What are the practical uses of voltage divider?
A typical voltage divider as the picture (I've attached) shows has two resistors, R1 and R2. However, if we put a load on Vout, the original R2 has become a parallel circuit with a different equivalent resistance than R2. So, if the ratio of voltage applied to the load is still depend on the load's resistance, why do we need R2 (or a voltage divider)? Why don't we just simply use the adequate R1 according to the load's resistance to supply the voltage we want? And by this way, we can save energy by not using R2.

 

Voltage dividers are seldom used to produce an output voltage that is intended to provide power to a load, for exactly the reasons you are getting at.

Instead, they are more commonly used to produce scaled voltages to serve as reference, sensing, or bias values. In these situations, the load they see is designed to be considerably higher (like 10x to 100x) in impedance than the parallel combination of R1 and R2 so that the loading effect is minimal.

For instance, in your circuit assume that Vcc was 50V and you wanted to measure it with a voltmeter that could only handle 5V. A simple way would be to use a voltage divider where the output voltage was less than 10% of the input voltage. Since the meter probably has a resistance of at least 1MΩ, you could use 91kΩ for R1 and 10kΩ for R2.

 

If you wanted to control one of the inputs of an op-amp, or the gate voltage to a MOSFET, you could do it with a voltage divider, because generally the inputs to these devices consume no current.

 

If you are trying to monitor a car battery with a 5 volt microprocessor, you can use a voltage divider to get a proportion of the battery voltage that is within the ability of the microprocessor.

If every microprocessor had exactly the same input impedance, which they don't, you could use a series resistor of rather high value, but you would then have a high enough impedance that nearby electrical noise sources would affect the measurement.

You need R2 to get the impedance of the circuit low enough to be immune to radiated energy and to lower the apparent impedance of the input pin enough that all microprocessor inputs appear to be the same impedance (on a production line basis).

 

You need R2 to get the impedance of the circuit low enough to be immune to radiated energy and to lower the apparent impedance of the input pin enough that all microprocessor inputs appear to be the same impedance (on a production line basis).

Can you explain more about the immunization to radiated energy?

Thanks.

 

A highly simplified way of looking at it is that radiated energy induces currents in wires and components. The higher the impedance, the higher the voltage is that results from a given induced current.

 

Consider this:
Grab a guitar cord (with the amplifier turned on) and touch only the metal tip of the connector. You get a buzzing sound in the speakers because the input impedance of the amplifier is so high that the radiated energy from the power wires in the walls is collected by your body and the input resistance of the amplifier is so high that the tiny bit of energy in your body creates a significant voltage.

Now, place a light bulb of any size from the tip of the connector to the shield of the connector. The buzzing will stop if you do this properly because the impedance of the light bulb is low enough that the current you can supply from radiated hum can not develop a significant voltage.