General rules for using resistors and capacitors?

Discussion in 'General Electronics Chat' started by RyanD, Jan 14, 2010.

  1. RyanD

    Thread Starter Member

    Jan 14, 2010
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    It makes sense when someone explains why they added that resistor/capacitor but when building my own circuits I'm stumped as to know when to add them. I understand that certain things can't accept certain current(like LEDs) and they need resistors but do all things have these, even IC's?
     
  2. lmartinez

    Active Member

    Mar 8, 2009
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    How about a start point! Do you have an electrical schematic where the fundamentals can be applied to in order to help you with your original question?
     
  3. RyanD

    Thread Starter Member

    Jan 14, 2010
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    Okay, lets start with this, capacitors seem to confuse me a little more. Take this schematic, if I was doing this I would have made the capacitors 100uf and 10uf but here they are 0.1 and 1 respectively. What am I doing wrong, would it matter that much?

    [​IMG]
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you can relate why you would have chosen those values, it would be interesting.

    Also, can you see why the resistors are being used?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    In the LM317 schematic you posted, I've found it curious that National Semiconductor would use 240 Ohms for R1, as to be able to achieve guaranteed regulation, Vout must have at least 10mA load current.

    Since Vref (the difference between Vout and ADJ) is nominally 1.25v, but can vary from 1.2v to 1.3v between devices, one must use a 120 Ohm resistor to guarantee that a mimimum of 10mA load will always be present at the output.

    Reasoning: I = E/R, and R=E/I, so 120 Ohms=1.2v/10mA and 10mA = 1.2v/120 Ohms

    This also means that R2 would have to be reduced by the same ratio to obtain the same range of output voltage.

    C1 is present from Vin to ground to bypass any high-frequency component from the input source. This is necessary if the distance between the supply filter capacitors and the regulator is more than a couple of inches.

    C2 is present to help insure stability, and to improve transient response. If C2 is increased significantly, you must use a diode back to the input, in case the input is shorted, or discharges rapidly. If V(Vout) is significantly higher than V(Vin), the LM317 may be destroyed.
     
  6. RyanD

    Thread Starter Member

    Jan 14, 2010
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    Those values are just what I know, I read something that said on power lines(which this is) you should use larger caps to provide more power in case of dips. Since the incoming line is higher(~12v) I use the 100uf and after the regulator when it's lower(5/3.3v) I use the 10uf. Would it be kosher to add a 0.1uf on the regulated side for quick burst of power to components?

    Yeah, I was able to figure out that the different values change the voltage you get on the output(from reading the datasheet). What I don't get is why can't I just use one, why must I use two?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Typically, the small 0.1uF caps have low inductance/impedance. You want them as close to the power/ground pins as possible. Larger caps you put just after the small caps.

    Generally, larger capacitors have higher ESR's, especially if you're buying cheap. So, the large caps fill up the small caps, the small caps take care of the high frequency stuff, the large caps take care of the low frequency stuff.
     
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