all about wiring diagram

Many times the hobbyist desires to have a simple, dual power supply for a project. Existing power supplies may be large either in power output or physical size. a simple Dual Power Supply is necessary.For most non-critical applications the best & simplest choice for a voltage regulator is the 3-terminal type.The three terminals are input, ground & output.

The 78xx & 79xx series can provide up to 1A load current & it have on chip circuitry to prevent damage in the event of over heating or excessive current. That is, the chip basically shuts down than blowing out. These regulators are cheap, simple to make use of, & they make it practical to design a method with plenty of P C Bs in which an unregulated supply is brought in & regulation is done locally on each circuit board.

This Dual Power Supply project provides a dual power supply. With the appropriate choice of transformer & 3-terminal voltage regulator pairs you can basically build a tiny power supply delivering up to amp at +/- 5V, +/- 9V, +/- 12V, +/-15V or +/-18V. You require to provide the middle tapped transformer and the 3-terminal pair of regulators you require:7805 & 7905, 7809 & 7909, 7812 & 7912, 7815 & 7915or 7818 & 7918.

The user must pick the pair they needs for his particular application.

Note that the + & - regulators do not must be matched: you can for example, use a +5v & -9V pair. However,the positive regulator must be a 78xx regulator, & the negative a 79xx. They have built in plenty of safety in to this project so it ought to give plenty of years of continuous service.

Transformer
This Dual Power Supply design makes use of a full wave bridge rectifier coupled with a centre-tapped transformer. A transformer with a power output rated at at least 7VA ought to be used. The 7VA rating means that the maximum current which can be delivered without overheating will be around 390mA for the 9V+9V tap; 290mA for the 12V+12V and 230mA for the 15V+15V. If the transformer is rated by output RMS-current then the worth ought to be divided by one.2 to get the current which can be supplied. For example, in this case a 1A RMS can deliver 1/(one.2) or 830mA.

Rectifier

They use an epoxy-packaged four amp bridge rectifier with at least a peak reverse voltage of 200V. (Note the part numbers of bridge rectifiers are not standardised so the number are different from different manufacturers.) For safety the diode voltage rating ought to be at least to times that of the transformers secondary voltage. The current rating of the diodes ought to be two times the maximum load current that will be drawn.

Filter Capacitor
The purpose of the filter capacitor is to smooth out the ripple in the rectified AC voltage. Theres dual amount of ripple is determined by the worth of the filer capacitor: the larger the worth the smaller the ripple.The two,200uF is an appropriate value for all the voltages generated using this project. The other consideration in choosing the correct capacitor is its voltage rating. The working voltage of the capacitor has to be greater than the peak output voltage of the rectifier. For an 18V supply the peak output voltage is one.4 x 18V, or 25V. So they have selected a 35V rated capacitor.

Regulators

The unregulated input voltage must always be higher than the regulators output voltage by at least 3V in order for it to work. If the input/output voltage difference is greater than 3V then the excess potential must be dissipated as heat. Without a heat sink three terminal regulators can dissipate about two watts. A simple calculation of the voltage differential times the current drawn will give the watts to be dissipated. Over two watts a heat sink must be provided. If not then the regulator will automatically turn off if the internal temperature reaches 150oC. For safety it is always best to make use of a small heat sink even in case you do not think you will need.

C4 & C5 improve the regulators ability to react to sudden changes in load current & to prevent uncontrolled oscillations.

The mono block capacitor C2 & C6 across the output provides high frequency decoupling which keep the impedance low at high frequencies.

Two LEDs are provided to show when the output regulated power is online. You do not must make use of the LEDs in the event you do not require to. However, the LED on the negative side of the circuit does provide a maximum load to the 79xx regulator which they found necessary in the coursework of testing. The negative 3-pin regulators did not like a zero load situation. They have provided a 470R/0.5W resistors as the current limiting resistors for the LEDs.

These protect chiefly against any back emf which may come back in to the power supply when it supplies power to inductive lots. They also provide additional short circuit protection in the case that the positive output is connected by accident to the negative output. If this happened the usual current limiting shutdown in each regulator may not work as intended. The diodes will short circuit in this case & protect the two regulators.

Anyone who’s spent much time searching the web for interesting circuits is likely to have found at least one TL431 based audio amplifier, the circuit being based on the principle that any comparator can be used in linear mode if it’s rolled off with enough negative feedback. Although the TL431 is often referred to as a programmable or adjustable zener, it is in fact a comparator with it’s own 2.5 V reference all neatly wrapped up in a TO92 package.

The problem with the TL431 amplifiers to be found on the web is that they simply roll it back with large nfb and leave it at that, which results in ver y low gain, to make mat ter s worse some such circuits make a bit of a hash of biasing the control input. 

 

Components C1 & R1 are entirely optional to selectively feed some unshunted feedback to reduce noise; 1.5 k? & 5.6 nF are as good a place as any to start off with. Initial set-up depends on the current drawn by the electret mic and the value for RL any-where between 200 and 2,000 ohms is good. R2 allows the TL431 cathode to swing despite the AC shunt, 1.2 k? was found to be satisfactory, P1 can be a 47 k? trimpot and is used to set the voltage drop on RL. In the case of moving coil speakers a compromise between volt-age swing and prebiasing the cone should be sought, with a resistive load adjust for 0.5 Vcc, once the operating point is determined P1 can be measured and replaced by an equivalent fixed resistor.

The circuit has a couple of handy features, firstly it wor k s ver y well on the end of a twisted-pair the output can be tapped off at the wiper if RLis a pot at the power supply end, secondly by salvaging the JFET from an old electret mic (some common types of JFET will work but not quite as well), just about any piezo electric element can be used as the transducer. Brass disc sounders give a good output (handy as vibration sensors if glued to a structure); even the quartz discs from clock crystals give some output, a phono crystal cartridge gives a high output and the piezo-ceramic pellet from a flintless cigarette lighter gives a huge output... the range of possible applications is awesome!

A surprising application is the ability to test the microphonic sensitivity of ordinary capacitors! Disc ceramic types don’t need to be tapped very hard to produce an output but rolled metalised foil types produce some out-put too. Link