Industrial Electronics 2nd Unit

INDUSTRIAL ELECTRONICS 2nd unit Regulated Power Supply System

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Transformer - steps down 230V AC mains to low voltage AC.

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Rectifier - converts AC to DC, but the DC output is varying.

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Smoothing - smooths the DC from varying greatly to a small ripple.

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Regulator - eliminates ripple by setting DC output to a fixed voltage.

Voltage regulator principle

Electronic symbol for Voltage regulator

A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage 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

is amplified and used to control the regulation element in such a way as to reduce the voltage error. This forms a negative feedback servo control loop; increasing the open-loop gain tends to increase regulation accuracy but reduce stability (avoidance of oscillation, or ringing during step changes). There will also be a trade-off between stability and the speed of the response to changes. If the output voltage is too low (perhaps due to input voltage reducing or load current increasing), the regulation element is commanded, up to a point, to produce a higher output voltage - by dropping less of the input voltage (for linear series regulators and buck switching regulators), or to draw input current for longer periods (boost-type switching regulators); if the output voltage is too high, the regulation element will normally be commanded to produce a lower voltage. However, many regulators have over-current protection, so entirely stop sourcing current (or limit the current in some way) if the output current is too high, and some regulators may 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 is made to act like a variable resistor, continuously adjusting a voltage divider network to maintain a constant output voltage. It is very inefficient compared to a switched-mode power supply, since it 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 output voltage, and a feedback circuit compares the output voltage to a reference voltage in order to adjust the input to the transistor, thus keeping the output voltage reasonably constant. This is inefficient: since the transistor is acting like a resistor, it will waste electrical energy by converting it to heat. In fact, the power loss due to heating in the transistor is the current times the 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 in it 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 and shunt regulators. •

Series regulators are the more common form. The series regulator works by providing a path from the supply voltage to the load through a variable resistance (the main transistor is in the "top half" of the voltage divider). The power dissipated by the regulating device is equal to the power

supply output current times the voltage drop in the regulating device. •

The shunt regulator works by providing a path from the supply 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 away from the load and flows uselessly to ground, making this form even less efficient than the series regulator. It is, however, simpler, sometimes consisting of just a voltagereference diode, and is used in very low-powered circuits where the wasted current is too small to be of concern. This form is very common for voltage reference circuits.

All linear regulators require an input voltage at least some minimum amount higher than the desired output voltage. That minimum amount is called the drop-out voltage. For example, a common regulator such as the 7805 has an output voltage of 5V, but can only maintain this if the input voltage remains above about 7V. Its drop-out voltage is therefore 7V - 5V = 2V. When the supply voltage is less than about 2V above the desired output voltage, as is the case in lowvoltage 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 through it is sufficient to take it into the zener breakdown region. The resistor R1 supplies the zener current IZ as well as the load current IR2 (R2 is the load). R1 can be calculated as -

where, VZ is the zener voltage, and IR2 is the required load current. This regulator is used for very simple low power applications where the currents involved are very small and the load is permanently connected across the zener diode (such as voltage reference or voltage source circuits). Once R1 has been calculated, removing R2 will cause the

full load current (plus the zener current) to flow through the diode and may exceed the diode's maximum 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 VS and inversely depending 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 voltage regulator and substantially improves the regulation of the circuit. Here, the load current IR2 is supplied by the transistor whose base is now connected to the zener diode. Thus the transistor's base current (IB) forms the load current for the zener diode and is much smaller than the current through R2. This regulator is classified as "series" because the regulating element, viz., the transistor, appears in series with the load. R1 sets the zener current (IZ) and is determined as -

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

where, IR2 is the required load current and is also the transistor's emitter current (assumed to be equal to the collector current) and hFE(min) is the minimum acceptable DC current 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 voltage due 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 VBE drop. Although this circuit has good regulation, it is still sensitive 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. This regulator is often used as a "pre-regulator" in more advanced series voltage regulator circuits.

Protection of voltage regulated supplies Over voltage protection The invention relates to a voltage regulator with over-voltage protection, which is mounted in an on-board supply system between the generator, which is arranged on a first voltage plane, and a battery which is arranged on the second voltage plane. Th generator is used as a generator, whereby the control voltage thereof can be freely selected within predefined limits. On the output side, the voltage regulator, which is configured as a series regulator, adjusts a regulated voltage which is used to supply the conventional on-board supply system user and for charging the battery. The series regulator is fitted with an electronic system and/or an intelligence system which comprises a microprocessor and evaluates supplied information relating to voltages, currents or load cut-outs, defines response criteria and introduces the necessary measures when the response criteria is met.

Short Circuit Protection series 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 this excessive current flow. You could place a fuse in the circuit, but in many cases, the transistor will be damaged before the fuse The main disadvantage of a series regulator is that the pass transistor is in blows. The best way to protect this circuit is to limit the current automatically to a safe value. A series regulator with a current-limiting circuit is shown in figure 4-50. You should recall that in order for a silicon NPN transistor to conduct, the base must be between 0.6 volt to 0.7 volt more positive than the emitter. Resistor

R4 will develop a voltage drop of 0.6 volt when the load current reaches 600 milliamperes. This is illustrated using Ohm's law:

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

When load current is below 600 milliamperes, the base­to­emitter voltage on  Q2 is not high enough to allow Q2 to conduct. With Q2 cut off, the circuit acts  like a series regulator.  When the load current increases above 600 milliamperes, the voltage drop  across R4 increases to more than 0.6 volt. This causes Q2 to conduct through  resistor R2, thereby decreasing the voltage on the base of pass transistor Q1.  This action causes Q1 to conduct less. Therefore, the current cannot increase  above 600 to 700 milliamperes.  By increasing the value of R4, you can limit the current to almost any value.  For example, a 100­ohm resistor develops a voltage drop of 0.6 volt at 6  milliamperes of current. You may encounter current­limiting circuits that are  more sophisticated, but the theory of operation is always the same. If you  understand this circuit, you should have no problem with the others. 

Thermal protection

The present invention relates to power supplies, and more particularly to a voltage regulator with thermal overload protection for use with electronic flash units. Numerous types of electronic photoflash units currently are available, including manual and automatic types. In a typical automatic electronic photoflash unit, a relatively large main capacitor 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 is dumped into the flash tube and causes the flash tube to emit light. In the automatic type unit, a light 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 light emission from the flash tube. In one class of electronic photoflash unit of the automatic type, a low 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 when sufficient 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 charging voltage to the main capacitor, which voltage typically is of the order of 330 volts dc. An example of a prior photoflash power supply using a series regulator between a dc source and main capacitor 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 then need to be recharged. Although present-day electronic flash units sometimes have a rapid charge capability 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, and which will provide the capability for more flash exposures. While such battery packs provide this capability, and the capability to rapidly recharge the main capacitor of the flash unit, they can damage the flash unit. When the photographer, using such a battery pack, takes a rapid sequence of 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 which includes a medium-to-long term heat integration feature to minimize the change of damage to the electronic 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 a series regulator circuit by causing the regulator to turn off if (a) the total current in (b) a given time 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 a responsive action to the time average electrical charge sent to the flash unit.