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multisim questions

Discussion in 'General Electronics Discussion' started by foTONICS, Oct 13, 2012.

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  1. foTONICS


    Sep 30, 2011
    Hey all, so I'm simulating this basic regulator using an LM117 and I have a few questions.

    1. I used a voltage divider with a trim pot to the input of the LM117 so I could see the effects of insufficient voltage on the output. This version of multisim I'm using has the feature of resistors that blow if the wattage is too low. I ran the simulation with R4 at the default quarter watt and it blew. But 60V/10000ohm=6mA. so the wattage through the R4 should only be .18 watts, is this enough to blow the .25watt resistor?

    2. I attached a screenshot of the oscilloscope (red is Vin / orange is Vout) and got a funny picture (2nd picture). After zooming in on the waveform (3rd picture) I saw the ripple. Now I completely understand ripple rejection and mains noise and all that jazz, but I was told that multisim doesnt account for ripple and that multisim DC sources are inherently why am I seeing ripple? C1?

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It sounds like it's clever enough to model the circuit oscillating when it has a high impedance voltage source and a very small output capacitor.

    Replace the voltage divider on the input with something that has a far lower impedance. Does multi-sim have variable voltage sources? Or voltage sources that are dependant on a value somewhere else in a circuit?
  3. Rleo6965


    Jan 22, 2012
    In normal practice. We don't place high value resistor in series between voltage source and Vin of IC regulator. This will result too much voltage drop below Vin required voltage resulting drop unregulated vout.
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