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Rabu, 05 Desember 2012

Power supply LM7812

7812 and 7912
Voltage Regulator Page




The LM7812 and LM7912 Regulator ICs are essentially the same (identical). The LM7812 is a Positive Voltage Regulator, while the LM7912 is a Negative Voltage Regulator. Both are capable of powering a 1.0 Amp Load (ideal conditions). Both devices generally come in a TO-220 Package. Some versions are available in the K Package, which is a TO-3 Power Case - but these devices cost at least $35.00 each (as opposed to the normal $1.85 for the TO-220 Packaged device).

















12V 1.0 Amp Power Supply - using an LM7812 Regulator IC:
Using a single 7812 IC voltage regulator, this power supply can deliver output load currents of up to 1.0 amps. The design is shown below:



As an alternative to a power transformer, a couple of 12 Volt car batteries could be used (but then you would need a battery charger to keep the batteries charged). Normally, the input voltage to the regulator must be at least several volts higher than the output voltage (12V) so that the regulator can maintain its output. The 7812 will only pass 1 amp or less of the output current, so this regulator/power supply configuration is best suited for small power applications. A 1 amp fuse in the regulators output prevents a safeguard.


12V 1.0 Amp Variable Power Supply - using an LM7812 Regulator IC:
This is a circuit modified to provide a variable voltage output using a 7812 Non-Variable Voltage Integrated Circuit. This power supply can deliver output load currents of up to 1.0 amps. The design is shown below:















This is essentially the same circuit as the LM7812 12volts Fixed Voltage regulator. This variable power supply is using a 7812 voltage regulator. This circuit offers excellent ripple rejection, eliminates mains hum, and has a design using a pi filtered C-L-C..

L1 is a powder core and has 32 turns of 0.75mm wire.

The power transformer should have a secondary rated 24V at 2A. The bridge rectifier contains 4 diodes, their current rating needs to be high with respect to the transformers output current; if not the current may damage the diodes. A 10-25Amp 50-100PIV Bridge rectifier is recommended. C1 is the main filtering capacitor, the supply is further smoothed by the combination of L1 and C3. Capacitors C2 and C4 are decoupling capacitors; their action is to further reduce the ripple factor.

The 7812 utilizes the action of zener diode which is in parallel with the potentiometer, R1. The tuning action of R1 produces a variable regulator output. The output voltage is variable from the regulator output to the regulator output plus the zener voltage. In other words if you were to use a 7805 regulator, and a 10V zener - you would get an output adjustable from 5 to 15 Volts. The regulator may be changed to provide different output voltages as may the zener. the zener should be rated a minimum of 1.3 Watts.

30-Amp Regulator Description:
Using a single 7812 IC voltage regulator and multiple outboard pass transistors, this power supply can deliver output load currents of up to 30 amps. The design is shown below:






























The input transformer is likely to be the most expensive part of the entire project. As an alternative, a couple of 12 Volt car batteries could be used. The input voltage to the regulator must be at least several volts higher than the output voltage (12V) so that the regulator can maintain its output. If a transformer is used, then the rectifier diodes must be capable of passing a very high peak forward current, typically 100amps or more. The 7812 will only pass 1 Amp or less of the output current, the remainder being supplied by the outboard pass transistors. As the circuit is designed to handle loads of up to 30 amps, then six TIP2955 are wired in parallel to meet this demand. The dissipation in each power transistor is one sixth of the total load, but adequate heat sinking is still required. Maximum load current will generate maximum dissipation, so a very large heat sink is required. In considering a heat sink, it may be a good idea to look for either a fan or water cooled heat sink. In the event that the power transistors should fail, then the regulator would have to supply full load current and would fail with catastrophic results. A 1 amp fuse in the regulators output prevents a safeguard. The 400mohm load is for test purposes only and should not be included in the final circuit. A simulated performance is shown below:



Calculations:
This circuit is a fine example of Kirchoff's current and voltage laws.
To summarise,
the sum of the currents entering a junction, must equal the current leaving the junction, and the voltages around a loop must equal zero.

For example, in the diagram above, the input voltage is 24 volts. 4 volts is dropped across R7 and 20 volts across the regulator input, 24 -4 -20 =0. At the output :- the total load current is 30 amps, the regulator supplies 0.866 A and the 6 transistors 4.855 Amp each , 30 = 6 * 4.855 + 0.866. Each power transistor contributes around 4.86 A to the load. The base current is about 138 mA per transistor. A DC current gain of 35 at a collector current of 6 amp is required. This is well within the limits of the TIP2955. Resistors R1 to R6 are included for stability and prevent current swamping as the manufacturing tolerances of dc current gain will be different for each transistor. Resistor R7 is 100 ohms and develops 4 Volts with maximun load. Power dissipation is hence (4^2)/200 or about 160 mW. I recommend using a 0.5 Watt resistor for R7. The input current to the regulator is fed via the emitter resistor and base emitter junctions of the power transistors.

Once again using Kirchoff's current laws,
the 871 mA regulator input current is derived from the base chain and the 40.3 mA flowing through the 100 Ohm resistor. 871.18 = 40.3 + 830. 88. The current from the regulator itself cannot be greater than the input current. As can be seen the regulator only draws about 5 mA and should run cold.



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