Power supply circuit » lm7812 http://apowersupply.com Sat, 04 Feb 2012 16:06:35 +0000 en hourly 1 http://wordpress.org/?v=3.1 Variable Power Supply with 78XX regulator http://apowersupply.com/variable-power-supply-with-78xx-regulator-242.html http://apowersupply.com/variable-power-supply-with-78xx-regulator-242.html#comments Wed, 11 Jan 2012 07:14:53 +0000 aPowerSupply.com http://apowersupply.com/?p=242 This variable power supply is using 7805, 7809, 7812 or 7815 voltage regulators, where the last 2 digits represents the maximum output voltage of the IC. This circuit offers excellent ripple rejection, eliminates mains hum, and has a design using a pi filtered C-L-C.

A core should be chosen to work within the specific frequency as stated by the manufacturer. L1 is a powder core and has 32 turns of 0.75mm wire.

Variable Power Supply Circuit Diagram

variable power supply 78xx regulator
The transformer has a 240V primary and has 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. C1 is the mainfiltering capacitor, the supply is further smoothed by the combination of L1 and C3. C2 and C4 are decoupling capacitors; their action further reduce ripple factor.

The regulator 78xxr, U1 utilizes the action of zener diode ZD1 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. E.G. A 7805 regulator and 10V zener give 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.

T1 Transformer 10:1 Secondary 24V @ 2A
BR1 Bridge Rectifier 50V PIV 2A rating
U1 7805 N.B. This may be changed for different output voltages e.g. 7812 for higher output voltage
ZD1 15V zener @ 1.3W

Source:http://www.zen22142.zen.co.uk/Circuits/Power/vpsu.htm

]]> http://apowersupply.com/variable-power-supply-with-78xx-regulator-242.html/feed/ 1 Increase regulator voltage outputs http://apowersupply.com/increase-regulator-voltage-outputs-249.html http://apowersupply.com/increase-regulator-voltage-outputs-249.html#comments Fri, 30 Dec 2011 02:40:40 +0000 aPowerSupply.com http://apowersupply.com/?p=249 It is often necessary to arrange an voltage regulator IC to give a higher output voltage than that set by the regulator alone. One method to achieve this is by connecting the “common” terminal to the mid-point of a potential divider but the problem with this method is that IC regulators have a small quiescent current (~10mA) flowing out of the common terminal to ground.

The magnitude of this quiescent current is not closely controlled and hence the total output voltage becomes somewhat unpredictable. Low divider resistor values help, but there are likely to be complications of heat dissipation and inefficiency.

24V from 12V regulator circuit diagram

12v to 24v regulator circuit diagram
The circuit presented here avoids the problem by using the transistor T1 to generate a low impedance at the regulator common terminal by emitter-follow action, while transferring the voltage divider from a relatively high-resistance divider network. The value of R3 is not critical but must be low enough to accept the highest quiescent current without causing T1 to turn-off.

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