Voltage Dividers

We’ve already looked at resistors and saw how they can impact voltage and current in a circuit.   In those cases, the resistor was placed in series with the other components.  While this is a typical means to deploy a resistor, it isnt the only way they can be used.  In my readings I came across what’s called a ‘voltage divider’.  A voltage divider is a simple circuit that turns a large voltage into a much smaller voltage.  You’re probably wondering how this is dfferent than a resistor (I wondered the same thing).  The bottom line is that ‘voltage divider’ describes a way to deploy resistors so that they act much more efficiently in reducing voltage.  For instance, let’s look at this simple circuit….

Hard to see – but this is just a simple series circuit with a 100 ohm resistor and a green LED.  As expected, we see about a 3.25 voltage drop when we measure across the LED leads (V = I * R, V = .015 * 100, V = 1.5 leaving 3.5 volts for the LED).  So this makes sense and its what we’d expect to see.  Now let’s look at the same circuit with a voltage divider in path rather than a resistor…

So what’s different?  Notice the addition of a second resistor going from the negative power bar to the positive side of the LED.  To clear this up a little bit, let me draw both these circuits out…

So what’s interesting about this is that the by adding another resistor of the same value to the circuit we are effectively cutting the voltage of the circuit down by half.  We know this because the total voltage of the circuit if 5 volts and the LED now only has a voltage drop of 2.5 volts.  If we checked the voltage drop of the first resistor (connecting from the positive lead of the power supply) we’d see that it also had a voltage drop of 2.5 volts.   Want to know something even more interesting?  It doesn’t matter what size the resistors are.  As long as they are the same size, the voltage will be cut exactly in half.  Why is this?  Well here’s the equation to determine voltage output of a voltage divider…

So as you can see – if the resistor values are the same, the voltage will always but cut in half regardless of what size the resistors are.  In the case of 100 ohm resistors or 1000 ohm resistors we get a voltage output of 2.5 volts through the divider.  To clarify what R1 and R2 are, let me draw the circuit a little differently…

There are lots of online calculators out there that will do the math for you.  Sparkfun has a great post on voltage dividers that includes a calculator as part of the article.

Now what about current?  Since we’re still dealing with a series circuit, the current calculation is the same as it was before.  We just need to account for the value of both resistors.  Let’s look at a couple of examples to show what I mean.  First we’ll do the math for when we use two 100 ohm resistors…

Now if we check this out in the lab, we can see that this works out as expected.  We can the voltage drop is 2.5volts as expected…

And the current in the circuit is drawing 25 milliamps (0.025 amps)

Perfect, not lets try changing the size of the R1 resistor to 330 ohms…

Now if we check this in the lab we should see similar results.  The voltage drop of the LED comes across as 1.18 volts which isnt enough to match the forward voltage of the LED so it doesnt light up.  It does match our math though!

And if we check out current draw, we can see that we’re puling about 11 milliamps which also matches our math…

Nice!  So voltage dividers are pretty awesome.  From what I can tell though, you shouldn’t use them for dropping the voltage for anything drawing significant load since the resistors are just dissipating the energy as heat.  Sounds like they’re pretty handy for dropping voltage when you’re checking it for signaling though.  More on that soon when we start looking at the Arduino stuff.

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