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Need help with simple electronics circuit

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  • #16
    You will want to experiment with resistor values in any case. All LEDs are not created equal. As the color type goes from red towards white the voltage drop increases, typically to almost double, and if you were me you would be likely using some unknown surplus or bargain basement device of unknown spec anyway. Although I see that PSTechPaul (whose circuit is a big improvement over the original) has used actual identifiable part numbers, if you want to take the trouble to source those.
    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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    • #17
      I found a spec sheet for that LED, and it is a "super bright" 0402 surface mount device, which is available in red, orange, yellow-orange, yellow, and yellow-green. All are specified as having a forward voltage of 2.0 to 2.4 volts at 20 mA. It is the lowest forward voltage of all the LEDs in the LTSpice database. Looking at all thru-hole LEDs from Mouser, only one has 1.5V Vf, and most others are 1.6 and higher. All are red. There are some SMT LEDs with Vf of 1.2 to 1.4, but all are infrared. For 1.5 to 1.7 volts, there are red, orange, and yellow.

      https://www.mouser.com/datasheet/2/3...ag-1195966.pdf

      http://pauleschoen.com/pix/PM08_P76_P54.png
      Paul , P S Technology, Inc. and MrTibbs
      USA Maryland 21030

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      • #18
        I assumed about a 1.75 Volt drop in the LED. If that is higher, then the 1.7 in my first equation would need to be increased to 2 or 2.5 or so.

        The Voltage drop in a given LED is fairly constant, regardless of the current flowing through it. It is the resistor or other circuit elements that determine how much that current is. And the calculation is just Ohm's law, pure and simple, nothing more. I worked in electronics for many decades and at least 90% of the calculations that I had to make were just using Ohm's law. It applies almost everywhere in electric circuits.

        If you don't know the forward Voltage drop for a given LED, you can use a power supply and a fairly large resistor, say 2K Ohms and just put a meter across the LED and measure it.



        Originally posted by PStechPaul View Post
        I found a spec sheet for that LED, and it is a "super bright" 0402 surface mount device, which is available in red, orange, yellow-orange, yellow, and yellow-green. All are specified as having a forward voltage of 2.0 to 2.4 volts at 20 mA. It is the lowest forward voltage of all the LEDs in the LTSpice database. Looking at all thru-hole LEDs from Mouser, only one has 1.5V Vf, and most others are 1.6 and higher. All are red. There are some SMT LEDs with Vf of 1.2 to 1.4, but all are infrared. For 1.5 to 1.7 volts, there are red, orange, and yellow.

        https://www.mouser.com/datasheet/2/3...ag-1195966.pdf
        Paul A.
        SE Texas

        Make it fit.
        You can't win and there is a penalty for trying!

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