This here is a special kind of buck converter and if you work with electronics, you must
know about double rail supplies.
Double rail means that it could give both positive and negative values, for example
+12V and -12V.
Usually my projects work with a battery or maybe some kind of adapter like this one here.
All these supplies right here give a steady positive DC voltage but sometimes we also
need negative values and these are the topics we will see in todays video.
And if you wonder, we must use this kind of dual rail power supply for audio amplifiers,
sometimes with operational amplifiers if you want negative and positive values, some ADCs
and DACs also need dual rail supply and even LCDs need a dual rail supply to switch the
pixels on and off.
If you remember, a few years back I made my own ultrasonic sensor.
To be able to vibrate the ultrasonic speaker I needed plus minus 20 volts and that’s
another example where I’ve used a dual rail supply.
So as you can see, is a quite important topic.
So what options do we have to get positive and negative supply?
We will check a few circuits and examples from a basic one to a bit more complicated
and understand how they work.
So guys let’s get started.
What’s up my friends, welcome back.
So, we need a positive and a negative voltage.
So let’s start with the easiest way to achieve that and then we go to some more complicated
solutions and you will have all the schematics in the video description.
Ok so we need two resistors and two capacitors.
Basically, if we add let’s say two 10K resistors in series like this, we will have a voltage
divider in the middle point of this connection.
if we take this middle point as a reference, then we would have a greater voltage on the
top side and a lower voltage than the reference on the low side.
I’ve made these connections on my breadboard.
As you can see on the oscilloscope, both references are in the middle and then we have a positive
value with yellow and a negative value with green.
Basically we get a two rail output, but is not that easy because the middle point would
be a so-called virtual ground!
As you can see when I add load to the output, the reference value changes and the values
start to go up or down.
To solve that we add an OPAMP as a buffer.
The main problem is that this would be unstable.
Is quite impossible to have two resistors with the exact same value.
And if we add a load and if just a little bit more current is flowing through one side
than the other, this would unbalance and we don’t want that.
To solve this we add an operational amplifier on the middle point.
The OPAMP will do everything possible to keep the voltage at the negative input to be the
same as the one on the positive input.
Like that it will work as a buffer so we have no current disturbance on the resistors and
now we get a more decent virtual ground.
I’ve mounted this on my breadboard and test it out.
As you can see we get a decent two rail supply with plus 5.7V volts and negative 5.7V volts
even if on the oscilloscope the value are a bit different.
This time if I add a load, the output is not affected anymore as you can see, so we solved
that problem.
But obviously, in this case the current limit is given by the operational amplifier, so
if the OPAMP could only deliver let’s say 50mA, that would be the maximum current the
dual rail supply would have.
To make this even better we could try this approach.
We add two BJT transistors at the output and the buffer negative input is not connected
at this point.
We also add the capacitors at the output.
Also add a diode at the input to protect the supply for reversed polarity.
Like this, the current would be provided by the transistors and not the OPAMP, so the
value would be higher this time, maybe something like 250mA or even more.
Ok but now let’s check the next solution.
This is a boost converter but you could also do it with a buck converter.
Anyway, as we all know, this is a switched mode kind of power supply.
Basically it means that in order to regulate the output, it has a dedicated IC that switches
on and off.
So let’s say you apply 5V and the input, it will give let’s say 12V at the output.
This circuit uses a coil together with a diode and a capacitor at the output to achieve that.
To learn more about buck or boost converters, check my videos on those topics, links are
below.
But anyway, if you add this other small circuit that is called a charge pump, at this point,
between the coil and the diode, we can also get minus 12V at the same time.
How you say?
Well, let's see.
Since this is a switched supply, that means we will have pules of voltage at this point.
When the voltage pulse is HIGH, this first capacitor will charge up following this path
through the diode towards ground.
That will charge the capacitor like this with positive on this side and negative on this
side.
But when the pulse is on the LOW value, this point will simulate a ground potential.
So basically, this second capacitor, since now the voltage is higher on this side than
the other side, it will charge up through this other diode.
But this time, it has the positive reference here, which is the ground point, and a negative
value here, and that’s how we get those negative 12V.
So basically this boost converter could now give 12V and negative 12V and the ground reference
will be in the middle.
I get some schottky diodes and some capacitors and I solder this same circuit at the output
of a boost converter module.
I power it up and check the output with the multimeter and as you can see, we get both
12V and negative 12V.
And is even more, using the potentiometer we can still change the voltage value just
as a normal variable boost converter.
Now this kind of supply has the same problems as any other switched mode supply, and that
is noise and oscillations.
If we check the output on the oscilloscope and zoom in, we can see the typical pulses
of a buck or boost converter.
And on higher loads, the peaks are even bigger so for example for an audio amplifier this
is not a good option since you don’t want that noise to pass to the audio.
And the same problem would be for a precision sensor or an instrumentation amplifier which
require very stable voltages.
You can buy directly such modules from AliExpress for example and they could deliver an output
from 50 up to maybe 500mA, which is not that much.
But this one here uses a different approach.
Instead of using a single buck converter IC and then add the diodes and capacitors to
create the negative rail, this one uses two buck converter ICs at the same time.
One is the MP 14 84 and the other one, from what I found on the internet, is the TD 27
86.
They can both deliver up to 3 amps of current so this time, the power output for both rails
could be a lot higher than the previous example.
As you can see it gives me plus and minus 15V.
So by now, as you can see, the current output and noise problems are something common for
a combination of a boost or buck converter together with a charge pump at the output.
But if you don’t need too much current and you are looking for something smaller, you
also have the ICL 76 62 which you could buy as a module or directly the IC and place it
on your circuits.
Look how small it is compared with the other modules.
This can input from 4 up to 20V DC and it will output from -4 down to -20V.
But the downside of this solution is the current output.
It can only deliver 10mA of current so you can use it as a negative voltage reference
but not as a powerful supply.
And then you also have the ICL 76 60 and the TPS 60 403 which could go up to 60mA of current
and is the same size.
This IC only requires a small capacitor at the input and another one at the output and
it requires no coils, diodes or any other component so consider this solution for any
of your future projects that must be very small.
Ok, but now let’s get serious.
How about when you need a steady and powerful negative rail supply?
You won’t use switched converters or these tiny ICs that could only deliver a few mA
of current.
To get a decent power double rail supply we use a so-called centered tap transformer.
This kind of transformer basically splits the secondary coil in half and we get the
middle point as a ground reference and we will have positive on one side and negative
on the other side.
When the magnetic flux is oscillating inside such a transformer, the current will be induced
on one side from ground towards one of the outputs and on the other side from the output
towards ground, that’s how we can get both positive and negative voltages.
Now the amount of windings on both sides must be the same if you want to get the same voltage
values on each side.
But to make sure we have the same voltage and also get rid of the noise, we add a filter
and a linear regulator at the output.
So the schematic would be something like this.
We have the mains voltage of let’s say 220V AC and this would be its shape on the oscilloscope.
Then we add the center tapped transformer and it will output plus and minus, let's say
20V and the shape would be the same as before but with a lower voltage.
Then we have a full bridge rectifier and basically get half value on each rail.
We would have the positive half of the wave on one side and the negative half wave on
the other side.
Then we add a filter using capacitors and we get a steady DC voltage.
Then we add a linear regulator to let’s say 12V and minus 12V.
That’s how we get a steady and decent power dual rail supply.
Have in mind that the used LDO for the negative side must be one that accepts negative voltages
such as the LM 79 12, otherwise it won’t work.
So this is a more powerful way to get a dual rail supply but as disadvantage is that this
time we use transformers which are very big, heavy and also work with mains supply so be
very careful because they are dangerous.
So guys, these were some solutions to get a double rail power supply for your projects
or to supply an audio amplifier, sensors, displays and much more.
I hope you have learned something new and if so, consider giving me a like or comment
below.
Thanks again and see you later guys