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Friday, January 10, 2014

Heat Sink Basics

As power transistor handle large currents, they always heat up during operation. Since transistor is a temperature dependent device, the heat generated must be dissipated to the surrounding in order to keep the temperature within permissible limits. Generally, the transistor is fixed on a metal sheet (usually aluminum) so that additional heat is transferred to the Aluminum sheet. The metal sheet that serves to dissipate the additional heat from the power transistor is known as heat sink.

heat sinks
Fig-1:  Heat Sink (Aluminum Sheet)


aluminum heat sinks
Fig-2: Heat Sink (Aluminum Sheet)


Heat Sink with IC
Fig-3: Heat Sink with Transistor/IC

Heat Sink with Transistor
Fig-4: Heat Sink with Transistor/IC

Most of the within the transistor is produced at the collector junction. The heat sink increases the surface area and allows heat to escape from the collector junction easily. The result is that temperature of the transistor is sufficiently lowered. Thus almost the entire heat in a transistor is produced at the collector-base junction. If the temperature exceeds the permissible limit, this junction is destroyed and the transistor is rendered useless.
Most of power is dissipated at the collector-base junction. This is because collector-base voltage is much greater than the base-emitter voltage, although currents through the two junctions are almost the same.

Heat sink is a direct practical means of combating the undesirable thermal effects e.g. thermal runaway. It may be noted that the ability of any heat sink to transfer heat to the surrounding depends upon its material, volume, area, shape, contact between case and sink and movement of air around the sink. Finned aluminum heat sinks yield the best heat transfer per unit cost. It should be realized that the use of heat sink alone may not be sufficient to prevent thermal runaway under all conditions. In designing a transistor circuit, consideration should also be given to the choice of (i) operating point (ii) ambient temperatures which are likely to be encountered and (iii) the type of transistor e.g. metal case transistors are more readily cooled by conduction than plastic ones. Circuits may also be designed to compensate automatically for temperature changes and thus stabilize the operation of the transistor components.
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NTSC PAL TV Signal Identifier

This circuit is able to identify PAL and NTSC video signals. Its output is high for an NTSC signal and low if the signal is PAL. This output signal can be used, for example, to automatically switch in a colour subcarrier converter or some other device while an NTSC signal is being received. One application is for the reception from satellites of free-to-air TV signals, which in Australia generally contain a mixture of 625-line PAL and 525-line NTSC programs. Operation of the circuit is as follows.

IC1 is an LM1881 video sync separator which takes the video input signal and generates vertical synchronisation pulses. 
 
For an NTSC signal, these pulses are 16.66ms apart, corresponding to the 60Hz field rate, while for a PAL signal they are 20ms apart, corresponding to the 50Hz field rate. The vertical sync pulses are fed into IC2a, the first of two dual retriggerable monostable multivibrators in the 74HC123A. IC2a has a period of very close to 17.9ms, set by the 200kO resistor and 0.22µF capacitor at pins 14 & 15. Because the monostable is retriggerable, NTSC sync pulses arriving every 16.66ms will keep its Q output, at pin 13, high.

NTSC-PAL TV Signal Identifier Circuit diagram:

ntsc-pal-tv-signal-identifier Circuit

However PAL sync pulses arriving every 20ms will allow the Q output to go low after 17.9ms, before being triggered high again 2.1ms later. Thus an NTSC signal will give a constant high output while a PAL signal will result in a train of pulses 2.1ms wide. The Q output from IC2a is fed to the inverting input of IC2b, the second monostable, which has a period of about 0.5s, as set by the 270kO resistor and 4.7µF tantalum capacitor at pins 6 & 7. With its input constantly high, resulting from an NTSC signal, IC2b is not triggered and its Q output remains low.

However, the pulse train from a PAL signal will constantly retrigger it, so its Q output will remain high. The period of IC2b also effectively makes it a low-pass filter which removes spurious switching due to any input glitches. The output signal is taken from the Q-bar (inverted) output, so that an NTSC signal gives a high output, while PAL gives low. For the particular application for which the circuit was developed, diode D1 and the resistor network shown drive the base of an NPN switching transistor and relay. A dual-colour 3-lead LED can also be fitted to indicate NTSC (red) or PAL (green). Note that with no video input, the output signal is high and will indicate NTSC.

Source : http://www.ecircuitslab.com/2011/08/ntsc-pal-tv-signal-identifier.html
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Top Bench Power supply Circuit Diagram

This is the Top Bench Power supply Circuit Diagram. A tapped transformer drives a diode bridge (D1-D4) and two 2500 µ¥ filter capacitors (Cl and C2), that provide a no-load voltage of 37 or 47 volts, depending upon the position of switch S2a. The unregulated dc is then fed to a pre-regulator stage composed of Ql and D5. 

Those components protect IC1 (the 723) from an over-voltage condition; the 723 cant handle more than 40 volts. The LED (LED1) and its 2.2 k current-limiting resistor (Rl) provide on/off indication. The current through the LED varies slightly according to the transformer tap selected, but thats of no real consequence.

 Top Bench Power supply Circuit Diagram

 Top Bench Power supply Circuit Diagram

 The series-pass transistor in IC1 drives voltage-follower Q2, providing current amplification. The transistor can handle lots of power. It has a maximum collector current of 15 amps and a maximum VCE of 70 V, both of which are more than adequate for our supply.
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