7 Replies Latest reply on Mar 22, 2019 2:44 AM by Jan Cumps

    Diodes - calculation, values, designation, size code, how to search them...


      I'm learning about Diodes lately all alone with little help from one gentleman/lady on Reddit who was only one to help until i have got this website and watched few videos about diodes, but still i do not get them fully to understand by their Values and Codes to do specific task.

      Since i understand how they function finally, i still can't get them by Values, i'll explain.


      First about Circuit i'm building:


      3.6V - 3x1.2V 2400mAh AA rechargable batteries


      5 Red LEDs 20mA 2V - 0805 ... resistor for LED's 21Ω

      5 Blue LEDs 20mA 2.8V - 0805 ... resistor for LED's 10.8Ω


      6 White LEDs 20mA 2.8V - 0805

      8 White LEDs 20mA 2.8V - 0805

      ... resistor from 1.st Switch 3.58Ω (I have found how to search code for SMD resistor or colour code for ordinary resistor... which is not my best side)

      = 14 White LEDs separeted from others with a diode (on pictures are 2 diodes but i have change that, i think it's better to separate with 1 diode)


      SPDT Switch micro

      SPST Switch micro


      Diode (the trouble)


      On pictures there's simple ordinary diode, but i have found and was adviced that Schottkey diode will be better and that's right, at first i have thought that if i put Vf 3.6V i'll get 2.8V for LED's as for example diode needs 0.8V or 800mV to work or something like that... ""that worked well""! hahaha... Later on Reddit person told me that that's wrong and i figured out somehow that i need diode Vf 0.8V or 800mV for powersource of 3.6V to gain 2.8V for LEDs but problem is not solved i need to regulate mA and i need 280mA (for full capacity of all LEDs) but i do not want use full capacity but for 3.5mA per LED less which means 14x3.5= 49mA less for all LEDs so in total i need 231mA. Now i'm not certain what to do for "If"... should i put If 231mA (closed number is 250 on one producer webpage) or put higher A (amps) then regulate it with current limiting resistors and will they have impact to V?


      1. What should I/We be carefull about when choosing Diode?

      2. How to read their designations, are they similar to resistors for example - SMD - 32X/S=21Ω, 04X/S - 10R8=10.8Ω?

      3. How to calculate Vf and If for needed circuit? As mentioned person form Reddit told me and i understood, "If" can be high to "A" but when are "mA" needed that's      how much "mA" will be drained from diode, is it correct?

          - For example if "If" is 3A, but if circuit drains 300mA, 300mA will be drained form diode no matter of If 3A as it is limit how much diode can withstand in total.

      4. How are their rating of their size and if we need for example 0805 diode how can we find same/similar diode with needed values?

        • Re: Diodes - calculation, values, designation, size code, how to search them...

          Hi Mihovil,


          What are the diodes needed for in your circuit? At a high level, they conduct electricity only in one direction, and require at least 0.6V to do so (known as the "forward voltage").

          LEDs and resistors are cheap, this is the type of circuit you could experiment with, along with a multimeter, to familiarise yourself with controlling LEDs, and understanding diodes in general.

          My first electronic components were 2 LEDs, and five resistors from Radio Shack. It was enough to start learning how it all works (and LEDs are a type of diode, with a higher forward voltage).

          To learn it better I needed books (Internet also helps of course, but a beginners book goes a long way).

          You can also simulate and measure virtually all the currents and voltages you wish, see here for a resistor example:


          1 of 1 people found this helpful
            • Re: Diodes - calculation, values, designation, size code, how to search them...

              Hi shabaz


              In this case diodes (on pictures) should separate White LEDs from other LEDs, recently i have change that to 1 diode but didn't take pictuore of it, so in conversation with one person i took Schottkey diode, as it is much precize, but what confuses me about them is how to search for them...

              I have learned that Vf should be in this circuit (for one diode) 800mV - 0.8V to reduce 3.6V to 2.8V (needed from 14 0805 white LEDs) but stil li'm concern about mA, if i take for example If 500mA in diode, but LEDs need 280mA for full capacity will they be supplied with 500mA or 280mA?

              That's what confuses me about values, and if i put resistor to lover it to 231mA would it change also 2.8V to for example 2.6V?


              Next, when i know Values that i need for "Vf" and "If" how to find size of diode or code of diode which will have closest or same values?

              For example i need SMD 0805 (Mostly i can find only LEDs with that size code not diodes) as i found out it should be for Diode SOD/SOT 523 but not sure (this was lucky)

              Why i ask? because for example SMD resistors are 0805 size (which i need) and their value code is for example 21Ω in code it's 21R0 or 32X or S ... is it simmilar for Schotky diodes?


              How they did coding for values on Diodes and how they designate them? There ara many of them but it seems it's mess. How do you search for them?


              There is alldatabase.com and there is almost everithing but what's the point when i do not know how i can search diodes with specific values when i can't put values in code or designation to find it directly, in all sizes. I can spend a year by sistematically search of names, designation and codes to find right one with needed values in Vf, If, and other.

              1 of 1 people found this helpful
                • Re: Diodes - calculation, values, designation, size code, how to search them...

                  Hi Mihovil,


                  I think I understand what you're line of thinking is, but there's some extra information that will show that this method of controlling LEDs is not good.

                  For reasons you'll discover in books/courses, Vf is not precise and (among other things) varies with current changes too. Change the current, and the Vf value changes : )

                  As a result, a normal diode used in this way makes a bad voltage regulator - so the circuit is not ideal if it is relying on diodes with particular Vf values to get to the correct voltage to drive your LEDs.


                  The thing you should be looking to control is the current through the LED, and there will be a recommended value for that, indicated on the LED datasheet. Once you know the desired current, you can estimate the voltage drop across the LED (it will be the Vf value in the LED datasheet), and then choose a resistor using Ohm's law.

                  This is covered here:

                  Ohm's law - LED resistor calculation | LEDnique


                  This is if I've understood your requirement and circuit correctly (I may not have, I have only finite time to dedicate to each issue). If you wish to explore more, some recommended books are here:

                  I Want to Do Electronics!  Where do I start?

                  Otherwise, I'd strongly recommend going through these videos: The Learning Circuit   , focus on Ohm's law, resistors and diodes sections, and trying out a few of the circuits (either for real or using the simulator), and confirming voltages and currents for yourself with a multimeter (real or simulated). It's the quickest way to get you confident with how to drive LEDs.


                  Also, until you're confident (or are happy experimenting) it is good to stick to existing circuit patterns, because there are many underlying reasons why particular circuits are used. You can look anywhere, and see LEDs and resistors in series, It isn't good to try to reinvent the wheel and use diodes to replace or augment the resistors, unless you're experimenting and learning, because it's an advanced thing to try to modify a circuit (it requires some studying), and usually baby steps should be taken (i.e. light an LED in the conventional manner with a resistor first), learn Ohm's law, and so on.

                  4 of 4 people found this helpful
                    • Re: Diodes - calculation, values, designation, size code, how to search them...

                      Thank you very much for clarfying to me ishue about diodes, i thought i'll be able to control Vf and If with them without using another resistor after Diode. I'm usually fast learner if something interest me highly, but sometimes i obviously overthink some things trying to find perfect simple solution (many times it works and don't, it helps me to learn).

                      Question: If i use this diode as example (diode) with Vf 0.3V If 3A when i connect it to source i have to measure Vf and If than calculate for proper resistor? The point is just to block current from white LEDs to get to others. Ofcourse i can do with 3 switches but there's no fun and need to check every time which one is closed/opened.

                      I'll watch all of these videos because i want to learn as much as possible for arduino but now i had to find solution for diodes.

                        • Re: Diodes - calculation, values, designation, size code, how to search them...

                          Check the Shockley equation https://en.wikipedia.org/wiki/Shockley_diode_equation , it is very close to the real thing. After understanding that, and also ohms law, you can figure out exactly how much current will pass through the diode with a specific resistor at a specific voltage source. Also be warned that semiconductors vary a lot, so with your proposed circuit some diodes could appear brighter (more current through them) than others.

                          • Re: Diodes - calculation, values, designation, size code, how to search them...

                            Hi Mihovil,


                            Try to build up a circuit with just a few series LEDs, and a resistor, and a switch, inside a simulator (e.g. the one mentioned earlier), and you can observe the current.

                            Then, try to build a circuit that is parallel to that (the only points where both circuits should connect, are at the battery terminals. and see if there is any need to block current.


                            It's unclear what you're trying to do, but if the aim is to have behaviour such as (say) turn on some LEDs and turn others off automatically with a single switch, then that needs to be achieved with a multi-throw switch (e.g. SPDT or DPDT), or with something like a relay.


                            Attempts to look for low Vf diodes are strange for switching purposes for the types of circuit you've shown. Basically if you need to look at the diode Vf with any accuracy, then the circuit is wrong. Your circuit should be achievable with switches, resistors and LEDs, and the simulator can help you put some of the theory (diode drop or Vf, and Ohm's law) in practice (or try with a real circuit too, e.g. with a breadboard, and real multimeter).

                            2 of 2 people found this helpful
                            • Re: Diodes - calculation, values, designation, size code, how to search them...
                              Jan Cumps

                              just a heads-up that you can’t put leds (any diode ) in parallel. They will not divide the current nicely amongst each other, because tolerances are not tight.

                              There will be leds taking more current than others. And they will get warmer.

                              And because of their temperature behaviour, these leds will then take even more current than the others.

                              This is a circuit that can’t be controled.

                              You will need a resistor in series with each led.

                              2 of 2 people found this helpful