Understanding voltage dividing [duplicate]

Question

I read here about voltage dividing. Let's look at this image for the discussion:

Ok so we have 2 resistors in the circuit, and Vout is of lower voltage. But I don't understand - voltage is always a difference between two points. So, with respect to what point is Vout has a lower voltage?

If you say with respect to ground - then, why in this article it says:

Keep in mind not to use a voltage divider to reduce the voltage to supply a device or load because you have to remember that an external device will result in a parallel circuit with R2.

So actually if you take Vout, connect it to the + terminal of a device, and the - terminal to GND, the voltage of this circuit will be dependent on the resistance of the device. So what is this arrangement of voltage dividing good for?

I'm actually asking this because I want to supply my ESP8266 with power from a 9V battery, so I thought of using resistors to divide the voltage. But for this, I need to know the resistance of my ESP8266, which is not shown in the datasheet.

Answer 1

So actually if you take Vout, connect it to the + terminal of a device, and the - terminal to GND, the voltage of this circuit will be dependent on the resistance of the device. So what is this arrangement of voltage dividing good for?

It's good for driving very high resistances, where the load resistance has no significant effect on the output voltage. It is no good for supplying power to loads, especially variable ones.

I'm actually asking this because I want to supply my ESP8266 with power from a 9V battery, so I thought of using resistors to divide the voltage. But for this, I need to know the resistance of my ESP8266, which is not shown in the datasheet

No, do not do this.

A load like an ESP8266 does not have a constant resistance. It draws more or less current as its activity changes.

Use a regulator, that can supply more or less current as the load's demand changes, while keeping the output voltage constant.

A 9 v battery is not a good choice where the load current is significant, like that of an 8266.

Answer 2

As Neil_UK's answer points out this arrangement is not good for power supply because it will change the resistance value if connected element is not high impedance element.

For power supply, you can use linear regulators like 7805 for 5V supply. You can also find 3.3V version of linear regulators. The higher voltage will be dissipated by mostly heat for linear regulators so it maybe more appropriate use switch mode regulators like lm2596. This regulators are more efficient and heat is not as problematic as in the case in linear regulators. But you should be careful about choosing capacitor and inductor values. There are cheap modules you can use which includes all capacitor inductor and resistance values for lm2596.

For your question about using resistor divider circuits they can be used for sensing circuits. For instance if you want to sense 9V with MCU, you can use voltage divider and since MCU digital pin will have higher impedance, there will be no significant change in resistance. But more appropriate usage will be by using voltage follower. Op-amp will have very high impedance and there will be no change. You can use it like;

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