'm building a circuit piece by piece (adding resistors etc as I go) how can I test what the amperage/voltage will be if my multimeter is always checking under the assumption of 'full load', which it seems to be doing? This would pull ~2 amps which would fry the circuit components in this case.

    Finally, sort of unrelated, capacitors have a negative band on the side. I'm unsure if this means the negative side faces towards ground or if the negative side indicates voltage going towards ground (from Vs+ to Vs-). I have seen it work, strangely, both ways. My LM386's capacitor is facing one way but on my breadboard I only get current if I face it the other way.

    Option A: Vs+ [- +] Vs-
    Option B: Vs+ [+ -] Vs-

    The [- +] is the capacitor. Let's put this baby to bed. Which is it?
    Last edited: Jul 15, 2012
  2. jcsd
  3. Jul 15, 2012 #2
    When you place ammeter across 12V supply you are loading it. When you place voltmeter across 12V supply you are *not* loading it.
    If you want to measure volts & amps through your circuit:
    1 - measure volts right across 12V (it will not load it).
    2 - Disconnect one of the 12V supply leads and place ammeter across this break. This will measure the current that your circuit is drawing from 12V supply. In other words place ammeter in series with your circuit.

    Capacitor polarity: negative band goes on negative. This may or may not be ground. If your supply is +12V then negative band will go on ground. If you want to double check, use voltmeter to determine which side is negative, and orient capacitor with band on that side.
  4. Jul 15, 2012 #3
    Makes sense. Thanks.
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