This is a 3.3 volt microcontroller board.
If all I have to power it is USB,
can I use voltage dividers instead of a regulator?
In this episode--
[RECORD SCRATCH]
No, wait.
I'm going to be very clear right now.
No, you cannot use a resistor voltage divider as a regulator.
However, there may be one other option to consider.
So in this AddOhms Tutorial, we will
see what happens when you try to use a voltage divider
as a regulator.
[MUSIC PLAYING]
If you have not watched it already,
I did a tutorial on dividers, but here's a quick review.
Say we have a 5 volt source and connect
two resistors in series.
If we measure the voltage at the point between them,
you'll get a ratio of the input voltage.
Using 10k for R1 in 20k for R2, the V-out
will be 3.3333333 volts.
It's really tempting to hook up this to a microcontroller,
isn't it?
Well, there's a couple of problems.
The first is what happens when that 5 volt changes?
Well, the output changes with it.
The voltage divider isn't stable.
Next, well, let's just hook it up and see what happens.
When I push the button, the LED should light up,
but it doesn't.
Using my multimeter, we can check the voltage.
Wait a minute, that's not even one volt?
So what happened?
Let's think about this for a second.
If the board draws 60 milliamps, then that 60 milliamps
has to go through R1.
To get 60 milliamps through R1, you would need like 600 volts,
so the resistor has to change.
But to what value?
You see, the next problem with the board
is that it's an active load.
So its current will always be changing.
Remember that the voltage drop across a resistor
is proportional to the current flowing through it.
That linear change is the basis of Ohm's law.
So we can't just pick a single resistor value for R1 or R2
that will make a stable 3.3 volts out.
Now at the beginning, I said there was one other divider
option.
You see, we need something to replace
R2 that doesn't have this linear change problem.
Something like a diode.
But not just any diode, a special one.
Diodes are unique because they appear as unopen
until enough voltage is applied to them, then they turn on.
Flip the diode around, and current flow
stops, at least until you reach the diode's reverse breakdown
voltage.
Normal diodes, like the one in 4001,
aren't designed to operate continuously in reverse.
Also, the reverse bias is like 50 to 100 volts.
There is a type of diode that is designed specifically
for this reverse bias circuit, and that's the Zener diode.
Like normal diodes, a Zener's forward voltage
can be around 0.7 volts.
However, if we look at these Zeners,
they have reverse voltages of 3.3, 5, 9.1, and 12 volts.
Kind of interesting numbers, huh?
Just like an LED, a Zener needs a current limiting resistor.
What's cool is that when we look at V-out of this divider,
the voltage across the Zener stays around 3.3 volts.
You see, this circuit takes care of the fluctuating
voltage problem from the resistor divider.
Now, we still have the problem that the current driving
the microboard is also going through R1.
So we need to do a little math to determine
a proper value for R1.
Zeners need about 5 milliamps to stay reverse bias.
So if we know the load we'll draw,
60 milliamps, and the Zener needs 5 milliamps,
then we can calculate something like 26 ohms.
I have a 22 ohm resistor on my kit,
so let's hook that up and see what happens.
Well, technically the circuit is working.
It resets and the push button works.
However, a quick look at the voltage,
and it's around 1.8 volts.
Here's the thing.
The Zener approach works well as long as the load is constant.
This microcontroller board has a lot
of active devices built into it.
And so while it's powered, its load is constantly changing.
Even with the stable voltage reference of the Zener circuit,
it is still difficult to pick a value of R1 that
will keep the circuit stable.
So while this is a low cost way to regulate voltage,
it isn't perfect.
You have to be careful when you use it.
It's best used when there is a very constant load.
An ESP8266 is not a good fit for a Zener voltage divider.
In fact, in most cases, a cheap linear regulator
is simply a better option.
Now honestly, I couldn't even think of a time
when to use a Zener as a regulator, so I'll ask you.
Have you ever used one before?
And if so, in what circuit?
When do you think it makes sense?
Leave a comment.
I'd like to hear from you.
In review, a resistor voltage divider
is good for situations with constant voltages
and minimal current, like IO shifting between an Arduino
and Raspberry Pi.
The Zener divider is good for creating stable voltage
references, like when you want to change
the analog reference for an analog-to-digital converter.
In a pinch, it can be used in some circuits
as a minimal voltage regulator.
If you aren't familiar with voltage regulators,
you can check out my tutorials on them.
I recommend starting with the linear regulator video.
If you have questions, leave them here
or drop by the AddOhms forums.
There you'll find show notes and links related to this episode.
And as always, remember, if your circuit isn't working,
just Add Ohms--
unless you're using a voltage divider as a regulator,
in which case, don't do that.
[MUSIC PLAYING]
Stuck around to the very end?
Cool.
Well, I have a secret, but don't tell the comments section, OK?
The board I'm using is a HUZZAH 8266 breakout from Adafruit.
And guess what?
It has a voltage regulator built in.
It's right there next to the 3.3 volt pin marked LDO.
So yeah, it is very easy to use this board with 5 volts.
I just didn't have anything else in the lab
that ran on 3.3 volts, so that's why I used this one.
Thanks again for watching.