Single button ON-OFF toggle with discrete transistors

     This circuit allows the switching appliances (loads) on and off with one momentary button. Once you press it, it turns on, press again to turn off. Usually this is built with binary divider or 555, but the disadvantage is the current consumption in OFF state and that this way is complicated. So I designed circuit without integrated circuits, with transistors only. In the off state it draws no current, because both transistors are closed. It is therefore suitable for battery power. 

     The working principle is simple: In OFF state the C1 is charged through the load and R2. After pressing the TL1, the voltage of C1 connects to T2 gate, it opens and turns on the load. At the same time T1 opens via R1 and further maintains the positive voltage on gate. C1 discharges through R2 and T1. When TL1 pressed again, gate of T2 is discharged into C1 (C1 has a much higher capacity than the gate). This closes the T2 and then T1. Resistance R3 keeps gate at 0V and the circuit remains in the OFF state until the next pressing of TL1. 

     As a switching element T2, N-type MOSFET was chosen, because it allows very little loss and in idle state its gate does not draw any unnecessary current. You can use any low-voltage MOSFET with Uds about 20 - 55V. The smaller on-state resistance, the better. Transistors rated at unnecessarily high voltages usually have the greater resistance. I used IRF3205 to test the circuit. You can also use MOSFETs from the PC motherboard. T1 is any small PNP like BC327, BC557 or 2SA733. In case of problems, ringing or too much capacity of gate, increase the value of C1. Maximum supply voltage is limited mainly by max voltage Ugs of T2 and maximum current by his allowable loss. The minimum supply voltage depends on the minimum voltage that turns T2 completely on. It is usually lower in LOGIC mosfets. In the case of inductive loads connect antiparallel diode to the output. When the load has a small current or its own switch, connect a parallel resistance of about 100k. The circuit is not suitable for loads with parallel capacity (modification would be necessary).

SIMPLE - IR RECEIVER

IR Receiver Circuit Diagram

IR Receiver circuit is very simple we just need to connect a LED to the output of the TSOP1738, to test the receiver. We have use BC557 PNP transistor here, to reverse the effect of TSOP, means whenever the output is HIGH LED will be OFF and whenever it detects IR and output is low, LED will be ON. PNP transistor behaves opposite to the NPN transistor, it acts as open switch when a voltage applied to its base and acts as closed switch when there is no voltage at its base. So normally TSOP output remains HIGH and Transistor behaves as open switch and LED will be OFF. As soon as TSOP detects Infrared, its output becomes low and transistor behaves as closed switch and LED will be ON. A 10k resistor is used for provide proper biasing to transistor and a 470ohm resistor is used at LED for limiting the current. So whenever we press the Button at IR transmitter, it is detected by TSOP1738 and LED will glow.

LDR based light control using LM358M

Pin Diagram

The LM358 datasheet specifies that it consists of two independent, high gain, internally frequency compensated operational amplifiers which were designed specifically to operate from a single power supply over a wide range of voltages. Operation from split power supplies is also possible and the low power supply current drain is independent of the magnitude of the power supply voltage. 

Features

• Available in 8-Bump micro SMD chip sized package, (See AN-1112)
• Internally frequency compensated for unity gain
• Large dc voltage gain: 100 dB
• Wide bandwidth (unity gain): 1 MHz (temperature compensated)
• Wide power supply range:
  • single supply: 3V to 32V
  • dual supplies: ±1.5V to ±16V
• Very low supply current drain (500 μA)—essentially independent of supply voltage
• Low input offset voltage: 2 mV
• Input common-mode voltage range includes ground
• Differential input voltage range equal to the power supply voltage
• Large output voltage swing

LDR and LM358

This is a simple dark sensor circuit. When you block light falling on LDR, the LM358 switches on the LED.

 Depending upon the light level of your room, you might have to adjust the variable resistor to adjust sensitivity of the circuit.

You can use relay to control other high rated devices.

Difference between kilowatt hour and amp hours

AMP HOURS VS KILOWATT HOURS

Amp Hour and Kilowatt Hour Differences Defined

Simply put,  Amp Hours are a measure of electric current and Kilowatt Hours are a measure of electric power.

However, there is a relationship between the two.  Specifically, electric current is a component of an equation that helps to determine electric power.  That equation is…

Power in Watts = Current in Amps x Voltage

Consider the following…

A battery rated for 100 amp hours will provide 5 amps for 20 hours.

If we have a 12 volt battery, we multiply 100 by 12 and determine that the battery will provide 1200 watt hours.

To apply the metric ‘kilo’ prefix, we divide the result by 1000 and determine that the battery can supply the 1.2 KW hours.

There is something to keep in mind. The Amp Hour rating is a 20 Hour rating, therefore it is necessary to treat any kilowatt conversion you make as a 20 hour rating as well.

In the case of our 1200 Watt Hour conversion, we need to understand that what is really being said is that the battery will provide 60 Watt Hours for 20 Hours.

Notes on Kilowatt vs Watts, KW vs KWH

The prefix ‘kilo’ indicate one thousand. Think of it as a multiplier. For example, if we have 1 Kilovolt (KV), we have 1000 volts.

Similarly 1 kilowatt equals 1000 watts.

The standard abbreviation for kilowatt is KW.

This differs from kilowatt hours. The abbreviation for kilowatt hours is KWH.

 Limitations on Kilowatt Hours as Tool.

Like Amp Hours, the Kilowatt Hour specification is a typically 20 hour rating. Like Amp Hours, the Kilowatt Hours are subject to a phenomenon known as Peukert’s Law.

What this phenomenon describes is the fact that any increase in the current draw on the battery will also result in a decrease on capacity. Similarly decreasing the current draw will result in an increase in battery capacity. More, the relationship is not linear.

From a practical perspective, this means the Kilowatt Hour rating is difficult to extrapolate when value as the current draw increases.

Kilowatt Hours and Series Batteries Systems.

In some systems, 12 volt batteries will be connected in series (end to end) to created a higher voltage. For example, two 12 volt batteries in series will provide 24 Volts.

Suppose two 12 volt batteries rated at 100 Amp Hours were connected in series. While the voltage would increase to 24 Volts, the Amp Hours would remain the same.

While amp hours remain the same, voltage does not. That means the power capacity has increased.

In the case of our 100 Amp Hour series batteries, the Watt Hours have increased to 2400 watts.

This particular characteristic can be a contributor to choosing 24 Volt Systems.

Wire size selection is largely based on the current that the wire must carry. The more current, the larger the wire. The larger the wire, the more expensive it is. Systems based on 24 volts can carry the same amount of power on smaller wire.

Solar night light - simple circuit

Simple Solar Circuits:

How to get started adding solar power to your small electronics projects. Use the sun to power small solar and battery powered night lights, garden lights, and decorations for halloween.

   

The first part of a solar circuit is  a device for collecting sunlight. To keep things simple, we’re using a single nicely made small solar panel for all of these circuits. The panel that we’re using for these circuits is this one, part number PWR1241 from BG Micro, about $3 each. This is a monolithic copper indium diselenide solar panel, apparently printed on a 60mm square of glass and epoxy coated for toughness. On the back of the panel are two (thin) solderable terminals, with marked polarity. (While you can solder directly to the terminals, be sure to stress-relieve the connections, e.g., with a blob of epoxy over your wires.) In full sunlight the panel is specified to produce 4.5 V at up to 90 mA, although 50 mA seems like a more typical figure.

[Before we move onto our first examples, a word of caution: These are small simple circuits. In building these, we will quite intentionally gloss over a number of minor details and issues that are unimportant at these low powers, but could become critical if you were to try to scale up.]

Direct Drive:

The most obvious way to use power from a solar panel is to connect your load directly to the output leads of the solar panel.

Here are a couple of examples of this in practice:

   

On the left, we’ve hooked up one of our little solar panels directly to a small motor taken from an old CD player. When you set it out in the sunlight or bring it close to a lamp, the motor starts to spin. On the right we’ve hooked one of the panels right up to a high-power blue LED. The reason that we’ve used a high-power LED here is that it can easily withstand 50-90 mA from the solar panel– a “regular” LED designed for 20 mA would be destroyed by that current. (The LED is the same type that we used for our high-power LED blinking circuit.)

Interruption-resistant direct drive:

The “direct drive” circuits work well for their design function, but are rather basic. They provide no energy storage, and so are quite vulnerable to blinking out when a bird or cloud passes overhead. For some applications, like running a small fan or pump, that may be perfectly acceptable. For other cases, like powering a microcontroller or other computer, a brief power interruption can be disruptive. Our next circuit design adds a supercapacitor as a “flywheel” to provide continued power during brief interruptions.

Instead of adding a single supercapacitor, you might notice that we’ve actually added two. That’s because the supercaps that we had on hand are rated for 2.75 V– not enough to handle the 4.5 V output of the panel when sunlight is present. To get around this limitation, we used two of the caps in series, for which the voltage ratings add, giving us a barely-okay total rating of 5.5 V. (Note: be careful adding capacitors of different values in series– the voltage ratings may scale in non-obvious ways.) When first exposed to the light, this circuit takes about 30 s to 1 minute to charge the capacitors enough that the LED can turn on. After it’s fully charged, the circuit can be removed from the sunlight and still drive the blue LED for about 30 s to 1 minute– a very effective flywheel for light duty applications.

Adding a battery

While interruption resistance is nice, a capacitor generally does not provide sufficient energy storage to power a solar circuit for extended periods of time in the dark. A rechargeable battery can of course provide that function, and also provides a fairly consistent output voltage that a capacitor cannot. In this next circuit, we use the solar panel to charge up a NiMH rechargeable battery and also LED off of the power, which will stay on when it gets dark out.

In this circuit the solar panel charges up a 3-cell NiMH battery (3.6 V). Between the two is a “reverse blocking” diode. This one-way valve allows current to flow from the solar panel to the battery, but does not allow current to flow backwards out of the battery through the solar panel. That’s actually an important concern because small solar panels like these can leak up to 50 mA in the reverse direction in the dark. We’re using a garden-variety 1N914 diode for reverse blocking, but there are also higher-performance diodes available that have a lower “forward voltage.”

In this design we are continuously “trickle charging” up the battery when sunlight is present. For NiMH batteries and sealed lead-acid batteries (the two types that are most suitable for this sort of un-monitored circuit) it is generally safe to “trickle” charge them by feeding them current at a rate below something called “C/10″. For our 1300 mAh battery cells, C/10 is 130 mA, so we should keep our charging below 130 mA; not a problem since our solar panels only source up to 90 mA.

The other thing to notice about this circuit is that it’s pretty darned inefficient. The LED is on all the time, whenever the battery is at least slightly charged up. That means that even while the circuit is in bright sunlight it is wasting energy by running the LED: a sizable portion of the solar panel current goes to driving the LED, not to charging the battery.

Detecting Darkness

To add a darkness detecting capability to our solar circuit is even easier, actually, because our solar panel can directly serve as a sensor to tell when it’s dark outside.

To perform the switching, we use a PNP transistor that is controlled by the voltage output from the solar panel. When it’s sunny, the output of the panel is high, which turns off the transistor, but when it gets dark, the transistor lets current flow to our yellow LED. This circuit works very well.

Connect battery directly to a solar panel?

is it ok to connect a solar panel straight to a battery to charge? 
do i just connect the positive to positive and negative to negative? 

also, lets say the battery is a 12V DC battery but has been discharged down to 6V. If the solar panel is getting 8V then its ok to connect them? 

 it is okay provided a couple of things. But if it were me, I would put a diode in the line. A diode is an electrical check valve, allowing current to flow in one direction, but not the other. This will allow the panel to charge the battery, but not let the battery discharge into the panel at night. Most panels today come with diodes already installed in the junction box. We have ours hooked up this way.

First, the voltage does not matter, as long as the panel voltage, or "Open Circuit Voltage," is higher than the battery's full charge voltage. This is why most panels today are wired for 18 volts, so they can charge a 12 volt battery. The panel voltage will simply sag to match the battery's charging curve voltage during the daytime. Second, if the panels max amps, or, "Short Circuit Current," rating is below 2% of the battery's amp hour capacity, then the panel will never overcharge the battery. You said your panel was putting out 8 volts? This is unusual, and I'm wondering if you checked the voltage while the battery was charging, or if you used the wrong contacts in the panels junction box for your voltage reading. A panel in direct sun without anything hooked to it should put out about 17 or 18 volts, unless it is wired for some other nominal voltage, other than 12. If you were charging a battery that was previously reading 6 volts, then 8 volts might be about right during the first phase of the charge, it will climb to 15 or so when finishing. Our system uses two 50 watt panels, with a max amperage of 6 amps, to charge 4 golf cart batteries, which are wired for an amp hour capacity of 440 AH, so we are well within the 2% window, and we do not use a charge controller. Our home is completely powered by the wind and sun, has been for 11 years now, but our little 12 volt system is still working along side of that, running some small lights and DC outlets in the home for small electronics. Just make sure if you do a direct hook up, that you keep the batteries watered at least once a month with distilled water, they will go through some electrolyte this way. 


Do not confuse a charge controller with a voltage controller. One is for charging batteries with a high current array, the other is used to send solar power directly to a device, like a water pump. There is a great discussion of all of this in Richard Perez's book, "The Complete Battery Book," look for it in your library. Richard happens to be the founding editor of Home Power Magazine, a good periodical on solar and wind power. I will put some ther sources below for you too. Be careful of what you read online about renewable energy, one thing I have learned over the years working on our home and teaching solar power courses at the local schools, is that there is a vast acreage of misinformation out there. Plenty of people are always willing to offer their advice on solar and wind, even though they have never laid a hand on a panel or wind turbine. I've had my hands on plenty of them, and they work fine once you understand them. Good luck Jt, and take care, Rudydoo

Opto Couplers – Types Based on Applications

Opto Couplers – Types & Applications

Opto-isolators or Opto-couplers, are made up of a light emitting device, and a light sensitive device, all wrapped up in one package, but with no electrical connection between the two, just a beam of light. The light emitter is nearly always an LED. The light sensitive device may be a photodiode, phototransistor, or more esoteric devices such as thyristors, TRIACs etc.

A lot of electronic equipment nowadays is using opt coupler in the circuit. An opt coupler or sometimes refer to as opt isolator allows two circuits to exchange signals yet remain electrically isolated. This is usually accomplished by using light   to relay the signal. The standard opt coupler circuits design uses a LED shining on a phototransistor-usually it is a npn transistor and not pnp. The signal is applied to the LED, which then shines on the transistor in the IC.

The light is proportional to the signal, so the signal is thus transferred to the photo-transistor. Opt couplers may also comes in few module such as the SCR, photodiodes, TRIAC of other semiconductor switch as an output, and incandescent lamps, neon bulbs or other light source.

Most commonly used is an opto-coupler MOC3021 an LED diac type combination. This IC is interfaced with a microcontroller and an LED is connected in series to the IC, which glows to indicate a logic High pulse from the microcontroller so that we can know that current is flowing in internal LED of the opto-IC. When logic high is given current flows through LED from pin1 to 2. So in this process LED light falls on DIAC causing 6 & 4 to close. During each half cycle current flows through gate, series resistor and through opto-diac for the main thyristor / triac to trigger for the load to operate.

The opto coupler usually found in switch mode power supply circuit in many electronic equipment. It is connected in between the primary and secondary section of power supplies. The opto-coupler application or function in the circuit is to:

1. Monitor high voltage
2. Output voltage sampling for regulation
3. System control micro for power ON/OFF
4. Ground isolation

This is the principle used in Opto−Diacs, the Opto-Diacs are available in form of ICs and can be implemented using a simple circuitry.

Simply provide a small pulse at the right time to the Light Emitting Diode in the package. The light produced by the LED activates the light sensitive properties of the diac and the power is switched on. The isolation between the low power and high power circuits in these optically connected devices is typically several thousand volts.

Opto-Diacs Pin Description:

4 Different Opto Couplers available

1. MOC3020

It comes in 6-pin DIP is shown in figure:

Working Principle of MOC3020:

The MOC3020 are designed for interfacing between electronic controls and power triac to control resistive and inductive loads for Vac operations. The principle used in opto-coupler is, MOC’s are promptly available in integrated circuit form and don’t require very complex circuitry to make them work. Simply give a small pulse at the right time to the LED in the package. The light produced by the LED activates the light sensitive properties of the diac and the power is switched on. The isolation between the low power and high power circuits in these optically connected devices is typically few thousand volts.

Features of MOC3020:

• 400 V Photo-TRIAC Driver Output
• Gallium-Arsenide-Diode infrared source and optically-coupled silicon triac driver
• High isolation – 500 Vpeak
• Output driver designed for 220 Vac
• Standard 6-terminal plastic DIP
• Directly interchangeable with Motorola MOC3020, MOC3021 and MOC3022

 Typical applications of MOC3020:

• Solenoid/valve controls
• Lamp ballasts
• Interfacing microprocessors to 115/240 Vac peripherals
• Motor controls
• Incandescent lamp dimmers

 Application of MOC3020:

The circuit shown below is a typical circuit used for AC load control from microcontroller, one LED can be connected in series with MOC3021, LED to indicate when high is given from micro controller such that we can know that current is flowing in internal LED of the opto-coupler.The idea is to use a power lamp whose activation requires mains AC as opposed to a DC voltage. That’s way, the mains AC power that we’re trying to switch the lamp and no external power supply is required. To switch the AC current to the lamp, we have to use an opto-coupled triac, lamp and a diac is shown in circuit below. A triac is said to be as an AC controlled switch. It has three terminals M1, M2 and gate. A TRIAC, lamp load and a supply voltage are connected in series. When supply is ON at positive cycle then the current flows through lamp, resistors, diac, and gate and reaches the supply and then only lamp glows for that half cycle directly through the M2 and M1 terminal of the triac. In negative half cycle the same thing repeats. Thus the lamp glows in both the cycles in a controlled manner depending upon the triggering pulses at the opto isolator as seen on the graph below. If this is given to a motor instead of lamp the power is controlled resulting in speed control.

 2. MOC3021

MOC3021 is an opto-coupler designed for triggering TRIACS. By using this we can trigger anywhere in the cycle, so can call them as non-zero opto-coupler. MOC3021 are very widely used and can be quite easily obtained from many sources. It comes in 6-pin DIP shown in figure.

Pin Description:

Pin 1: Anode

Pin 2: Cathode

Pin 3: No connection (NC)

Pin 4: Main terminal

Pin 5: No connection (NC)

Pin 6: Main terminal

Features:

• 400 V Photo-triac driver output
• Gallium-Arsenide-Diode Infrared Source and Optically-Coupled Silicon triac driver
• High Isolation 7500 V Peak
• Output Driver Designed for 220 Vac
• Standard 6-terminal plastic DIP

There are many applications of MOC3021 such as solenoid/valve controls, lamp ballasts, interfacing microprocessors to 115/240 Vac peripherals, motor controls and incandescent lamp dimmers.

Application of MOC3021:

From the below circuit, the most commonly used is an opto-coupler MOC3021 with an LED diac type combination. Additionally while using this with microcontroller and one LED can be connected in series with MOC3021, LED to indicate when high is given from micro controller such that we can know that current is flowing in internal LED of the opto-coupler. When logic high is given then the current flows through the LED from pin 1 to 2. So in this process LED light falls on DIAC causing 6 and 4 to close. During each half cycle current flows through gate, series resistor and through opto-diac for the main thyristor / triac to trigger for the load to operate.

3. MCT2E

The MCT2E series of opto-coupler devices each consist of gallium arsenide infrared LED and a silicon NPN phototransistor. They are packaged in a 6-pin DIP package and available in wide-lead spacing.

Pin 1: Anode.

Pin 2: Cathode.

Pin 3: No connection.

Pin 4: Emitter.

Pin 5: Collector.

Pin 6: Base.

Features:

• Isolation test voltage 5000 VRMS
• Interfaces with common logic families
• Input-output coupling capacitance < 0.5 pF
• Industry standard dual-in-line 6 pin package
• Compliant to RoHS directive 2002/95/EC

The opto-coupler usually found in switch mode power supply circuit, read relay driving, industrial controls, digital logic inputs and in many electronic equipments

Application of MCT2E:

It is a combination of 1 LED and a transistor. Pin 6 of transistor is not generally used and when light falls on the base-emitter junction then it switches and pin5 goes to zero.

• When logic zero is given as input then the light doesn’t fall on transistor so it doesn’t conduct which gives logic one as output.
• When logic 1 is given as input then light falls on transistor so that it conducts, that makes transistor switched ON and it forms short circuit this makes the output is logic zero as collector of transistor is connected to ground.

4. MOC363

The MOC3063 devices consist of gallium arsenide infrared emitting diodes optically coupled to monolithic silicon detectors performing the functions of zero voltage crossing bilateral triac drivers. It is also a 6-pin DIP shown in figure:

Pin Description:

Pin 1: Anode

Pin 2: Cathode

Pin 3: No connection (NC)

Pin 4: Main terminal

Pin 5: No connection (NC)

Pin 6: Main terminal

Features:

• Simplifies logic control of 115/240 Vac power
• Zero crossing voltage
• dv/dt of 1500 V/µs typical, 600 V/µs guaranteed
• VDE recognized
• Underwriters Laboratories (UL) recognized

Applications:

• Solenoid/valve controls
• Static power switches
• Temperature controls
• AC motor starters and drivers
• Lighting controls
• E.M. contactors
• Solid state relay

 Working of MOC3063:

From the circuit, we have an opto-coupler MOC3063 with an LED SCR type combination. Additionally while using this opto-coupler with microcontroller and one LED can be connected in series with MOC3063 LED to indicate when high is given from micro controller such that we can know that current is flowing in internal LED of the opto-coupler. When logic high is given then the current flows through LED from pin 1 to 2. The LED light falls on SCR causing 6 and 4 to close only at the zero cross of the supply voltage. During each half cycle current flows through SCR gate, external series resistor and through SCR for the main thyristor/triac to trigger for the load at the beginning of the supply cycle always to operate.

Optocoupler Characteristics, Types and Application

Optocoupler Characteristics, Types and Application

We know from our tutorials about Transformers that they not only provide higher or lower voltage differences between their primary and secondary windings, but they also provide “electrical isolation” between the higher voltages on the primary side and the lower voltage on the secondary side.

In other words, transformers isolate the primary input voltage from the secondary output voltage using electromagnetic coupling by means of a magnetic flux circulating within the iron laminated core. But we can also provide electrical isolation between an input source and an output load using just light by using a very common and valuable electronic component called an Optocoupler.

An Optocoupler, also known as an Opto-isolator or Photo-coupler, is an electronic components that interconnects two separate electrical circuits by means of a light sensitive optical interface.

The basic design of an Optocoupler consists of an LED that produces infra-red light and a semiconductor photo-sensitive device that is used to detect the emitted infra-red beam. Both the LED and photo-sensitive device are enclosed in a light-tight body or package with metal legs for the electrical connections as shown.

An optocoupler or opto-isolator consists of a light emitter, the LED and a light sensitive receiver which can be a single photo-diode, photo-transistor, photo-resistor, photo-SCR, or a photo-TRIAC and the basic operation of an optocoupler is very simple to understand.

Phototransistor Optocoupler

Assume a photo-transistor device as shown. Current from the source signal passes through the input LED which emits an infra-red light whose intensity is proportional to the electrical signal.

This emitted light falls upon the base of the photo-transistor, causing it to switch-ON and conduct in a similar way to a normal bipolar transistor.

The base connection of the photo-transistor can be left open for maximum sensitivity or connected to ground via a suitable external resistor to control the switching sensitivity making it more stable.

When the current flowing through the LED is interrupted, the infra-red emitted light is cut-off, causing the photo-transistor to cease conducting. The photo-transistor can be used to switch current in the output circuit. The spectral response of the LED and the photo-sensitive device are closely matched being separated by a transparent medium such as glass, plastic or air. Since there is no direct electrical connection between the input and output of an optocoupler, electrical isolation up to 10kV is achieved.

Optocouplers are available in four general types, each one having an infra-red LED source but with different photo-sensitive devices. The four optocouplers are called the: Photo-transistor, Photo-darlington, Photo-SCR and Photo-triac as shown below.

Optocoupler Types

The photo-transistor and photo-darlington devices are mainly for use in DC circuits while the photo-SCR and photo-triac allow AC powered circuits to be controlled. There are many other kinds of source-sensor combinations, such as LED-photodiode, LED-LASER, lamp-photoresistor pairs, reflective and slotted optocouplers.

Simple homemade optocouplers can be constructed by using individual components. An Led and a photo-transistor are inserted into a rigid plastic tube or encased in heat-shrinkable tubing as shown. The advantage of this home-made optocoupler is that tubing can be cut to any length you want and even bent around corners. Obviously, tubing with a reflective inner would be more efficient than dark black tubing.

Home-made Optocoupler

Optocoupler Applications

Optocouplers and opto-isolators can be used on their own, or to switch a range of other larger electronic devices such as transistors and triacs providing the required electrical isolation between a lower voltage control signal and the higher voltage or current output signal. Common applications for optocouplers include microprocessor input/output switching, DC and AC power control, PC communications, signal isolation and power supply regulation which suffer from current ground loops, etc. The electrical signal being transmitted can be either analogue (linear) or digital (pulses).

In this application, the optocoupler is used to detect the operation of the switch or another type of digital input signal. This is useful if the switch or signal being detected is within an electrically noisy environment. The output can be used to operate an external circuit, light or as an input to a PC or microprocessor.

An Optotransistor DC Switch

As well as detecting DC signals and data, Opto-triac isolators are also available which allow AC powered equipment and mains lamps to be controlled. Opto-coupled triacs such as the MOC 3020, have voltage ratings of about 400 volts making them ideal for direct mains connection and a maximum current of about 100mA. For higher powered loads, the opto-triac may be used to provide the gate pulse to another larger triac via a current limiting resistor as shown.

Triac Optocoupler Application

This type of optocoupler configuration forms the basis of a very simple solid state relay application which can be used to control any AC mains powered load such as lamps and motors. Also unlike a thyristor (SCR), a triac is capable of conducting in both halves of the mains AC cycle with zero-crossing detection allowing the load to receive full power without the heavy inrush currents when switching inductive loads.

Optocouplers and Opto-isolators are great electronic devices that allow devices such as power transistors and triacs to be controlled from a PC’s output port, digital switch or from a low voltage data signal such as that from a logic gate. The main advantage of opto-couplers is their high electrical isolation between the input and output terminals allowing relatively small digital signals to control much large AC voltages, currents and power.

An optocoupler can be used with both DC and AC signals with optocouplers utilizing a SCR (thyristor) or triac as the photo-detecting device are primarily designed for AC power-control applications. The main advantage of photo-SCRs and photo-triacs is the complete isolation from any noise or voltage spikes present on the AC power supply line as well as zero-crossing detection of the sinusoidal waveform which reduces switching and inrush currents protecting any power semiconductors used from thermal stress and shock.

Characteristics of an Optocoupler: 

  

optocoupler-characteristics

  

Current Transfer Ratio (CTR). One of the most important parameters of an optocoupler device is its optocoupling efficiency. This parameter is maximized by closely matching spectrally the LED and the phototransistor (which usually operate in the infra-red range). The optocoupling efficiency of an optocoupler may be conveniently specified by the output-to-input current transfer ratio (CTR) i.e., the ratio of the output current Ic (measured at the collector terminal of the phototransistor), to the input current IF flowing into the LED. 

Input-to-Output Isolation Voltage (Viso). This is the maximum potential difference (dc) that can be allowed to exist between the input and output terminals. Typical values range from 500 V to 4 kV. 

Maximum Collector-Emitter Voltage, VCE (max). This is the maximum allowable dc voltage that can be applied across the output transistor. Typical values may vary from 20 to 80 volts.  

Bandwidth. This is the typical maximum signal frequency (in kHz) that can be use­fully passed through the optocoupler when the device is operated in its normal mode. Typical values vary from 20 to 500 kHz, depending on the type of device construction. 

Response Time. Divided into rise time tr and fall time t*. For a phototransistor output stages, tr andtr are usually around 2 to 5 us. 

A simple isolating optocoupler uses a single phototransistor output stage and is usually housed in a six-pin package, with the base terminal of the phototransistor externally available. In nor­mal use the base is left open circuit, and under such a condition the optocoupler has a minimum CTR value of 20 % and a useful bandwidth of 300 kHz.