The optocoupler is a circuit/circuit component that optically couples the signal from one circuit to the other circuit and provides electrical isolation (Galvanic isolation) between the two circuits.

Since it provides the electrical isolation between the two circuits, it can be used for protecting very important low voltage circuit block from other circuits which is prone to the high voltage spike, noise or ground loops.

Applications of Optocoupler

1. To provide electrical isolation between two electrical circuits
2. Prevent very important low voltage circuit from noise, ground loops, and high voltage spikes
3. To control the high voltage circuit using a logic circuit or micro-controller (keeping electrical isolation between them)
4. Communication Systems
5. Solid State Relays
6. Power Supplies

What’s Inside Optocoupler?

A typical optocoupler consists of two elements. The Infrared LED and Photosensitive device. The photosensitive device can be a photo-transistor, photo Darlington pair, photo-SCR, or photo Triac. The LED and photosensitive devices are integrated into a single package and for optimal coupling, their spectral response or wavelengths are tightly matched.

How Optocoupler Works?

On one side of the optocoupler, when an electrical signal is applied to the LED, the LED converts the electrical signal into an optical signal. The LED light falls on the photosensitive device and it converts the optical light into the electrical signal. (It generates the photocurrent). When light falls on the photo-sensitive device, it conducts and allows the flow of current. And the same current also flows through the external circuit which is connected with the photodetector. In this way, the optocoupler optically couples the signal of one circuit to the other circuit.

The electrical signal could be an analog signal or a digital signal. For analog signals, to achieve good linearity between the input and output, linear optocouplers are used.

Optocoupler Specifications

While selecting the optocoupler for a specific application, one should also check the specifications of the optocoupler. Here is the list of some of the important specifications of optocoupler.

1. Forward Current and the Forward Voltage

In the datasheet, the absolute maximum rating for various parameters is specified. One such parameter is the forward current of the LED. The current through the LED should be less than the maximum specified limit.

In the datasheet, the typical value of the forward voltage drop across the LED is also mentioned.

Based on the input voltage and the typical forward voltage drop across the LED, one can decide the series resistor for the LED for the specific current. But the current should not exceed the maximum specified value of the current in the daatsheet.

2. Current Transfer Ratio (CTR)

The current transfer ratio is the ratio of the output collector current (In case of the photo-transistor) to the input forward current of the LED in the optocoupler.

The CTR changes with photo-sensitive devices. The different photo-sensitive devices (like photo-transistor, photo SCR, photo Darlington pair) have different output currents and hence different current transfer ratios. But for the given, photo-sensitive device, it is the function of temperature, the forward current of the LED, and the output biasing voltage.

3. Switching Characteristics

When optocoupler is used for the switching application then this characteristic is very important. Under this characteristic, the typical rise-time and fall-time for the optocoupler is specified in the datasheet. The rise-time and the fall-time decide the maximum switching frequency of the optocoupler.

4. Maximum Isolation Voltage

It is the maximum RMS voltage up to which it provides the isolation between the two sides of the optocoupler. Typically, this isolation voltage is specified in kV. The datasheet also mentions the peak transient voltage. It is the peak transient voltage up to which it provides the electrical isolation between the two sides of the optocoupler.

5. Common mode Transient Immunity

The optocoupler should be able to reject the common mode noise and common mode transient noise. The common-mode noise is noise which presents on the both input and output side of the optocoupler. The optocoupler datasheet mentions the common-mode transients (V /µs) up to which it provides immunity.

Difference between Relay and Optocoupler

Functionality wise the relays are very similar to the optocoupler as they provides the electrical isolation between the two circuits. And using relay, it is possible to control the high voltage circuit using low voltages. But there are some differences between relay and optocoupler.

1. Optocouplers are typically faster than Relay (Relays are used for slow switching applications)
2. Optocouplers work with both analog and digital signals. The optocouplers can be used in switching applications as well as to transfer the analog signal from one circuit to the other circuit. While relays are typically used for switching applications.
3. The relays can work with high current and high power loads. The optocouplers are typically used for low power applications.
4. Since optocoupler does not contain any mechanical part like relay, the life span of optocoupler is longer than relay.

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The capacitor is one of the widely used electrical circuit elements in electrical and electronic circuits. In fact, it is hard to imagine any circuit without capacitors.
Depending on the type of dielectric material and the construction of the capacitor, there are various types of capacitor which are used in different applications, like decoupling, timing and wave shaping, filtering, etc. Depending on the different characteristics or the electrical properties of the capacitor, different capacitors are used for different applications.

Fig. 1 Various Types of Capacitors

Here is a list of some of the important specifications or the electrical characteristics of the capacitor.

1. Rated Capacitance
2. Rated Voltage
3. Tolerance
4. Temperature Coefficient
5. Leakage Resistance
6. Volumetric Efficiency
7. Effective Series Resistance / Dissipation Factor

These electrical characteristics or the specification decides which type of capacitor should be used for a specific application.

Types of Capacitors

There are various types of capacitors that are used in electrical and electronic circuits. But here is the list of widely used capacitor types.

1. Electrolytic Capacitors
2. Ceramic Capacitors
3. Film Capacitors
4. Mica Capacitors
5. Supercapacitors
6. Trimmer Capacitors (Variable Capacitor)

The capacitors can be classified as polarized and non-polarized capacitors. Electrolytic capacitors and supercapacitors are polarized capacitors, while ceramic, film, mica, or trimmer capacitor are non-polarized capacitors.

Electrolytic Capacitor

The electrolytic capacitors are polarized capacitors where the anode or the positive terminal of the plate is made up of metal and through anodization, the oxide layer is created. This oxide layer acts as an insulator. The electrolyte covers the surface of the oxide layer and acts as a cathode or the negative plate of the capacitor. Depending on the type of material used for the electrode, there are three types of electrolytic capacitors.

1. Aluminum Electrolytic Capacitor
2. Tantalum Electrolytic Capacitor
3. Niobium Electrolytic Capacitors

Aluminum Electrolytic Capacitor

Fig. 2 Aluminum Electrolytic Capacitors

In aluminum electrolytic capacitors, the anode or the positive terminal is made up of aluminum. The aluminum oxide acts as a dielectric. And the electrolyte acts as a cathode of the capacitor. Due to the very thin oxide layer, it is possible to achieve very high capacitance in these aluminum capacitors.

Here is the typical characteristics of the Aluminum Electrolytic Capacitor

• Cheaper than other types of capacitor
• Large Capacitance (0.1 µF to 2.2 F)
• Voltage Rating ( 2.5 V to 700V)
• Typical Tolerance (± 10% to 20 %)
• High Effective Series Resistance (ESR)
• High Leakage current
• Lower Lifetime than other types of capacitor (Particularly for wet electrolytic capacitors)

Usage: Since these capacitors are polarized they are typically used in DC applications. They are used in the DC power supply filtering and the de-coupling application.

Tantalum Capacitor

Fig.3 Tantalum Capacitors

In the tantalum capacitor, the tantalum metal is used as electrode and thin tantalum oxide Is created on top of it, which acts as a dielectric. This tantalum capacitors are available in lead type and as well in the chip form for the surface mounting.

Here is the typical characteristics of the Tantalum Electrolytic Capacitor

• Capacitance Range: 10 nF to 100 mF
• higher Volumetric Efficiency than Aluminum Electrolytic Capacitors
• Rated Voltage: 2V to 500 V
• Tolerance : ± 5% to 20 %
• Lower ESR than Aluminium Capacitors (can withstand higher ripple currents than aluminum capacitors for the same capacitance and voltage rating)
• Very stable over the temperature range and very reliable
• Longer shelf life
• Very sensitive to reverse polarity (Even a small amount of reverse voltage can lead to the destruction of the capacitor)
• Very sensitive to the high voltage spikes

Usage:

Due to reliability and good volumetric efficiency, the tantalum capacitors are used in applications where good reliability is required and there is a size constrain.

1. Sample and Hold Circuits
2. Medical Devices
3. Power Supply Filtering Circuits in laptops and other compact devices
4. Military and Space Applications

Niobium Capacitors

Fig. 4 Niobium Oxide Capacitors

Niobium Capacitors are not as popular as tantalum and aluminum capacitors but, in some capacitance, and voltage range they are used over tantalum capacitors because of their lower price.

Ceramic Capacitors

Fig. 5 Ceramic Capacitors

In a ceramic capacitor, the dielectric material is the ceramic material. They are available in the lead from and the surface mount from. The ceramic capacitors are available in wide capacitance (0.1 pF to 100µF) and the voltage range ( 2V to 50 kV). The ceramic capacitors are non-polarised capacitors and can be connected either way in the circuit.

Depending on the type of dielectric material of ceramic capacitors, there are two types of ceramic capacitors.

1. Type- I Ceramic Capacitors
2. Type-II Ceramic Capacitors

Class-I Ceramic Capacitors

Class-I ceramic capacitors are used in the circuits where high stability and low losses are required.They are very accurate and provides very stable capacitance over the change in the temperature, biasing voltage or the frequency.

Here is the typical characteristics of the Class-I Ceramic Capacitor

• Good Stability and Reliability
• Low ESR and Low leakage current
• Low tolerance
• Low Volumetric Efficiency (Typically available in low capacitance value)

Usage: Class-I ceramic capacitors are used in high-frequency applications like resonant circuits and oscillators.

Class-II Ceramic Capacitors

Here is the typical characteristics of the Class-II Ceramic Capacitor

• Low Stability and accuracy than Class-I Capacitors
• High Volumetric Efficiency than Class-I Capacitors
• Non-linear change in the capacitance over the operating temperature range
• Change in capacitance with biasing voltage
• Large Tolerance than class-I capacitors

Usage: Class-II ceramic capacitors are used in applications where stability and the accurate value of capacitance are less important. Typically, they are used in DC power supply filtering circuits and for coupling and decoupling in various circuits.

Multilayer Ceramic Capacitors (MLCC)

Fig. 6 Multilayer Ceramic Capacitors (MLCC)

Now a days, the surface mount type multilayer ceramic capacitor or the MLCC is very popular and billions of such capacitors are produced every year. As shown in Fig. 7, MLCCs are made up of alternating layers of metal electrodes and dielectric ceramic.

Fig. 7 Construction of Multilayer Ceramic chip Capacitor (MLCC)

The resulting capacitor basically consists of many smaller capacitors connected in parallel, and that increases the overall capacitance. MLCCs consist of 500 or more such layers. For type-I MLCC, the capacitance is in the range of pF to nF. And they are used in applications where precision and stability are required. The type-II MLCC are available with capacitance up to 100 µF, and they are used in power supply filtering and general-purpose applications.

Film Capacitors

Fig. 8 Film Capacitors

In the film capacitor, the plastic film is used as a dielectric material. Depending on the different film materials, there are different types of film capacitors. (e.g polypropylene, polyester, and polystyrene)

The film capacitors are used in applications where good reliability and stability are required. In general, the film capacitors have low ESL and ESR or low dissipation factor. They are available in pF to mF capacitance range with a voltage rating range from 10V to 10 kV.

Depending on the film material, different types of film capacitors have different electrical characteristics.

Polypropylene Film Capacitors

Polypropylene film capacitors are one of the widely used type of film capacitors. Here are typical characteristics of Polypropylene film capacitor.

• Low Leakage Current
• Low Dissipation Factor
• Good Temperature Stability
• Can withstand high voltage and high current surges
• Low Volumetric Efficiency

Usage: They are used in the oscillator and the frequency selective circuits where precise capacitance is required. Due to the low leakage current, it is also used in the sample and hold circuits.

In power electronics,they are used for EMI suppression, power factor correction and in the pulse applications.

Polyester Film Capacitors

They have higher volumetric efficiency than polypropylene capacitors but the capacitance is temperature and frequency dependent. They are typically used in the general-purpose applications.

Supercapacitors

Fig. 9 Supercapacitors

Supercapacitors are also known as ultra-capacitor. It is used to store a large amount of electrical charge. Typically its capacitance ranges from a few Farad to 100s of Farad. And Maximum charge voltage is 2.5 to 2.9V. Sometimes you also find a supercapacitor whose voltage rating is higher. But in such case, two or more capacitors are connected in series internally.

The super capacitors are used to quickly transfer the charge or for fast charge and discharge time.Batteries usually take up to several hours to reach a fully charged state, while supercapacitors can be brought to the same charge state in less than two minutes.

It finds its application in the automotive industry and in low-power applications where a large life cycle and quick recharge are important. (e.g photographic flash and SRAM)

Mica Capacitors

In the Mica capacitor, a silver mica is used as a dielectric material. These capacitors provide good temperature stability and very accurate capacitance. And therefore they are used in RF applications.

Due to the high breakdown voltage, these capacitors are used in high voltage applications. Because of their large size and high cost, now a days these capacitors are replaced by other capacitors.

Trimmer Capacitors

Fig. 10 Trimmer Capacitor

Trimmer capacitors are variable capacitors. They are used for the initial calibration of equipment during manufacturing or servicing. In this type of capacitor, by changing the distance between the two plates or by changing the overlapping area between the two plates, the capacitance is varied in certain range.

These capacitors are used to initially set oscillator frequency values, latencies, rise and fall times, and other variables in a circuit. There are two types of trimmer capacitors: air trimmer capacitor and ceramic trimmer capacitor.

The minimum capacitance of these capacitors is around 0.5 pF and it can be varied up to 100 pF. These capacitors are available up to the voltage rating of 300V.

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