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Thread: amp meter "fixing" led strip power supply overload....

  1. #1
    Junior Member
    Join Date
    Feb 2017
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    amp meter "fixing" led strip power supply overload....

    Last night I was happily trying out my new octows2811 adapter and teensy. I connected 4 strips of 92 leds and the teensy to a 10 amp 5v switching power brick. Now I read all the warnings about not underestimating the power usage of these strips, but having little experience with them so I thought it would be enough for a simple test using the basictest example from teensyduino.

    Through the first few colorwipes everything went perfect. Then full white came along and the power supply switched off . After that I wanted some more info on (average) current draw and put an ampere meter in series with the power cables for the strip and the power supply. I measured the power draw for the single colors to be between 5 and 7 amps. Then full white came along again and I measured 12 amp current draw. However this time the power supply did NOT switch off and everything just continued on perfectly stable! I then removed the current meter and connected everything directly. As soon as it hit white, the power supply switched off (into overload I guess). I reconnected the amp meter and everything was stable again.

    To me this makes little to no sense. The amp meter surely is not magically supplying the extra amps. Anyone here has a suggestion as to what is going on? What kind of influence could the meter have on the circuit...


    p.s. I will off course add I second power brick, but I'm curious if anyone can explain my weird observation.

  2. #2
    Senior Member Ben's Avatar
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    Jul 2013
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    Germany
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    401
    Hi,

    the amp meter measures the current by letting it flow through a low-value resistor ("shunt resistor") and measuring the voltage drop across this resistor. So by introducing the meter into your circuit you effectively created a resistor in series to the rest of your circuit. That seems to have lowered the current consumtion enough to prevent the over-current protection of your power brick from engaging.
    You can actually measure this shunt resistor by simply measuring into your amp-meter with a second multimeter set to ohms.

    Ben

  3. #3
    Senior Member PaulStoffregen's Avatar
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    Nov 2012
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    21,354
    Yup, without the meter the current is probably even more than 12A, and your poor 10A supply just can't keep up.

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