Why is the current always measured negative in LTSpice?

Discussion in 'General Electronics Chat' started by thakid87, Jul 20, 2010.

  1. thakid87

    Thread Starter Active Member

    May 23, 2009
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    Whenever I check the current coming out of a voltage source, it always comes up as a negative value.

    Am I doing something wrong?
     
  2. Ghar

    Active Member

    Mar 8, 2010
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    It uses the same convention for sources and loads, which is that + power is absorbing and - power is delivering.
    Since the voltage is positive the current is negative to give you a negative power.

    If you switched it up for loads and sources you'd just get confused by sources that are really sinking and such...
    If it really bothers you you can double click the plot and add a - sign in front of I(xxx)
     
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  3. thakid87

    Thread Starter Active Member

    May 23, 2009
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    I might be doing this wrong, but look at the circuit below.

    I @ R2 is coming up as -10mA and I @ R1 is nearly 1mA. Based on what you've told me, the current at R1 would then be 11mA?

    Sorry for my ignorance. I'm learning transistors right now...
     
  4. Ghar

    Active Member

    Mar 8, 2010
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    The 'plus' terminal of a resistor isn't shown on the schematic so you have no good way of knowing which direction the current actually is.
    If you don't know you can check out the voltages, those must be correct.

    If the direction keeps bothering you you can rotate the resistor and even though it looks identical on the schematic it will reverse the displayed current direction.

    The current in R1 is 1 mA, and the current in R2 is 10 mA, it does show the correct current in the component. The only iffy part is the direction.
     
  5. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    When in doubt put the current probe back on the component and look at which way the arrow is pointing.
     
  6. Ghar

    Active Member

    Mar 8, 2010
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    Hah, can't believe I forgot LTSpice shows the arrow.
     
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    I am sure there is a simple way to change the directionb of that arrow but for now here is an amplifier circuit. You can't learn transistors without an amplifier circuit.

    Gosh darn it confused me again.
    I have to save to the desktop or it won't upload.
    Anyways, hope this is useful.
     
  8. thakid87

    Thread Starter Active Member

    May 23, 2009
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    Thanks for the help. I can't begin looking at that right now, lol. I will check it out tomorrow morning. I'm sure it will be helpful.

    Are transistors useful mainly with two different sources? Can they be used effectively with one source?

    Thanks!
     
  9. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    You would normally use two sources with paired transistors in a Bridge.

    You could also use them for some other uses like a DC coupled (no capacitor on the output) Class A Amp.
     
  10. thakid87

    Thread Starter Active Member

    May 23, 2009
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    If you wouldn't mind, can you show me some examples of a transistor being used with a single supply?

    What are applications that would use 2 different power supplies? Is this seen often?
     
  11. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    I posted a Transistor Amp using a single supply before.

    Here is a Bridge Amp. With any bridge the emitter resisters can tell you a lot. Check currents R1 and R2 with the output Resistor Current. You can see because the peaks don't match that the output is out of balance.

    The output is also low compared to the 2V input because I used bigger emitter resistors than anyone would want if they were building for power output. But since the design is only using small transistors it would be better to increase the size of those emitter resistors or just add an output series resistor to reduce the currents and power output.

    I think you wanted to see a single transistor used in a double ended supply The trick I can think of that would make that useful is balancing the output to ground and I think that is trickier with a single Transistor. I will have to think about an example for that.

    And for anyone, please feel free to point out anything that has information value, but don't pick on my poor Bridge Circuit too much. I was trying to make something simple and sane at midnight in a few minutes so I can go to sleep. It is just a demo and a few flaws like the ones I pointed out might actually make it a better demonstrator.
     
  12. thakid87

    Thread Starter Active Member

    May 23, 2009
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    Thanks once again. I will continue to take more looks at those circuits to see if I can get a better understanding...

    I still keep getting confused with the source/sinking currents of LTSpice. I will post something up tomorrow if I can as to where my confusion lies.

    Thanks, guys.
     
  13. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    92
    I am also wondering if you are looking for a circuit that works with just one voltage source. In the first Amps I add examples for that is actually two amps, or more accurately two version of the same Amp. The second Amp I just removed the emitter Bias Resistor and bypass capacitor.
    If you do a frequency response curve you will see that it was causing some loss of gain and phase delay at frequencies below KHz, but it prevents some distortion.

    The two amps are a copy including the voltage supply which is not really needed twice. Both amps could have used the same voltage.

    And there is another voltage - the signal source. An amplifier typically needs some AC to play with.
     
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