Resistors?

Discussion in 'The Projects Forum' started by Gadersd, Jul 9, 2013.

  1. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
    Things like diodes and transistors need resistors while things like motors and ics don't need resistors. How can I find out if something needs a resistor or not? Whenever I look at data sheets I see the max current. I don't know whether this is the max current it will draw without a resistor or if it is the max current that can sadly pass through it. Where in the data sheets can I find out if it needs a resistor or not?
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,363
    There is no rule that says diodes and transistors need resistors.
    Show a circuit diagram and then we will say if it is correct or not.
     
  3. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
    I want to connect a laser diode to a 2.5v power supply. The data sheet says that it needs a min of 45ma and a max of 60ma. It is a diode so it defiantly needs a resistor, but some electronic components don't need a resistor. They have enough eternal resistance to prevent a short circuit and keep the current from going too high. How would I be able to tell from the data sheet that it has enough external resistance to not need a resistor?
     
  4. Ramussons

    Active Member

    May 3, 2013
    557
    92
    Thumb rule:
    If the specs define a Max or Min value for a parameter (voltage, current, wattage, temperature, ...), then this parameter is to be controlled externally.

    In the present case, the " ...data sheet says that it needs a min of 45ma and a max of 60ma ..." means that the current needs to be externally limited normally by a resistor or current source.

    Ramesh
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,056
    To be honest, it's mostly experience. After a while, you come to "know" and hardly give it any thought. You'll get there.

    In the meanwhile and for everyone, a useful way to look at things is to assume everything needs current limiting until you prove otherwise. Any path for current to flow is a potential problem for the components in that path, until you can identify something in the circuit that is restricting current flow to within the specifications of each component, especially the weakest link. ICs that need power are usually meant to be connected directly to the power rails without current limiting. (Their outputs may need protection, but not their power supply.) LEDs need a specific limit on current. Transistors, resistors, diodes and such all have limits on how much current they can take (usually it depends on the power they must dissipate) and you need to make the calculations to be sure you're OK. These components cannot on their own protect themselves.
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,363
    Basically, spec sheets alone do not tell you when you need resistors.
    Know-how will.
    Learn the fundamentals of electronics and you will know the answer.
     
  7. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
    Data sheets don't specify the internal resistance do they? To calaculate the current that will pass through it I would need to know the internal resistance.
     
  8. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    For a laser diode you probably need something a bit better than a resistor, especially if you want it to run at the same power all the time. Search for:
    laser diode driver circuit
    Some of the links will probably explain why it is a good idea.

    <ed> Looking at your other thread I see you already have </ed>
     
    Last edited: Jul 10, 2013
  9. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,238
    ICs normally take the current they require without any external impedance required. The current they take is given in the data sheet, sometimes with min and max limits. The internal resistance may be quite non-linear, i.e. the IC will have an operating current that varies only slightly with applied voltage. That's typical of op amps, for example.

    If an IC requires an external impedance it should be apparent from the data sheet.
     
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