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5033 Views 7 Replies Latest reply: Sep 24, 2014 3:07 PM by kelliemoak123 RSS
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Aug 12, 2014 9:07 PM

General Question on Resistors

Good day

 

I just have a very novice question, aside from wattage rating, is there any other consideration for choosing a resistor, for both AC and DC circuits? Simply, if you were designing, do you have an AC resistors and DC resistors container or are they are both completely  interchangeable in usage and you just have to mind the wattage rating of your circuit?

 

Thanks!

  • KISS Novice 4 posts since
    Jul 19, 2011
    Currently Being Moderated
    Aug 13, 2014 8:39 PM (in response to Riley)
    General Question on Resistors

    Every component has parasitics.  Unwanted capacitances and inductances.  Even short leads have inductances and they matter at microwave frequencies.

     

    There are "non-inductive" resistors. and they would be a good choice for current monitoring for motor loads.

    I read of a Tektronics circuit that had 4 identical resistances in series and the circuit will not work if a single resistor is used.    The parasitics were important in the circuit.

     

    Resistors have volltage ratings too.  Soemtimes resistors are placed in series to increase the voltage rating.

     

    You have to know when to ignore stuff,  Waving a wire in the air generates a current and in one set up that was a nusence to me.  The wires had to be taped down so they would not move.  The wire is moving in the Earth;s magnetic field, so yes, it generates a current.

     

    Coax cables generate a voltage when flexed.  Some have graphite between the shield and the outer jacket to help reduce those effects.  I had to deal with those effects.

  • KISS Novice 4 posts since
    Jul 19, 2011
    Currently Being Moderated
    Aug 17, 2014 9:49 AM (in response to Riley)
    General Question on Resistors

    yep, every project has different design constriants. With DC, low voltage and low frequency there aren't too many problems.  But if you add VLF frequencies for radio transmission, or 0.8V logic voltages or very long battery life you might have different issues to solve.

  • Scott594 Novice 6 posts since
    Aug 21, 2013
    Currently Being Moderated
    Sep 10, 2014 5:09 PM (in response to Riley)
    General Question on Resistors

    One consideration often overlooked is voltage rating. In this day of low voltage everything, resistors have evolved to carry fairly low voltage ratings. I build tube audio gear, and it's easy to exceed a 300v rating common on film and carbon resistors. They don't last long if you exceed the rating.

     

    On the AC/DC end of things, most are mildy inductive, with film and wirewound ones being the worst. A careful read of the spec sheet will tell you how much inductance to expect, and it matters a lot in RF equipment, even in switching supplies depending on circumstances.

     

    There is a resistor tailored for every application, that's why there's hundreds of pages of them in the catalog. A 1/2W carbon used to be pretty universal, but today, you can get VERY specific with a resistor spec. It can drive you nuts reading when you just want a dropping resistor for an LED or something.

  • JimHarrison Apprentice 84 posts since
    Sep 9, 2011
    Currently Being Moderated
    Sep 11, 2014 12:05 PM (in response to Riley)
    General Question on Resistors

    Important resistor considerations are the tolerance (from ±10% to maybe ±0.001%), Temperature coefficient (maybe 50 ppm/°C), power rating (say 1 W), and size (surface mount or through hole).

  • AnneC Novice 6 posts since
    Sep 20, 2014
    Currently Being Moderated
    Sep 20, 2014 11:35 PM (in response to Riley)
    General Question on Resistors

    Also, about the power rating. It's not enough to just select a power rating that is above the E x I. Which would be fine per see on a breadboard. But when you consider your design going into a enclosure, your resistors will generate heat in the box. You have to make sure that this will not adversly affect your circuit' stability. The same concept applies for all components responsible for heat generation in your design.

  • kelliemoak123 Novice 8 posts since
    Sep 22, 2014
    Currently Being Moderated
    Sep 24, 2014 3:07 PM (in response to Riley)
    General Question on Resistors

    Waving a wire in the air generates a current and in one set up that was a nusence to me.  The wires had to be taped down so they would not move.

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