Saturday, January 14, 2017
TIP2955, TIP3055 (NPN), TIP2955 (PNP) Complementary Silicon Power Transistors. It is a Darlington, that means good current gain. See Darlington transistor - Wikipedia. When current in R1 10E goes more than 50mA a voltage of 50mA X 10E = 500mV is applied across Emmiter-Base junction. So lower than 500mV no bias the tap is turned off, 500mV-700mV the tap starts turning on depending on type of transistor.
Power Supplies Section
The transistor is like a water Tap. So TIP2955 carries the major current burden thru the load allowing LM317 to do the decision making when to turn-on or off. It is analog control, it is not On-Off but linear-proportional. The LM317 is very cool as the burden is passed off to TIP2955 who will need a heatsink to keep going and deliver the power you want.
Mains Voltage Power Transformers
If you build it and wire it without designing a PCB, then make all wiring and connection very sound. The test of this ability you can know easily, If your project stops working after the last screw of the cabinet is tightened, Then we need to improve.
Friday, November 18, 2016
This is a General Purpose Chip, Series Regulation. It can be varied or trimmed. There is an Internal temperature compensated reference. The minimum trim value is around 1.2. In case you want a Low value voltage like 0.5 with a good current, then use a good negative supply to offset the 1.2 V.
Power supply with battery backup for DMM
The transformer can be s Split Bobbin with Pri-Sec copper shield foil. This can be earthed along with the metal enclosure. C4 sends any hi-freq components to earth. It is better if you do not earth the ground but use such capacitors. A Supply should simulate a a battery with both ends floating wrt Earth. A option to eartth the ground is fine. This also helps the user to configure his own dual supplies.
Q1 and R5 form a Short Circuit OR current fold back OR constant current mechanism. TP1 can be used to vary the output voltage. Better use something like a Bourns 10T trimpot. An open preset may introduce a noise due to dust and vibration. I don’t remember why i added a zener DZ1, a diode may suffice.
See more at my Power Supplies Section.
Wednesday, February 04, 2015
Voltage Regulators LM7812 and LM317
You could use LM7824 or LM7815 or LM7812 for 24V, 15V and 12V respectively. You could use LM7924 or LM7915 or LM7912 for -24V, -15V and -12V respectively.
The Filter capacitor C1 4700uF has an impedance of Xc = 1 / (2 * 3.14 * f * C) which comes to 0.6 ohms at 50 Hz.
Power Electronic Circuits
The impedance of the load at 2A for 24V is R = V / I that is 12 Ohms which is more than 20 times the impedance of the capacitor at 50 Hz. That means less than 1 / 20 of ripple current will flow thru the load. The Regulator also reduces the ripple a little.
Monday, January 05, 2015
Look at R2 a 10 Ohm resistor, when the current in your load to the power supply is less than 70mA the voltage across R2 is less than 10E * 70mA = 700mV right. The base emitter junction of Q1 will be biased or turned on around 700mV, less than 700mV the transistor just does nothing.
When the current in your load goes over 70mA the voltage across R2 goes above 700mV and a small base current Ib flows from emitter to base of Q1 turning on the transistor. Now a collector current Ic flows from emitter to collector and then to your load supplying the excess demand. The Ic = Ib * hfe where hfe or beta is the DC gain value.
From my Power Electronic Circuits
Some transistors will have only AC gain specified which is lower than DC gain. TIP2955 has a gain of 20 so for an Ib of 50mA the Ic will be 1 Amp which saves the regulator from heating up or shutting down as the main current flows thru the transistor. Q1 should be provided with a good heatsink.
Thursday, October 12, 2006
Every electronic gadget primarily needs a D.C, power supply to energize it. It also forms the basic requirement for any constructional project. consequently there is a need to obtain multiple voltage values for cost reduction, convenience and compact arrangement for all the above applications
List Of Components For Power Supply.
X1-6-0-6 (500 ma), X2-12-0-12 (500ma)
IC6-7805, IC7-7808, IC8-7908, D1 to D10-IN4007, D11 and D12 - 12v, 1W, Zener
R1 and R2 - 100E 1/2 W CFR
4. Capacitors. C 40v
C5 and C8 - 1000 Mfd , C1 - 2200 Mfd, C5 and C7 - 0.1 Mfd, C9 to C12 - 100Mfd
F1-250ma, N1-Neon, 3-Pin Mains Chord.
The required D.C. power supply is usually obtained by means of a transformer. It is also possible to have transformerless power supplies. Though the elimination of the transformer makes the circuit compact, economical and simple, also facilitating quick assembly and built in short circuit protection, certain drawbacks creep in. These power supplies are useful only for low current applications. Special safety precautions ? are to be followed while using them. Physical contact should be strictly avoided, since the output terminals are not isolated from A.C. mains supply.
This obviously necessitates the use of a transformer. By suitable modification it is possible to obtain multiple/ fractional dual voltages from a transformer. Different not-so obvious voltage values can also be obtained from the transformer by rectification circuits. The output so obtained from a transformer secondary is unregulated. For good load regulation, the internal impedance of any power supply should be as low as possible. The regulation can be improved either by resistor zener method or series regulator method.
However, the three-terminal regulators greatly simplify the power regulation problem. These regulators need no external components. They employ internal current limiting and thermal shutdown which make them tough. For simplicity, compactness, convenience and accuracy the use of three terminal regulators is ideal. These IC voltage regulators are freely available in various ranges both positive and negative. A functional schematic of a three terminal regulator is shown in the datasheet. It can be seen that the device is a complete regulator, with built-in reference, error amplifier, series pass transistor and protection circuits. The protection circuits include current limiting, safe area protection to limit dissipation in the series pass transistor and thermal shut down to limit temperature.
Low power IC voltage regulators of the 78L series used in our measuring instrument are now so cheap that they represent an economic alternative to simple zener-npn stabilisers. In addition they offer the advantages of better regulation, current limiting/short circuit protection at 1000 mA and thermal shunt down in the event of excessive power dissipation. In fact, virtually the only way in which these regulators can be damaged is by incorrect polarity or by an excessive input voltage. Regulators in the 78L series upto the 8v type will withstand input voltages upto about 35v, whilst the 24v type will withstand 40v. Normally, of course, the regulators would not be operated with such a large input-output differential as this would lead to excess power dissipation. All the regulators in the 78L series will deliver a maximum current of 1000mA provided the input-output voltage differential does not exceed 7v. Otherwise excessive power dissipation will result, causing thermal
Two transformers have been used to step down the voltage from 230-250v a.c. mains input. One of the transformers produces an output of 6-0-6v at the secondary terminals. This output is fed to a full wave rectifier and a capacitive filter. The filtered output is fed to IC6 which is a 3 pin voltage regulator which gives a regulated output of + 5v. This is used to activate the DPM circuit. It is also fed to the temperature network as a precision voltage reference source.
The other transformer produces an output of 12-0-12v at its secondary terminals. The centre tap is grounded like in the previous case. The other two terminals of the secondary are fed to a bridge rectifier constructed using diodes. The rectified output is filtered by using capacitor C5 and C6 fed to IC7 and IC. The IC7-8 which is are 3 pin voltage regulators gives an output of ±8v. These two voltages are fed to the signal generator. The -8v source output is fed to the temperature network, also as voltage reference. It is also necessary to produce a +12v and -12v supply for application to operational amplifiers. This can be conveniently done by means of 12v zener diodes. The output of the bridge rectifier is clamped to +12v and -12v respectively using two zener diodes. The zener output is fed to the operational amplifier supply terminals. Since the supply to
the operational amplifiers need not be very efficiently regulated to + 12v, the use of zener diodes proves economical.
For the testing of electronic components a voltage of above 50 V is required. This can be achieved by means of a voltage quadrupler circuit. It consists of four diodes and four electrolytic capacitors. The secondary ungrounded terminal of the 12-0-12v is connected to the quadrupler circuit. The output of the quadrupler circuit is 68v with respect to ground.
The two transformers can be controlled by the power supply switch PS 1 The switch also controls a neon lamp, which lights up once the transformer supply is on. The instrument is prevented against short circuits-excessive voltages by fuses. When the a.c. power supply exceeds beyond 250 volts resulting in any overload or damage, the fuse F1 blows out thus saving the rest of the circuit within the instrument.
The a.c. power is drawn from a 3 pin plug connected to a cable of 1000 mm to activate the instrument. The ground terminal in the 3 pin plug is earthed to the chassis, while the other two terminals are connected to the primaries of the two transformers.
D7 one end should be grounded.
Power supply part of the mesureall instrument i built 20 years back, ocr errors may be there, like I - 1 and 0 - O, other errors also uncorrected, reference textbooks, appn notes, datasheets
Reference Resources and Extra Reading