A lot of people have been asking me how to build a 12 volt DC to 120 volt AC inverter.
Normally I try stay away from high voltage projects that could
harm people, but since so many people have been asking for
an inverter tutorial I figured I would give it a try. Unfortunately, the results weren't
great, and I'm making this video to warn you guys about the limitations
of this design, because it seems to be all over the
internet, and there are people with thousands of views who are showing it off like it's
the greatest thing on earth. This video builds upon material I covered
in my transformer video so make sure you've watched that
one first.
If you search for inverter circuits you get many results, but they are all based on the
same basic idea. You have an oscillator switching a pair of transistors,
and that pushes and pulls current through a transformer,
which allows you to convert DC to AC. This design is an especially popular one, and it
has many variants. You have a CD4047 IC configured as an astable
multivibrator. This resistor and capacitor set the oscillation
frequency, and since we want to recreate mains voltages, you want 50 or 60Hz. A potentiometer
will help you fine tune the exact frequency. The CD4047
has two outputs, one normal, and one inverted. Here is
what that looks like on the oscilloscope. You can see me getting a relatively stable
60Hz square wave, and the two outputs are the opposite. When one
wave is high, the other is low, and vice versa.
This is really handy for driving two transistors and ensuring that both of them are never on
at the same time.
So let's say this top transistor is on, and the bottom one is off. Current will flow from
your 12 volt source, through one winding of the transformer, through
the transistor, to ground. Now let's change things. Let's
turn the top transistor off and turn the bottom one on. Now current will flow from the 12
volt supply, through the other winding of the transformer,
through the transistor, towards ground. When you
constantly push and pull current through the windings like this, you get alternating current
through the transformer, and that will create an AC voltage
on the other side of the transformer. I'm deliberately not
going into much detail in this video because this circuit is actually not very good, and
you'll see why soon.
Here's what it looks like on a breadboard, and notice that I'm using the exact same transformer
from my transformer video. It's meant to be a 120
volt to 24 volt center tapped step down transformer, but you can
use transformers like this in reverse to step up voltage.
So now I'm going to power up the inverter with 13.8 volts, which is what you'd expect
to get from a fully charged 12 volt battery, and I'm going to
power a 40 watt light bulb. The bulb lights up, and we're getting
106 volts AC RMS on the output, which is pretty close to the 110 to 120 volts we ideally want.
So... what's the problem?
First, take a look at how the frequency changes depending on your input voltage. If I lower
the input from 13.8 volts down to 10 volts, the frequency
changes from 60Hz to 56Hz. This is somewhat expected because
the oscillator is simple, and only based on a resistor and a capacitor. So if you were
running this inverter off of a battery or a solar array, you'd never
get a stable frequency from it. Okay, that's not a big deal, all
that means is you couldn't run any circuitry that requires a stable input frequency, like
a cheap alarm clock. Are there any other problems?
Well, check this out. I'm going to remove the light bulb, dial the input voltage down
under 7 volts and show you the unloaded output on my oscilloscope.
Do NOT try this at home. If you look closely, you can see that
even though the output from the transformer is a relatively low 60 volts RMS, we have
a peak to peak voltage of 290 volts! If we zoom in, you can
see that it's because the square wave input causes unwanted
high frequency ringing on the output. If you were to power this circuit
from 12 volts and try to measure it on your oscilloscope you
would get about 600 volts peak to peak, which could damage
your oscilloscope or the probe. A normal sinusoidal 120 volt mains electricity waveform should
have a peak to peak voltage of around 340 volts,
and we've got almost double that.
Now remember that this circuit doesn't have any kind of voltage regulation or monitoring
at all, and the output voltage is highly dependent on the
input voltage. So if you were to use it in an environment where
you have 15 volts coming from a battery charger, alternator, or solar panel, you'd end up with
700 volts peak to peak or more on the output, and well
over 1000 volts if you used a 240 volt transformer. It would
be ridiculously dangerous to use this design with any low load device like a phone charger.
Okay, but what if we put a load on it? This variant of the circuit recommends a 0.1uF
high voltage rated capacitor on the output, and that should not
only put a light load on the inverter, it should also help filter
out some of the high frequency ringing. Let's see what happens. Uh... well... so much for
that!
After doing some tests, I found that 3 microfarads significantly reduced the spiking. Not a big
deal, only 30 times more than what the internet recommended.
But now the inverter draws about 14 watts of power
from the input even when it's powering no additional load. That's about as efficient
as the TSA.
Finally, I want to show you what happens when you change the load on the output. Right now
I have things set up to give me an ideal 120 volts RMS with
no load. Now when I put a load on it, it's 103. It performs
even worse with dynamic loads. This circuit is like a box of chocolates, you never know
what you are going to get.
So... what was the point of this video? It's not to discourage people from posting schematics
and projects on the internet, and it's not to discourage
hobbyists from trying out schematics that they find on the
internet. After all, it is a very good circuit for powering light bulbs. The only point I
am trying to make is that there are a lot of electronics websites
that talk very confidently about the designs they show you, and
they don't always tell you about the limitations of the design. Be extra careful when working
with high voltages, because something that may look
good in a Youtube video, and may seem safe with your
multimeter, could actually have serious problems that could destroy your oscilloscope. Hopefully
this video will have saved somebody the hassle.
Thank you for watching, if you liked this video please give it a thumbs up, and check
out my other videos about electronics!