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prateeksikka

OPTOCOUPLERS

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The coupling of a 4N25 optocoupler and many others are guaranteed to be at least 100%, so it has pretty good "efficiency".
Its output switches off in 2us, so it can have its emitter (LED) modulated with any audio frequency and low-speed data as signals.

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hi audioguru!
my question still remains.what are these optocouplers.i only know one line pertaining to them "they have same relation to 2 ground points isolated from each other"which i could search from google.com
reply soon
prateek

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Hi Prateek,
Optocouplers provide electrical isolation:
1) You connect the output of your computer directly to the gate of a Triac to turn on and off a mains-powered lamp. But the ground wire of the lamp and the Triac are accidently connected to 220VAC instead of to the neutral wire of the mains. Goodbye computer!
With an optocoupler the computer and Triac would be electrically isolated from each other so that nothing bad will happen.

2) You don't have a proper current transformer so you simply connect a low-value resistor in series with the neutral wire of a big electric motor in a facory that has many big electric motors. You connect the resistor to your computer to measure the voltage drop across the resistor so that you can calculate the current of the motor. But the motor or other motors in the factory have heavy currents in the neutral wire and therefore their neutral wire swings many volts above and below the ground of your computer. Goodbye computer!
With an optocoupler the computer and motor would be electrically isolated from each other so that nothing bad will happen.

There are many more applications for optocouplers but I don't use them so I can't think of any more.  ???   

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hi audioguru!
u gave the example of a triac controlled lamp earlier. 
GROUND IS A GROUND I.E ZERO WITH RESPECT  TO LIVE NOW WHETHER IT IS COMPUTER'S 5 V GROUND  OR AC 230V GROUND WHAT DIFFERENCE DOES IT MAKE?
WHAT "BAD" CAN BE CAUSED BY SHORTING 2 ZERO POTENTIAL POINTS?
WHICH MAKES U SAY "GOODBYE COMPUTER"PLZ DO REPLY SOON
PRATEEK

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Can you trust that all your electrical outlets or the lamp with a triac are wired correctly?
If one is wired backwards then its ground is really live. If it is connected to your computer then many Amps will flow through the connection until the fuse or breaker blows.

Or maybe your grounded computer in the office connects to a sensor in the factory. The big electric motors in the factory cause a voltage drop across their wiring, raising the voltage of their ground. Again, many Amps will flow through your computer but nothing will blow except the computer.
Optocoupling is important.

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Optocouplers are used primarily in digital applications. The reason is that their intensity with respect to voltage is cannot be uniform. It would be nice to use them for coupling signals but because they are transducers, they are not as linear as a transistor arrangement. Some say they are used for isolation. Whereas you would want to transpose a signal that exists in one system into another. Sometimes it can be helpful to condition a signal in a medium that is quieter or more in balance with the signal. But because they were intended for digital applications, they have proven most useful in transfering data short distances.

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...It would be nice to use them for coupling signals but because they are transducers, they are not as linear as a transistor arrangement. Some say they are used for isolation....


Kevin, they are also called "opto-isolators" because of this purpose. In fact TI lists them as "Optocoupler/Optoisolators".
They are very good at isolation as well as coupling signals.

MP

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MP just have a look at the articles section in Electronics-Lab. They have optocouplers in there and they say they are digital devices. Now you can send analogue if it's modulated. They suggest PWM where the pulse width can be directly altered for each frequency. I have to wonder how they do that.

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Kevin,
An optocoupler passes audio and even DC pretty well. You can use a 2nd optocoupler for negative feedback to reduce distortion. They are not used only for digital. Maybe you don't understand how they work.
If you wonder about pulse-width-modulation you can look at it in our Articles section. Usually its frequency doesn't change, it is used as a carrier and its modulation has frequency and amplitude changes.

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hello everybody!
as audioguru said in his last post that optocouplers prevent expensive devices
like computers from burning out.but there is one thing that if we have high voltage across say a motor winding with respect to ground and we still short the two grounds of computer and motor what difference does it make?
audioguru! do u mean that if i have ideal carefully wound devices with  correct polarity of ground and live,i dont need optocouplers?
prateek

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Hi Prateek,
If the motor is powered from live and neutral but has leakage current to its ground, then a voltage would develop across its ground wire due to the current in its resistance. Therefore the ground at the motor isn't at the same voltage as the ground at the computer.

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Hi Prateek,
I don't know about your country, but in Canada the ground wire is used for safety so that metal-cased appliances have their case grounded. If a live wire inside touched the case then the breaker would blow. The appliance is powered from the live and neutral wires. The neutral wire is separate from the ground wire at each receptacle because current in the neutral wire creates a voltage across its resistance and therefore at an operating appliance is not at earth potential. The neutral wires and ground wires are joined at the breaker box and earthed there.
If an appliance developed some leakage current from its live wire to its ground wire, its ground potential will be low and wouldn't hurt you but its current could be high enough to destroy a grounded computer that it is connected with.

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hi audioguru!
you rightly said it couples the signals through a beam of light.Actually a phototransistor is switched on.But do you know that why is the transistor not damaged by high current  on motor side.(i take the motor computer example u gave)
reply soon how could an electronic device like a small signal transistor bear such high
current?
prateek

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Oh yeah? Do you think that the tiny phototransistor in an optocoupler switches the entire load all by itself? Duh!  :o ::)

Firstly, the LED in a optocoupler has a max current rating of about 60mA.
Secondly, the optocoupling isn't perfect so the phototransistor can conduct only about 40mA when the LED has its max current. The phototransistor has a max voltage rating of about 30V.

The tiny phototransistor turns on a real transistor that turns on a power driver transistor that turns on a bunch of paralleled power transistors. If the paralleled power transistors can't handle the load then they switch a high-power relay that drives the load.  ;D

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