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The need for resistance

Discussion in 'General Electronics Discussion' started by Dustin fausnaugh, Nov 18, 2014.

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  1. Dustin fausnaugh

    Dustin fausnaugh

    Nov 18, 2014
    Why do you need resisitance ?or do you not need it?
  2. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    Aug 27, 2013

    Welcome to EP!

    Well to quip a quote from the Borg...."Resistance is futile!"....

    OK, Seriously, resistance is one of three PASSIVE properties associated with electricity, the other two being Capacitance and Inductance....As a component in an electronic circuit resistors might be used to limit current or create a voltage some cases they might even be used as heating elements....for instance in a hair-dryer a special type of wire with a relatively high resistance is used to generate heat from the power dissipated by a current flow through it....In other cases resistance is the enemy of efficiency....for instance in High Power transmission lines from power generating stations to homes and businesses resistance in the large copper conductors means that some amount of power is lost just getting from one place to in this case resistance is "bad"....

  3. Dustin fausnaugh

    Dustin fausnaugh

    Nov 18, 2014
    Thank you fish , I'm new to electronics and I'm just trying to learn what it's all about.
    Last edited by a moderator: Nov 18, 2014
  4. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012

    Electronics is all about having fun!
    Arouse1973 likes this.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Sometimes resistance causes losses (an example is the resistance in electrical wires supplying your house). In this case resistance is something we try to minimise.

    In other cases we use resistance to limit current, or to drop a voltage. Examples of these are simple timing circuits and voltage dividers (like volume controls). In this case, without resistance the circuit could not operate.
  6. LvW


    Apr 12, 2014
    Electrical "resistance" is nothing else than a property of each material under influence of an electrical voltage. And this "property" determines how much current can exist - as a result of this voltage. Some materials have very large resistances (so called "isolators" with R>1E12) and other materials have medium or very low resistance values. In particular the latter group is used in electronic circuits to fulfill some specific tasks - as outlined in the former answers. .
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi Dustin and welcome to Electronics Point :)

    Continuing from what Steve wrote...

    As Steve said, resistance, as a phenomenon, is not necessarily useful. When we specifically want resistance in a circuit, we use a resistor.

    The behaviour of a resistor is defined by Ohm's Law, which says:

    I = V / R

    I = current flowing through the resistor, measured in amps;
    V = voltage across the resistor, in volts;
    R = the resistance of the resistor, in ohms.

    Ohm's Law is used when R is constant. In the real world, no resistor is perfect, but imperfections and tolerances are a normal part of all kinds of engineering, and real world resistors, even ones you can buy for less than $0.01 each, are actually fairly close to perfect, so the approximation that R is constant is nearly always close enough.

    Given that R is constant, Ohm's Law says that I (current through the resistor) and V (voltage across the resistor) are directly proportional to each other. Doubling one will double the other, and so on. So it's fair to say that a resistor converts a voltage into a current, or converts a current into a voltage.

    Resistors in real world applications are used to do both of those things, but the first one is much more common. A voltage is applied across a resistor, and this causes a current to flow. The current is proportional to the voltage, and the quantity that defines the relationship between them is the resistance of the resistor.

    A very common use for a resistor is to limit, or set, the current through another component when a voltage is present; a resistor used in this way is called a current limiting resistor or a current setting resistor. A common example is making an LED glow:

    Battery supplying LED via resistor.png

    In this circuit, battery BT1 supplies a fixed voltage - say 9V, which I've labelled VTOTAL. This total voltage appears across the series combination of resistor R1 and LED LED1. Because voltages in series add together, VR1 + VLED1 = VTOTAL.

    Current will flow from the positive terminal of BT1, through R1, through LED1 and back to the battery. To make this particular LED glow at the desired brightness, we want that current to be around 10 mA.

    Because of the characteristics of LEDs, the voltage across an LED (called the forward voltage, VF) falls within a fairly narrow range. For a typical red LED, VF is around 2V.

    Since VTOTAL is 9V and VLED1 is about 2V, VR1 must be about 7V. We can now use Ohm's Law to calculate the value to use for R1.

    We know that there will be about 7V across the resistor, i.e. V = 7. And we know that we want about 10 mA to flow through the resistor (and through the LED; currents in a series circuit are all the same because it is the same current flowing through all parts of the circuit).

    Ohm's Law rearranges to R = V / I. Plug in the values:

    R = V / I
    = 7 / 0.01 (remember, the current, I, must be in amps, not milliamps)
    = 700Ω

    Resistors aren't available in all possible values; they're only made in ranges called the "preferred value series", and you usually have to choose the closest preferred value. Say we choose 680Ω.

    The resistor is being used to set the LED current. We know that there will be about 7V across it, so we chose a 680Ω resistor to give a current of about 10 mA. This is a classic example of a resistor being used to limit or set the current flow when the voltage across it is roughly known.

    Resistors are used in several other ways but that was an introduction to a very common case.
    hevans1944 likes this.
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