The post explains how to connect common 78XX voltage regulator ICs such as 7805, 7812, 7824 etc in an electronic circuit for getting the intended fixed regulated output voltages at 5V, 12V, and 24V depending on the selected 78XX specification
Importance of 78XX voltage regulator in circuits
A varying voltage can cause drastic consequences to a sensitive electronic circuit, for example a TTL, LS and HC series of ICs cannot tolerate more than 5 volts and can get immediately damaged.
A CMOS IC cannot stand more than 16 to 18 volts.
A relay if operated at voltages more than its rating can become hot and waste electricity unnecessarily.
There are several other issues which might be faced with electronic circuits if the applied is an unregulated one.
For solving the above issue, many high grade yet very simple to configure chips have been designed and are available cheaply and plentifully in our electronic markets.
The 78XX voltage regulator series for example, comes with most of the standard voltage ratings which can be used in conjunction with an ordinary power supply DC for obtaining high grade, clean voltage controlled outputs.
Technical specifications of the 78XX series IC
- Output voltage tolerances are around ±2% at Tj = 25˚C and ±4%
- Line regulation is around 0.01% of VOUT/V of ∆VIN at 1A load
- Internal circuitry is thermal and overload protected
- Internal short-circuit current limit protections are also included
- Output transistor safe area protection is also one of the features of these ICs
Identifying 7805/7812/7824 ICs Pinouts
A classic example can be seen in this article where a 7805 IC is used as a cell phone charger regulator.
Referring to the above circuit diagram
- These ICs have just three leads, making it very easy to understand and connect. The leads are assigned as input, ground and output respectively.
- Keeping the printed side toward you, the left side lead is the input, the center one is the ground and the right side lead is the output.
- The DC from any standard power supply is applied across the input and the ground leads of the IC, the positive goes to the input while the negative is connected to the ground.
- The output is acquired across the output and the ground pins of the IC, the positive being received from the "output" pin and the negative from the common ground line.
IC 7805, 7812, 7824 Pinout Specifications
Most of the common voltage regulator ICs beginning with the 78 prefix, such 7805, 7812, 7824 typically have identical pinout assignments as shown below:
However, in the above chart, we can also see that except the 78LXX, the other variants have slightly different pinout specifications, and needs to be connected exactly as per the given details otherwise the IC may fail to work and produce unexpected outcomes.
The ICs beginning with 78XX are positive voltage regulators, meaning these will accept a positive input voltage across across their input/Gnd terminals and regulate the same across their output/Gnd terminals with the specified fixed voltage output.
Conversely, the 79XX ICs will accept a negative voltage and produce a negative fixed voltage across their relevant output terminals.
The package of the ICs also signify a vital information. The ICs with TO220 package are rated to handle and produce a maximum of 1 Amp current whereas the smaller 78LXX version are rated to handle only upto 100mA.
We all very well know regarding the building procedures a DC power supply circuit using a transformer, a bridge rectifier and a filter capacitor.
It just requires connecting four diodes in a bridge configuration and connect it to the secondary of the transformer, the capacitor goes to the output of the bridge terminals.
The output produced across the capacitor is approximately equal to the rated voltage of the transformer, rather a few volts higher than the transformer spec.
However the voltage obtained from the above simple configuration is never regulated and stabilized, meaning the output from it will never be constant and will vary with the varying input mains voltage levels, which we know is never constant.
How to Connect 7805, 7812, 7824 in an Electronic Circuit
In order to regulate an existing supply to a fixed level, we normally use these 78XX ICs, and theses can be very easily connected with any supply source in the following shown manner:
ICs 7812 and 7824 can be also connected exactly in the above shown manner, the only difference being the input/output voltage specifications which will vary as per the IC's ratings.
For example a 7812 will require an input above 13V and will produce a fixed 12V at its output.
Similarly a 7824 will require a input of not less than 26V, and will offer an output voltage fixed at 24V, and so on.
What does the Capacitors do?
We can some capacitors attached across the input and output terminals of the ICs, these are just included to rectify any residual DC spikes and ripples that may exist in the supply line.
According to the datasheet of the IC, the input capacitor is required only if the input source is at a significant distance away from the IC, may be at over a meter away. The output capacitor may be included if you want an improved transient regulation, meaning protection from noise spikes.
The value of these capacitors is not critical any value between 1 uF and 100 uF can be used for rectifying higher frequency ripples, whereas smaller capacitors in the range of 0.1 uF to 0.47 uF can also be attached in parallel to control any possible high frequency entry along the supply rails.
How to Get 10 Amps from 7805 IC
A 7805 IC is designed to deliver 5 V at 1 Amp maximum current. So if you want to get more than 1 amp from this IC it may be impossible to achieve this, directly from the IC.
However, the following transistorized circuit not only upgrades the IC 7805 specifications to generates up to 10 Amp current but also allows the user to get a voltage as high as 15 V from the 7805 IC.
The following diagram suggests the entire procedure for achieving 10 amps from the IC 7805 and a variable output voltage from 5 V to 15 V.
Here's another circuit which can convert an ordinary 7805, or 7812 IC into a high current delivering circuit. The design presented also has a current control feature which ensures that the output transistor is always safeguarded from short circuit or high current over loads.
High Current from 7805 and 7812 IC with Short Circuit Protection
The circuit functions in the following way:
When the voltage drop across R1 exceeds 0.6 V, Q2 begins conducting and it bypasses the voltage regulator IC to provide the full high current from input supply to the output terminals.
Q2 works like a current limiter stage. If the current through Q1 tries to exceed over the maximum tolerable range, the voltage drop across R2 also exceeds 0.6 V turning ON Q2.
Q2 collector shorts circuits the Q1 base with the positive supply inhibiting its conduction and thus switching it OFF. In this way the current is controlled strictly to the desired maximum range.
R1 and R2 can be calculated using the following formulas:
R1 = 0.6 / 0.5 = 1.2 ohms 1/2 watt
R2 = 0.6 / maximum current limit