Voltage divider tutorial


This tutorial is about voltage dividers. A voltage divider circuit uses resistors
to reduce voltage. You can use voltage dividers to do a
lot of things, like create a volume control circuit or generate
reference voltages and much more. If you apply Ohm’s law you’ll find that
one volt spread across two equal value resistors gives you
half a volt in the middle. So you are getting half the voltage. And as you would expect, four equal value resistors would give
you 1/4 volt, 2/4 volt, and 3/4 of a volt. For the sake of simplicity, let’s
just use two resistors. If you use two resistors of different
values, the voltage gets divided up in proportion to the amount of resistance
there is. In this example with twenty percent of
resistance on the bottom, you get twenty percent of the
input voltage on the output. If swap the position of these resistors, now with eighty percent of the
resistance on the bottom, you get eighty percent of the input voltage
on the output. By the way, voltage dividers will work
fine with both AC and DC input voltages. And this is the equation you should use
to calculate the output voltage voltage depending on the resistors
you are using. And if you are not getting the output
voltage that you want, try the equation with some different resistor
values, or rearrange the equation. Alright, here are some more examples. Here I am taking ten volts and dividing
it down to one volt. Here I’m taking five volts and dividing
it down to four volts. And here I am taking 100 volts and dividing
it down to one volt. You might have noticed I’m using extremely different resistance values
here… sometimes a few ohms… sometimes a few megaohms… but it is
ultimately the ratio of resistance values that determines the output voltage. Now that does not mean that when you
build a real world voltage divider you should use low value resistors. Whenever you create a voltage divider you
have to be aware of the overall power consumption of the circuit. And you calculate it using this equation. With this circuit, the resistors are
dissipating 0.1 Watts which is very little so everything is fine. But in this circuit the resistors are
dissipating a thousand watts! So the resistors are going to catch fire! The output voltage the same in both cases, but the power consumption is totally
different. Also before you create the voltage divider,
you have to be aware of how much current your voltage divider has to supply… because voltage dividers are not the same
thing as regulated power supplies. There is going to be a voltage drop
as soon as you connect your device to
the voltage divider, depending on how much load there is. For example, here is an unloaded voltage
divider giving me five volts. When I connect a microcontroller that draws
just one milliamp, the voltage drops to 4.56 volts. And that’s not so bad… the microcontroller
will still work… But if I try to power this motor that would
normally draw 100 milliamps from 5 volts, the voltage drops down to 0.5 volts,
and the motor won’t work. So this voltage divider would be a terrible
power supply for motor. In general, you shouldn’t be
using a voltage divider if you have to supply more than ten milliamps. But if you
want to screw around with resistor values and see what you can get away
with, you can calculate the Thevenin equivalent resistance of the voltage
divider using this equation. And yes it also happens to be the
same as the parallel resistance equation. Once you have calculated the Thevenin
equivalent circuit, you can calculate how much voltage your
load is really going to receive using the voltage divider equation from earlier. Now what are some real-world circuits where
you could use a voltage divider? Well an easy one is a volume control
circuit that you could use before an amplifier. And all you are doing here is just
dividing voltage with a potentiometer. In this amplifier circuit, I’m using the
voltage divider to add a DC bias to an audio signal so that the transistor can
turn on and amplify the signal properly. And finally the most obvious use of all is
powering devices that expect a certain voltage and only draw tiny
amounts current, like a cheap calculator or something. And you should really be using a voltage
regulator to do this, but what can I say some people are cheap. Alright I’m done with voltage dividers and
thank you for watching!