Filter Circuit and Need of filters in Electronics
Table of Contents
A device or circuit, which converts pulsating DC output received from a rectifier, into a steady solid-state DC, is called a filter. The basic function of a filter circuit is to eliminate ripple contents found in the rectifier’s output DC.
We know that the output of various rectifier circuits uses to be pulsating in nature that is it contains some value of AC contents apart from DC value (due to which rectifier’s output consists of impure DC). The presence of AC contents in DC output is called ripples. Pulsating DC is useful for some needs (e.g. for operating different relays or battery charging), however, this pulsating Dc is not suitable for operating sensitive, delicate and precious electronic circuits or gadgets (e.g. TV, radio, amplifier, etc.), because such appliances require a very steady-state DC. (Heat produces in such gadgets due to pulsating DC, due to which their performance deteriorates). Therefore, certain circuits are used for emitting / filtering out AC contents from the rectified output, which are called filter circuits. Thus, a circuit that converts the pulsating output of a rectifier into a steady-state level or pure output is called a filter (figure 1)
The filter circuit is composed of just a capacitor or only an inductor or different combinations of these two. For using a capacitor as a filter, it has to be provided parallel pulsating voltages, due to which the process of charging and discharging occurs. Capacitor charges during an increase in the voltage value i.e. it stores energy in the form of an electrostatic field. When it gets fewer voltages, the capacitor returns its stored energy to the circuit, thus it reduces ripple by discharging through the load. Thus, deficiency in voltage is compensated through capacitor discharging. Capacitor charges again during the next pulse and this process of charging and discharging moves on.As a capacitor, opposes a change in voltage, nearly constant voltages are found parallel to it. In figure 2, the capacitor’s filtering action whereas in figure 3 capacitor filter’s input and output have been displayed. If an inductor or choke is inserted in the pulsating voltage series for the purpose of filtering, continuously changing voltages, cause a change in current according to their own change. When this changed current passes through the choke coil, a voltage is produced as per Lens law, which opposes the change occurring in the current. Thus, the tendency of change in current minimizes, and achieving constant voltages become possible from the inductor’s output. In other words, the inductor abolishes changes in current by storing and returning energy deposited in the magnetic field. This inductor blocks AC contents.
Types of Filter Circuits
Filter circuits manufactured through a combination of inductor and conductor, are of the following types
- L-Filter Circuit
- Pie- Filter circuit (π-Filter)
- T-Filter circuit
There are further two types of L-Filter circuits according to the mutual arrangement of capacitor and inductor.
- Inductor Input L-Filter circuit
- Capacitor Input L-Filter circuit
Inductor Input Filter circuit
Inductor input filter is also known as choke input filter or LC filter. In such filters, first an inductor and then a capacitor is being applied, therefore, it is called an inductor input filter. Inductor fixed on the circuit, creates too much reactance or resistance in the way of AC contents found in current, while DC passes through it without any resistance. Contrary to this, when the capacitor is fixed after the inductor, gets the inductor’s output. It lets ripples pass through it offering very little reactance in the way of remnant output’s AC contents. Thus, ripples get bypassed through the capacitor and steady-state or smooth DC becomes available on load. Remember that capacitor works as a short circuit for AC and an open circuit for DC whereas inductor functions as a short circuit for DC and an open circuit for AC. Choke’s input filter circuit along with the voltage waveform has been illustrated in figure 4
We know it is the property of a choke or inductor that opposes changes in AC current passing through it. The output achieved from the rectifier (which has to be filtered), is composed mainly of DC contents apart from some AC components (i.e. it is pulsating) Inductor. Inductor offers a lot of resistance in the path of flow of AC contents and allows DC contents to pass without any resistance (zero reactance). Thus, DC succeeds to pass through inductor L while a significant quantity of AC drops parallel to it. Thus, AC cannot reach the output. However, after passing through L, remnant AC contents get filtered through a shunt capacitor “C” fixed parallel to load, before reaching the output, because the capacitor provides a very low reactance or resistance path to AC, while for DC, it acts as an open circuit (i.e. capacitor causes high resistance in the path of DC). Thus, only DC passes through the load, however, a tiny quantity of ripples is left behind in the output voltage. Remember, due to a choke in the inductor input filter,there is less current charging the capacitor, due to which the capacitor cannot charge to its peak value. Therefore, less output voltage we get compared to the capacitor input filter. Moreover, if the value of L is small, there will be more ripples in output and if the L value is increased, ripples will subside. Due to being costly, heavy, and large in size, the inductor or choke input filter is not so vastly used.
Capacitor Input Filter circuit
A filter circuit, in which a capacitor has been fixed in the beginning and an inductor fitted subsequently, is called a capacitor input filter. It has been shown in figure 5
It is visible from the diagram, immediately after the rectifier circuit, a parallel capacitor and then an inductor in the load series, have been fixed. As this arrangement of combining capacitor and inductor is inverted L shaped, that’s why capacitor input filter is also called L filter.
We know that property of a capacitor that it opposes any changes in voltage, (or capacitor works as a short circuit for AC and an open circuit for DC). Pulsating output (which has to be filtered) obtained through a rectifier, when supplied on capacitor input circuit, the capacitor mounted at the beginning of filter circuit, charges to its peak value during conducting half-cycle (i.e. energy is stored inside it) In other words, capacitor charges during an increase in the value of input voltages and preserves energy within it in the form of an electrostatic field. On the contrary, when it receives fewer voltages, (i.e. during the non-conducting half cycle), the capacitor starts discharging through load i.e. it returns energy stored inside, and thus, deficiency in voltages is compensated through capacitor discharging. The capacitor recharges during the next pulse, and thus this process of charging and recharging continues. Resultantly, such voltages are found parallel to capacitors, which have very small ripple contents. Thus, when pulsating DC voltages are supplied on the capacitor, it smoothens or filters out ripples to a great extent. After this, the capacitor’s output is passed on to the inductor adjusted alongside it. This inductor drops the rest of the ripples present in the capacitor’s output, due to its high reactance. Therefore, the inductor mounted in a series of loads prevents current variations (i.e. AC passes through the inductor quite easily whereas the remnant quantity of AC drops parallel to it). Consequently, AC contents or ripples cannot reach output and output becomes available in pure DC form.
Remember, a capacitor is charged to its peak value in a capacitor input filter, therefore, more output voltages are achieved through applying a capacitor input filter instead of a choke input filter in the power supply. Capacitor input filters are most widely used in power supplies.
Pie Filter circuit
A filter, in which two capacitors and an inductor are connected in the shape of a pie (π) is called a pie filter or CLC filter. In this circuit, first an inductor after a capacitor, and then another capacitor is being mounted. In other words, the pie filter is composed of an inductor and two capacitors mounted on both of its parallel ends. As the three components are interconnected in the form of the Greek word pie (π), therefore, it is called a pie filter or capacitor input filter. In figure 6, the pie filter has been illustrated both through its output and input waveforms.
A value of input capacitor C1 is selected, which provides exceptionally little reactance to ripple frequency, therefore, the bulk of filtering is concluded through C1, while the rest of the ripples come to an end through mutual interaction between L and C2. When pulsating DC voltages received from the rectifier, are provided on the first capacitor C1, it is charged to its peak value. The charge gradually leaks or discharges through the load during the rest of the input cycle. The charge leakage process occurs between central periods of two continuous peaks following which, the charging current transmits through C1 once again, by means of which it is recharged to its peak value once again. Thus, such voltages appear parallel to C1 which are fewer variations compliant as compared to a full rectified wave (i.e. very little AC contents exist in such voltages, detail of which has been given in the previous pages). Thus, the choke installed within the series, halts current variations and C2 bypasses remnant AC components, thus C2 also keeps the output voltage constant.
This circuit performs filtering in an exceedingly better manner as compared to the LC filtering circuit. However, C1 which is mounted directly parallel to the supply requires high pulses current in case of an excessive current load. As these high peak current pulses can damage the rectifier, therefore, this filter is used mostly in low current devices.
Though this filter provides some large output voltages, its voltage regulation is inferior to the LC filter.
As the name suggests, it is a filter in which two interconnected inductors and a conductor are arranged in such a way that it resembles the English alphabet “T”. In other words, basically, a T-filter consists of an LC filter and an inductor. At the start, there exists an LC filter, while an inductor is mounted on the end of the LC filter. The inductor placed in the beginning is called L1, while the one mounted at the end, is called L2. T-filter completes by fixing a capacitor C parallel to inductors L1 and L2 (figure 7).
In a T-type filter circuit, drop voltages parallel to reactance in the LC filter (which has been installed at the starting point), output peak voltages are less compared to input peak voltages. However, due to the smoothing or filtering action of L1 and L2, the output resulting from this filter contains only a few ripples compared to a pie- filter. Normally, compared to a pie type filter, a T type filter provides low output voltages on a specific input voltage. However, it is better in case of plummeting ripples.
The first inductor L1 mounted in the filter circuit, through a huge resistance, blocks AC components found in the input, while the capacitor placed nearby this inductor, lets the AC components pass through it offering very little resistance. Thus, ripples vanish to a great extent in the first half of the T filter or through filtering action in the LC filter. Despite this, if the LC filter’s output contains some small quantity of AC components, the inductor L2, mounted near LC, thwarts even this negligible quantity of ripples through a high reactance, as a result, nearly uncontaminated DC becomes obtainable on the T filter output.
A small variation in DC output voltage of a filtered rectifier caused via charging and discharging of the filter capacitor is called ripple voltage.
We know that when a capacitor filter is mounted with a rectifier’s output, the capacitor charges very rapidly at the beginning of the positive cycle, however, after the positive peak has passed (i.e. when the diode has become reversed bias), it discharges gradually. Therefore, these variations in output voltages, resulting from the capacitor’s charging or discharging, are called ripple voltages. It has to be kept in mind that ripple is an unnecessary thing, therefore, the output that has more ripple value, implies that its filtering process is ineffective or not better. On the contrary, the process of filtering in rectified output, causing minimum ripples, is more effective (figure 8). In figure 9, a comparison of ripples has been reflected in situations of half wave or full wave through uniform filter capacitor, uniform load, and uniform sign wave inputs.
The ratio of an output voltage’s DC or an average value (VDC) obtained via a peak to peak ripple voltage (Vr) and filters circuit, is called ripple factor (figure 9)
It is the ratio of the ripple voltage to the dc output voltage.
Ripple Factor= peak to peak ripple voltage/ dc value of filter’s output voltage
Or r= Vr/ VDC
Remember, the ripple factor, in fact, reflects a power supply’s effectiveness to measure the reduction in ripple voltages. A filter with a lower ripple factor is considered better. Ripple factors’ value can be lowered by increasing the value of load resistance or filter capacitor.
The extreme current which rushes through a filter capacitor at the time of powering on a rectifier circuit is known as a surge current. It is the initial rush of current through a filter capacitor when the power is turned on. We know that prior to turning a rectifier unit on, the filter capacitor mounted alongside keeps uncharged. Due to being uncharged, when power is turned on the capacitor works as a short circuit initially. Therefore, the value of the initial charging current is incredibly high. At that time voltages are at their maximum or peak value. Peak voltage also exists on transformers winding. Therefore, both transformer’s winding and diodes’ resistance attempt to confine the high initial value of the charging current. However, in a short stint of time, when the capacitor starts charging progressively, this high-value initial current tends normally. The high value of initial current which rushes through a capacitor at the time of powering on a supply is called surge current. Initial extreme increase in current is highly undesirable, designers of a power supply select a diode with a high current rating, which could withstand this surge current. Normally, a surge resistor is applied for bringing the surge current to a secure level by limiting its initial impact. The value of surge current also depends on the size of the capacitor. If the value of a capacitor is low, it charges quickly and thus the value of surge current is also low. However, several cycles are required for charging a large-sized capacitor, due to which the duration of the surge current also gets longer and so long as the capacitor does not charge, there remains a danger of the diode becoming useless.
In figure 10 (a) rush of surge current in a capacitor, whereas in figure 10 (b), the limitation of surge current through mounting a surge resistor has been illustrated. According to figure (a), the filter capacitor remains uncharged before the switch is closed. As soon as the switch is closed, the bridge receives voltages and the capacitor looks short. Thus, there is a rush of initial high value surge current through two forward-biased diodes. If the switch is closed at the secondary voltage peak value, the value of the surge current becomes maximum, which can make the diode ineffectual. In figure (b), surge current has been limited through a surge resistor. Remember, the value of surge resister should be less than RL and the maximum forward surge current ratings should be to such an extent that it could tolerate an initial temporary surge of current.
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