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Topic: Very simple resistors question (Read 697 times) previous topic - next topic


Hello :)

I have a simple question about calculating resistor.

I have lots of leds driven by negative, wired with one resistor for each one and 2 leds wired together. I can't modify this as it's enclosed in a box. This is the schematic on the left.
I need to lower the current in the leds, to lower the light amount.

My only option is to add a resistor, like on the right schematic. One resistor for 2 leds+resistors.
How can I calculate the new current flowing through the leds or equivalent resistor? Everything I've found on internet is parallel or serial network, but never parallel + serial.

Thank you :)


You know the voltage drop across the left hand network (5V) and you can measure the current flowing through it. That tells you its resistance (Ohm's law).

Now imagine the whole thing is just a big resistor with that value... the rest is easy.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)


James C4S

The problem is kind of complicated by the LEDs since they aren't linear like the resistors.

This is a good time to make use of Falsted's simulator... :)

Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com


If the LEDs are different colours you are likely to run into problems this way - if the same
colour then it will work better (but current distribution won't be as balanced - this may not matter)

Another approach is to PWM the supply to the existing LEDs - reducing the average brightness
without changing the resistors.
[ I won't respond to messages, use the forum please ]

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