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salesm21

Attatched I have a photo of a small circuit I set up (I know its simple but I am just starting out) and realized something weird. When I put my test leads over my resistor I got very low resistance on my resistor which I thought would take the majority of the drop since the led offers little resistance. Also odd was when I turned my MM onto amps the DC power supplies current reading went up. Why is that? image

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  • You've got a 3.2V voltage source (your PSU) driving a resistor and LED in series. The LED behaves like a diode with quite a high forward voltage drop.  I found a nice cheap blue LED a bit like yours on Farnell ( 2377467) - if you download the data sheet you'll see that they specify the forward voltage as 3.3V typical at 20mA. Diodes and LEDs are very non linear for for their forward voltage/current relationship but for your diode to drop about 2.7V at 3mA (which I can read on you psu) seems quite reasonable.

     

    Ohms law doesn't apply to diodes ! The current rises much more rapidly as the forward voltage increases than it does with a resistor.

     

    Here's a tutorial on LEDs (looks OK but their are many others):  https://www.electronics-tutorials.ws/diode/diode_8.html

     

    Hope this helps.

     

    MK

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  • You've got a 3.2V voltage source (your PSU) driving a resistor and LED in series. The LED behaves like a diode with quite a high forward voltage drop.  I found a nice cheap blue LED a bit like yours on Farnell ( 2377467) - if you download the data sheet you'll see that they specify the forward voltage as 3.3V typical at 20mA. Diodes and LEDs are very non linear for for their forward voltage/current relationship but for your diode to drop about 2.7V at 3mA (which I can read on you psu) seems quite reasonable.

     

    Ohms law doesn't apply to diodes ! The current rises much more rapidly as the forward voltage increases than it does with a resistor.

     

    Here's a tutorial on LEDs (looks OK but their are many others):  https://www.electronics-tutorials.ws/diode/diode_8.html

     

    Hope this helps.

     

    MK

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