industrial electronics 2nd unit

8/14/2019 Industrial Electronics 2nd Unit

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INDUSTRIAL ELECTRONICS2nd unit

Regulated Power Supply System

Transformer - steps down 230V AC mains to low voltage AC. Rectifier - converts AC to DC, but the DC output is varying. Smoothing - smooths the DC from varying greatly to a small ripple. Regulator - eliminates ripple by setting DC output to a fixed voltage.

Voltage regulator principle

Electronic symbol for Voltage regulatorA voltage regulator is an electrical regulator designed to automatically maintain a constantvoltage level.

It may use an electromechanical mechanism , or passive or active electronic components.Depending on the design, it may be used to regulate one or more AC or DC voltages.

With the exception of passive shunt regulators, all modern electronic voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference
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is amplified and used to control the regulation element in such a way as to reduce the voltageerror. This forms a negative feedback servo control loop ; increasing the open-loop gain tends toincrease regulation accuracy but reduce stability (avoidance of oscillation, or ringing during stepchanges). There will also be a trade-off between stability and the speed of the response tochanges. If the output voltage is too low (perhaps due to input voltage reducing or load currentincreasing), the regulation element is commanded, up to a point , to produce a higher outputvoltage - by dropping less of the input voltage (for linear series regulators and buck switchingregulators ), or to draw input current for longer periods (boost-type switching regulators ); if theoutput voltage is too high, the regulation element will normally be commanded to produce alower voltage. However, many regulators have over-current protection, so entirely stop sourcingcurrent (or limit the current in some way) if the output current is too high, and some regulatorsmay also shut down if the input voltage is outside a given range

Linear voltage regulator (series and shunt)In electronics, a linear regulator is a voltage regulator based on an active device (such as a

bipolar junction transistor , field effect transistor or vacuum tube ) operating in its "linear region"(in contrast, a switching regulator is based on a transistor forced to act as an on/off switch) or

passive devices like zener diodes operated in their breakdown region. The regulating device ismade to act like a variable resistor , continuously adjusting a voltage divider network to maintaina constant output voltage. It is very inefficient compared to a switched-mode power supply , sinceit regulates the voltage by burning off "unwanted" voltage as heat.

Overview

The transistor (or other device) is used as one half of a potential divider to control the outputvoltage, and a feedback circuit compares the output voltage to a reference voltage in order toadjust the input to the transistor, thus keeping the output voltage reasonably constant. This isinefficient: since the transistor is acting like a resistor, it will waste electrical energy byconverting it to heat. In fact, the power loss due to heating in the transistor is the current timesthe voltage dropped across the transistor. The same function can be performed more efficiently

by a switched-mode power supply (SMPS), but it is more complex and the switching currents init tend to produce electromagnetic interference . A SMPS can easily provide more than 30A of current at voltages as low as 3V, while for the same voltage and current, a linear regulator would

be very bulky and heavy.

Linear regulators exist in two basic forms: series regulators andshunt regulators. Series regulators are the more common form. The series

regulator works by providing a path from the supply voltageto the load through a variable resistance (the main transistoris in the "top half" of the voltage divider). The powerdissipated by the regulating device is equal to the power
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supply output current times the voltage drop in theregulating device.

The shunt regulator works by providing a path from thesupply voltage to ground through a variable resistance (the

main transistor is in the "bottom half" of the voltage divider). The current through the shunt regulator is diverted awayfrom the load and flows uselessly to ground, making thisform even less efficient than the series regulator. It is,however, simpler, sometimes consisting of just a voltage-reference diode , and is used in very low-powered circuitswhere the wasted current is too small to be of concern. Thisform is very common for voltage reference circuits.

All linear regulators require an input voltage at least some minimum amount higher than thedesired output voltage. That minimum amount is called the drop-out voltage . For example, acommon regulator such as the 7805 has an output voltage of 5V, but can only maintain this if theinput voltage remains above about 7V. Its drop-out voltage is therefore 7V - 5V = 2V. When thesupply voltage is less than about 2V above the desired output voltage, as is the case in low-voltage microprocessor power supplies, so-called low dropout regulators (LDOs) must be used.

When one wants a voltage higher than the available input voltage, no linear regulator will work (not even an LDO ). In this situation, a switching regulator must be used.

Simple zener regulator (shunt regulator)

Simple zener voltage regulator

The image shows a simple zener voltage regulator. It is a shunt regulator and operates by way of the zener diode's action of maintaining a constant voltage across itself when the current throughit is sufficient to take it into the zener breakdown region. The resistor R1 supplies the zener current I Z as well as the load current I R2 (R2 is the load). R1 can be calculated as -

where, V Z is the zener voltage, and I R2 is the required load current.

This regulator is used for very simple low power applications where the currents involved arevery small and the load is permanently connected across the zener diode (such as voltagereference or voltage source circuits). Once R1 has been calculated, removing R2 will cause the
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full load current (plus the zener current) to flow through the diode and may exceed the diode'smaximum current rating thereby damaging it. The regulation of this circuit is also not very good

because the zener current (and hence the zener voltage) will vary depending on V S and inverselydepending on the load current.

Simple series regulator

Simple series voltage regulator

Adding an emitter follower stage to the simple zener regulator forms a simple series voltageregulator and substantially improves the regulation of the circuit. Here, the load current I R2 issupplied by the transistor whose base is now connected to the zener diode. Thus the transistor's

base current (I B) forms the load current for the zener diode and is much smaller than the currentthrough R2. This regulator is classified as "series" because the regulating element, viz., thetransistor, appears in series with the load. R1 sets the zener current (I Z) and is determined as -

where, V Z is the zener voltage, I B is the transistor's base current and K =1.2 to 2 (to ensure that R1 is low enough for adequate I B).

where, I R2 is the required load current and is also the transistor's emitter current (assumed to be equal to the collector current) and h FE(min) is the minimum acceptable DCcurrent gain for the transistor.

This circuit has much better regulation than the simple zener regulator, since the base current of the transistor forms a very light load on the zener, thereby minimising variation in zener voltagedue to variation in the load. Note that the output voltage will always be about 0.65V less than the

zener due to the transistor's V BE drop. Although this circuit has good regulation, it is stillsensitive to the load and supply variation. It also does not have the capability to be adjustable.Both these issues can be resolved by incorporating negative feedback circuitry into it. Thisregulator is often used as a "pre-regulator" in more advanced series voltage regulator circuits.
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Protection of voltage regulated supplies

Over voltage protection

The inventionrelates to avoltage regulator with over-voltage

protection, whichis mounted in anon-board supplysystem betweenthe generator,

which is arrangedon a first voltage

plane, and a battery which isarranged on thesecond voltage

plane. Thgenerator is usedas a generator,whereby the control voltage thereof can be freely selected within predefined limits. On theoutput side, the voltage regulator, which is configured as a series regulator, adjusts a regulatedvoltage which is used to supply the conventional on-board supply system user and for chargingthe battery. The series regulator is fitted with an electronic system and/or an intelligence systemwhich comprises a microprocessor and evaluates supplied information relating to voltages,currents or load cut-outs, defines response criteria and introduces the necessary measures whenthe response criteria is met.

Short Circuit Protectionseries with the load. If a short develops in the load, a large amount of current

will flow in the regulator circuit. The pass transistor can be damaged by thisexcessive current flow. You could place a fuse in the circuit, but in manycases, the transistor will be damaged before the fuse The main disadvantageof a series regulator is that the pass transistor is in blows. The best way toprotect this circuit is to limit the current automatically to a safe value. Aseries regulator with a current-limiting circuit is shown in figure 4-50. Youshould recall that in order for a silicon NPN transistor to conduct, the basemust be between 0.6 volt to 0.7 volt more positive than the emitter. Resistor
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R4 will develop a voltage drop of 0.6 volt when the load current reaches 600milliamperes. This is illustrated using Ohm's law:

Figure 4-50. - Series regulator with current limiting .

When load current is below 600 milliamperes, the base-to-emitter voltage onQ2 is not high enough to allow Q2 to conduct. With Q2 cut off, the circuit actslike a series regulator.

When the load current increases above 600 milliamperes, the voltage dropacross R4 increases to more than 0.6 volt. This causes Q2 to conduct throughresistor R2, thereby decreasing the voltage on the base of pass transistor Q1.

This action causes Q1 to conduct less. Therefore, the current cannot increaseabove 600 to 700 milliamperes.

By increasing the value of R4, you can limit the current to almost any value.For example, a 100-ohm resistor develops a voltage drop of 0.6 volt at 6milliamperes of current. You may encounter current-limiting circuits that aremore sophisticated, but the theory of operation is always the same. If youunderstand this circuit, you should have no problem with the others.


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Thermal protection

The present invention relates to power supplies, and more particularly to a voltage regulator withthermal overload protection for use with electronic flash units.

Numerous types of electronic photoflash units currently are available, including manual andautomatic types. In a typical automatic electronic photoflash unit, a relatively large maincapacitor is used as the power source for the electronic flash tube. With the main capacitor suitably charged, the flash tube can be triggered, whereupon energy from the capacitor isdumped into the flash tube and causes the flash tube to emit light. In the automatic type unit, alight sensor circuit is provided which senses the light reflected from the subject being

photographed, and when sufficient light has been received this circuit operates to extinguish lightemission from the flash tube. In one class of electronic photoflash unit of the automatic type, alow resistance path is applied across the flash tube to thereby shunt the remaining capacitor energy therethrough. In another type, a switch is used in series with the flash tube, and whensufficient light has been received, this switch is turned off to thereby extinguish the flash of light.

Typical flash units of this nature include a builtin power supply with either disposable or rechargable batteries, along with an appropriate converter circuit for supplying a chargingvoltage to the main capacitor, which voltage typically is of the order of 330 volts dc. An exampleof a prior photoflash power supply using a series regulator between a dc source and maincapacitor of a flash unit is shown in U.S. Pat. No. 3,819,893.

The usual rechargeable batteries have sufficient energy for exposing a roll or so of film, and thenneed to be recharged. Although present-day electronic flash units sometimes have a rapid chargecapability for the rechargeable battery system, there are instances when photographers desire a

portable power supply or battery pack which can be connected with the electronic flash unit, andwhich will provide the capability for more flash exposures. While such battery packs provide thiscapability, and the capability to rapidly recharge the main capacitor of the flash unit, they candamage the flash unit. When the photographer, using such a battery pack, takes a rapid sequenceof pictures, because of the number of times the main capacitor of the flash unit is recharged over a short period of time, damage to the electronic flash unit may result easily.

Accordingly, the present invention provides a voltage regulator in the form of a portable power supply or battery pack which may be used with conventional electronic flash units and whichincludes a medium-to-long term heat integration feature to minimize the change of damage to theelectronic flash unit. That is, the voltage regulator incorporates thermal overload protection for the electronic flash unit load rather than for the voltage regulator itself. This is accomplished in aseries regulator circuit by causing the regulator to turn off if (a) the total current in (b) a giventime reaches a potentially destructive level. This is accomplished through thermal coupling

between one or several components of the series regulator and a thermal switch to provide aresponsive action to the time average electrical charge sent to the flash unit.

industrial electronics 2nd unit

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