Adjustable voltage source bipolar
+ 15V, -15V, 1.5A
For all electronic project need a voltage source, also known as power supply, which provides power for the circuit work, there are many types of voltage sources, this time I present a simple source the simplest of which has the following characteristics:
• source is adjustable; this indicates that the voltage may vary from 0 to 15 volts, the voltage at which the vast majority of projects work.
• It's a bipolar source; this indicates that the source provides two voltages; of 0-15 volts positive and negative 0-15 volts, very useful in projects with operational amplifiers.
• The amperage is 1.5A which is provided by regulators, at least half of the electronics will not need more than 1 amp.
• It has a fan for the circuit is cooled apart from that regulators have their sink.
• It is a basic source commonly used for electronic circuits perfect for personal or for laboratory testing of prototypes.
A controllable source comprises a knob that adjusts the voltage has two terminals one for positive (+) and one for negative (-). The measured with a multimeter between the two terminal blocks value is the potential difference between them.
How does a voltage source?
A voltage source functions as a battery, both provide power to an electronic circuit, but a voltage source converts the alternating voltage supplied to an outlet at a lower voltage and continuous, and because it is connected to an outlet current voltage source is not downloaded compared to a battery, but of course the source must always be connected.
To understand the operation of the voltage source will see that consist of several stages:
Transformer stage
The transformer can reduce the voltage from the outlet (which is 120VAC 60Hz at any home) at a lower voltage with the same frequency for better handling (in this case the transformer drops to 12VAC 60Hz). A transformer consists of two coils wound on a closed soft iron core, the first winding is known as the primary and the second winding is known as secondary.
In the picture above is seen as the primary and secondary winding of a transformer, used in this project:
Its specifications are:
Primary: 110-120 V AC
Secondary with center tap, some transformers has some not, the difference is that we get 12VAC or 24VAC due to the secondary center tap.
Livewire performing the simulation shown in the following figure decreased voltage 120V to 12V. In the simulation the value is the RMS value of the AC voltage that produces the same effect of heat dissipation is required that the equivalent voltage, therefore:
Pk = 120V
Vrms = Vpeak * 0.707 = 120V * 0.707 = 84.84V
Grinding Stage
Even at the stage of the transformer we have an AC voltage but with a much lower value, the grinding stage is responsible for converting the alternating voltage live through diodes, specifically through a diode bridge or full-wave rectifier. At the entrance of the bridge we have an alternating voltage while its output voltage will have a direct positive and negative pole.
Seeing photography transformer has at its output 12V, this voltage is known as an effective voltage, and the multimeter will measure the maximum voltage or measuring voltage peak, which is given by the following equation:
Vmax = V * 1.4142
Vmax = 12V * 1.4142
Vmax = 16.97 V
This being the voltage of 12V and ensure that every component however small consume voltage, for example the rectifier consists of four silicon diodes which consume 0.7V each, making the score at the end and still have 14.17V does not fall below 12V.
When the secondary voltage is in the positive half cycle the diodes D2 and D3 conduct, being the output voltage of positive diodes of the bridge as the secondary. When the secondary voltage is in the negative side D1 and D4 conduct, so that the input voltage of the secondary causing the voltage output of the diode bridge is positive again reverses. Here we have a positive voltage but still is not continuous, as seen in the image below.
Filter stage
As shown in the picture above the voltage increases from zero to a peak value and then descend back to zero, this type of voltage is not suitable for different electronic circuits now this voltage must be continuous for this filter is used or capacitor which is also known as electrolytic capacitor.
When the diodes conduct the capacitor is charged to the peak voltage (Vmax), once the positive peak is exceeded opens the capacitor as the capacitor has a voltage Vmax at their ends and the voltage at the transformer secondary is a bit less than Vmax, the cathode of the diode voltage is over the anode because it starts downloading the capacitor through the load, at that time the diode does not conduct and the capacitor has to "hold" and make the load voltage not less than Vmax. When the voltage is lower than the transformer secondary diode will conduct and thus charging the capacitor again, so that the voltage is as follows:
The voltage regulator stabilizes the voltage reducing curling and provides a continuous output voltage of a certain value, the most common is the regulator 3-pin TO-220 package, the LM317 for this source was used for the positive voltage and the LM337 for negative voltage.
Both adjustable voltage regulators are capable of supplying 1.5A in a range of 1.25 to 37 volts, these regulators are used in the basic voltage sources, these have overload protection.
And identifying encapsulated pin LM337T
And identifying encapsulated pin LM317T
The objective is to provide a fan vented to atmosphere aid circuit and regulators to have a good temperature.The fan consumes 12V 0.12A resistance that this must have is:
R = V / I
R = 12V / 0.12A
R = 100 Ohms
But by using a value that the fan works at least in this case, use a resistor of 68 ohms and the fan runs at a regular speed. Vateaje use that strength?
P = IxIxR
P = (0.12x0.12) 68
P = 0.97W = 1W
2W resistor was used since it is not desired that the resistance remains at a high temperature.
Circuit Diagram
Instead of implementing a diode bridge can be used commonly used 4 diodes 1N4004 or 1N4007 regarding all the capacitors must be greater than 35 volts because it must exceed the voltage used which are 15 volts.
The ventilator circuit has a diode configuration called half-wave rectifier, where a diode rectified AC signal to DC capacitor charges and allowing the fan rotate without stopping, has its own fuse for any altecargado likely to have the fan.
Because it is a power bipolar voltage the two lines of the transformer as the center tap, both lines enter the diode bridge and the center tap is used as common or 0 volts, one line is the positive side for use LM317T regulator and the other line (more negative than 0 volts) downside for the LM337T regulator.
There is a formula to determine the output voltage of the regulator:
Vout = 1.25V (1 + VR1 / R2)
And if R2 is set to a value of 220 ohm and wish you a Vout = 16V our variable resistor VR1 should be:
VR1 = R2 * ((Vout / 1.25V) - 1)
VR1 = 220 * ((16V / 1.25V) - 1)
VR1 = 2596 = 2.6Kohms
So how can vary the voltage LM317T with this formula and clear that the transformer can provide this voltage, if possible place that value potentiometer but as I could not find her place potentiometers 5K was the smallest that I got, it is possible to provide even more voltage to reach 24V to change it but it must take into account that the voltage may be high but the current decreases.
Printed circuit
Equipment needed to assemble voltage source
1 Puente 4A 200V diode
2 electrolytic capacitors 4700uF 35V
2 50V 47nF polyester capacitors
1 LM317T TO-220 package
1 LM337T TO-220 package
3K two potentiometers
two 220 ohms resistors 1 / 4W
3 Diodes 1N4004
2 50V 1uF electrolytic Capacitore
2 Heatsink for TO-220 package
1 Resistance 68 ohms 2W
1 Electrolytic Capacitor 35V 1000uF
12V 0.12A 1 Fan
1 Fuse 1A
Fuse for one plate
to plate 2 Borneras triples
1 double terminal block for plate
1 Transformer 120V primary and 24V secondary center tap
one copper plate (see PCB) Extra Materials recessed three terminal blocks (2 red, 1 black) 1 Box for installing the board and transformer 1 Power connector chassis computer with one switch lever (for power source) 1 Fuse 3A (for network protection) 1 Fuse box for embedding in
Photographs of the completed project
The following link may download the complete documentation, theory, images, and printed circuit diagram.
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