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Forward Current, Forward Voltage, Reverse Voltage...

Discussion in 'General Electronics Discussion' started by TehMaxwell, May 21, 2015.

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  1. TehMaxwell


    Mar 21, 2015
    Having decided recently that I wanted to do electronics more at home, I decided to order in some parts that I had been using at school. When the arrived I looked at the data sheets to see the voltage and current requirements for the IC's and other components etc. I keep seeing the words Forward Current, Forward Voltage and Reverse Voltage. Can anyone explain to me what each of these terms means?

    Also with current, I understand that is an extremely bad idea to pass too high a current through a component. For example when driving an LED with an output from an IC, you use a resistor in series with the LED. I have always been told its a "protective resistor", but what are you actually protecting? An LED has very low resistance and thus will draw a high current. So are you protecting the IC?

    Finally, with current. I understand that you draw current? Does this means that a component will circuit will only draw the current it needs? For example if you have a 500ma power supply, would a circuit only draw 300ma from it?

    Its a really fundamental bit of knowledge, and I feel kind of stupid for asking. But I guess I have never really needed to know about this kind of circuit design/construction.

    Thanks for all the help,

  2. davenn

    davenn Moderator

    Sep 5, 2009
    The resistor is protectinv the LED from excess current ev a standard LED requires around 20mA at about 2.5V. So if the LED is bsing supplied with 12V the resistor value is calculated drop the current max to 20A. There is a section on driving LEDs in the resources section of the forum.

    Drawing current and current needed is one of the most asked questions on forums like this
    The current through a given circuit is determined by the resistance (load) it presents to the power supply and the voltage of the supplythis is all basic Ohms Law ...... Learn it well !!! :)
    Eg a 10V supply and a 10 Ohm esistor across it ...... What is the current flow?
    Current I = Voltage / resistance
    So I = 10V / 10 Ohms
    so I = 1 Amp

    Theres the basics ..... Exleriment with the values of voltage and resistance and see how it affects the current

    hevans1944 likes this.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    Please excuse typos .... Im typing on my fone from a hospital bed
    hevans1944 likes this.
  4. davenn

    davenn Moderator

    Sep 5, 2009
    Hi hop

    Took me ages to type all that lol
    Arouse1973 and hevans1944 like this.
  5. Gryd3


    Jun 25, 2014
    I should probably throw this in here as well:
    * Linked from

    LEDs are a unique creature when it comes to drawing current...
    Most devices have a resistance, or a voltage rating so it's incredibly easy to say with certainty that :
    "I will draw 5mA of current if I hook this 1KΩ up to a 5V battery"
    - or -
    "This LM555 timer can work with anything from 4.5V to 16V"

    The tricky part with LEDs is how they behave.. they don't have a voltage ratings like ICs and they don't have a resistance like other components. In reality their 'resistance' will vary based on the voltage present and the temperature. This can make it very difficult to expect how much an LED will draw. Take a look at the image above. For a red LED, changing the voltage by only 0.5V can result in a wide swing in the current draw! (anywhere from 10mA to 30mA). Remember that temperature, manufacturing and color can affect this graph...
    That's why LED 'forward voltage' is provided as a range. If you strictly control the current flowing through an LED at 20mA it could have anywhere from 1.5 - 1.8V across it. Now.. you can use or build a constant current supply for LEDs, and it will make sure it is always putting out 20mA, or you can cheap out and simply use a resistor. The resistor works almost as a buffer. It's entire job is to drop voltage across itself so that if the LED's forward voltage changes, the current flowing through will not change very much. This resistor is calculated essentially by guessing based on the forward voltage drop provided by the LED manufacturer. You can't guarantee that 20mA are flowing through this way, but you can be reasonably certain that no MORE than 20mA will flow through with normal use. A resistor that drops more voltage will be a better buffer than a resistor that only drops a little... There is a catch though. These resistors waste power, so you may find them smoking or burning if you want to drop 7-8 Volts across a resistor to power a 500mA LED. (More on that later if you're interested)
    Personally, when I calculate a resistor value, I always use the 'smallest' expected voltage drop in an LED, and the 'largest' voltage I could possibly see from the power supply. This will help ensure that I don't go over the 20mA specification of the LED.

    Now that I've said all that, there are exceptions to this rule. Many LED accessories, like strip lights, pucks, and even addressable LEDs actually have a working voltage. They are built with drivers or resistors built in and will operate properly as long as you use the voltage provided in the specs. (As that is the voltage they used when they designed the product)
    poor mystic and hevans1944 like this.
  6. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Forward Current, Forward Voltage, and Reverse Voltage are typically specified for diodes, and in bi-polar junction transistors or BJTs for the base-emitter and base-collector junctions, which are also diodes. The forward current is always accompanied by a forward voltage and represents the current the device is rated to conduct without internal damage, provided it has a proper heat-sink. The amount of power created as heat is the product of the forward voltage and forward current. For small devices the heat is conducted to the surrounding air and escapes mostly by convection. Larger devices may require a metal heat-sink to increase heat flow by conduction as well as forced air across the heat-sink to remove internally generated heat by forced-air convection. If the heat-sink gets hot enough to become incandescent, you are probably dissipating too much power as heat and the device will probably fail, unless it is made of silicon carbide which has an insane maximum junction temperature. In any case, what is important is the maximum junction temperature which must not be exceeded. There is always thermal resistance between the semiconductor junction and the outside world, so the junction temperature will always be hotter than the case temperature even if you cool the case with a cryogenic liquid.

    The Reverse Voltage is the maximum voltage that can be applied to a diode junction without electrical breakdown occurring, typically by means of avalanche conduction. Some devices, such as the base-emitter junction of a BJT, are quite sensitive to reverse voltage breakdown and should never be operated with reverse bias so as not to cause reverse breakdown to occur. Other diodes, zener diodes and avalanche diodes for example, are designed to operate in the reverse breakdown region, but with current limiting resistors or other circuitry to prevent failure from excessive current. The reverse current of a diode junction prior to breakdown is typically very small, microamperes or smaller. Once breakdown does occur, only external circuitry limits the reverse current.

    @davenn gave you a good explanation of why a series resistor is necessary when driving an LED from a voltage source. Here is a link to a discussion on the Electronics Point Resources Forums. There is a lot of information on that forum to get newcomers started and old salty dogs refreshed. As for current drawn from a power supply, it always depends on Ohm's Law, as Dave mentioned, until you try to exceed the maximum current capabilities of the power supply, at which point things get complicated and you are likely to let the "magic smoke" out that makes all electronics work FB (fine business). Heh. Avoid also anthropomorphizing about current or anything electronic. Current has no "will" and does not "seek the path of least resistance". It is what it is. Learn Ohm's Law first and, later, Kirchoff's Laws for circuit analysis. Both sets of Laws are applicable to both DC and AC circuits with the proper math to account for inductive and capacitive reactance.

    Oh, I know others have said this, but I will too: Welcome to Electronics Point!
    poor mystic likes this.
  7. poor mystic

    poor mystic

    Apr 8, 2011
    Ohm's Law would make a very useful app, whether for Android or Windows.
    hevans1944 likes this.
  8. Kiwi


    Jan 28, 2013
    hevans1944 likes this.
  9. TehMaxwell


    Mar 21, 2015
    Thanks very much for all the help guys, this makes much more sense now than it did! It's nice to be able to understand how things work rather than just doing what your told to do! :D
    Gryd3 likes this.
  10. Martaine2005


    May 12, 2015
    Hi Tehmaxwell. Some really good explanations and advice above.
    If you are anything like me, (a little slow), there is a guy on youtube who explains with scenarios until you end up SHOUTING ' Yes, I got it '.
    Here is a link to his Ohms law video.
  11. Gryd3


    Jun 25, 2014
    We appreciate it, we really do.
    And while some get a little ... impatient, it helps to remember that at one point we were all beginners.
    I think the biggest reason you sparked so much detail from Dave, Hevans and myself is that you have shows interest in understanding and have already begun to experiment.
    Quite often we get simple questions that can be googled, and the OP expects a one liner response.
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