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I am trying to connect two components to the same 12V/1000mA DC power source. The devices are 300mA/9V (D1, auxiliar device) and 500mA/3.3V (D2, ESP8266) respectively. See the diagram below:

enter image description here

Given that R1/D2 and R2/D2 are connected in parallel I can simplify the resistance in the circuit using the following expression:

$$\frac{1}{R}=\frac{1}{R1+30} + \frac{1}{R2+6.6}$$

And the final expression using the formula R=V/I=12V/1000mA is:

$$\frac{(R1+30)(R2+6.6)}{R1+R2+36.6}=\frac{12V}{1A}$$

Is there any way to get proper values of R1 and R2? Thanks in advance!

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  • \$\begingroup\$ No, there isn't, because the loads D1 and D2 will change. Use voltage regulators instead. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 24 '15 at 3:07
  • \$\begingroup\$ Didn't have in mind that loads could change, thanks \$\endgroup\$ – JFonseca Aug 24 '15 at 14:25
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If the resistance of D1 and D2 does not change and 12v power supply is stable, you can simply calculate R1 by ignoring R2 and D2:

R1=(V1/I1)-D1

R1=(12/0.3)-30=10 Ohm

You can calculate R2 by ignoring R1 and D1 as well.

BUT, actually the resistance of the load (ESP8266) will change when it turn on. Connecting the resistor to 12v supply will risk damaging the devices. Use voltage regulator instead. You can use LM2576 regulator adjusted to 9V and 3.3v.

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  • \$\begingroup\$ Many thanks Oka, nice explanation. Excuse me, could I use a NTE1904 instead of the LM2576? \$\endgroup\$ – JFonseca Aug 24 '15 at 14:22
  • \$\begingroup\$ Yes, NTE1904 works for 3.3v, with a proper heatsink. It has much lower power eficiency than LM2576 \$\endgroup\$ – Oka Aug 24 '15 at 14:48
  • \$\begingroup\$ Perfect, one last question please :) The output current limit of the NTE1904 is 1000mA but the ESP8266 supports 500mA only, should I place a 3.3V/1A = 3.3Ω resistance in between? \$\endgroup\$ – JFonseca Aug 24 '15 at 18:35
  • \$\begingroup\$ No, we do not need any resistor in between.1A is maximum current, the actual current is lower depends on the load resistance (I=V/R). \$\endgroup\$ – Oka Aug 25 '15 at 0:12
  • \$\begingroup\$ Without any additional resistor, ESP8266 already has resistance much higher than 3.3 Ohm \$\endgroup\$ – Oka Aug 25 '15 at 0:15
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The ESP8266 is a WIFI radio and microcontroller IC. As with most radio related devices, the current will change dramatically when transmitting and when not. Below is a graph of one. It jumps from anywhere between 20mA and 360mA. Using a voltage divider to regulate current will NOT work, unless you want to fry the chip.

http://blog.hekkers.net/wp-content/uploads/2015/03/INA219_Sampler_650.jpg

A pair of switching regulators will be much better.

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