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Thread: Hot capacitor with 18m2 low power board (CHI030)

  1. #61
    Senior Member
    Join Date
    Dec 2010
    usa: west of the mississippi


    Quote Originally Posted by hippy View Post

    Around 20mA-30mA would seem as expected for the setup when not playing. Not sure why it won't play with the meter in-line. I use a bench supply which shows current drawn so am not familiar with the pro's and con's of measuring in-line.
    The effect of an in-line amp meter is the series resistance it adds to the circuit. Using my cheap Craftsman DVM, in the 20A range the resistance is only 0.4 ohms. The milliamp range adds 4 ohms, and the microamp range adds about 100 ohms. Whether this matters in a particular circuit will depend on many factors, but in all cases the effect will be to reduce the voltage to the circuit under test. This reduction will equal follow Ohm's Law. V=R*I


  2. #62
    Senior Member
    Join Date
    Jan 1970
    Colorado USA


    Well noted tom... however have you tried 20 amps on that meter? 20 x 20 x .4 = 160 watts.. should keep your hand warm using it!

  3. #63
    Senior Member
    Join Date
    Feb 2012



    Quote Originally Posted by geezer88 View Post
    in the 20A range the resistance is only 0.4 ohms.
    I make that an 8 volt drop at 20A and a dissipation of 160 watts.

    Sounds like it should release the magic smoke rather well.

    To endorse what others have said; Ohms Law and Power = Volts x Amps are the first things to consider in almost any electronics design.. When Power > 1 watt things might start to get quite warm (and many components are only good to around 250 mW).

    Cheers, Alan.


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