Understanding Optocoupler circuit

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In summary, the conversation is discussing a circuit with an optocoupler that generates light and isolates different parts of the circuit from each other. If the primary switch is open, there will be no light generated in the optocoupler and Vo will be the same as Vcc. If the primary switch is closed, the phototransistor will conduct and Vo will drop to something below one volt, depending on the primary current. Connecting the emitter of the phototransistor to ground has no effect on the circuit's operation.
  • #1
shahper
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Hello all,

If in this http://en.wikipedia.org/wiki/File:Optocoupler_Circuit.svg" [Broken], Emitter is not connected to the ground, what will be the voltage at Vo and and at the Emitter?

Thanks
 
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  • #2
The primary section of this circuit only generates light in the optocoupler, so it can be connected to whatever voltage you like. It does not have to be grounded at all.

That is what optocouplers do. They isolate different parts of circuits from each other.

In this case, the switch in the primary is open, so there is no light being generated in the optocoupler.
So, the phototransistor is not conducting so Vo is the same as Vcc.
 
  • #3
I'm talking about the emitter of the phototransistor. If it is not connected to the ground, then what will be the voltage at Vo. In this case, I believe primary side of the circuit will be working.

And further more, if transistor is in cut-off mode (i.e not conducting), than I believe Vo will be 0 volt.

Thanks
 
  • #4
You need to measure voltage from somewhere to somewhere else.

In this case, as shown, the primary switch is open so there is no current flowing in the primary (ie through the LED).

Connecting the emitter of the phototransistor to ground has no effect as far as the operation of the circuit is concerned as long as there is a return circuit for VCC back to the emitter of the phototransistor.

If the primary circuit is open circuit because the switch is open, there will be no light falling on the phototransistor.

So the phototransistor will be a very high resistance.
If R2 is any value a lot less than this very high resistance, there will be a voltage divider action where most of the supply voltage will be across the phototransistor and very little or none of it will be across R2. So, V0 will be very close to Vcc, relative to the emitter of the phototransistor.
 
  • #5
Oh, I mean the situation where the switch is closed, and LED is on (i.e the primary side of the circuit is conducting).

I'm extremely sorry for the confusion.
 
  • #6
No confusion at all.

In the circuit you referred to, the switch is open. Have a look.

If you close the switch, the phototransistor will conduct. Vo would then drop, but would not go to zero relative to the emitter of the phototransistor. It will drop to something below one volt, though, if the primary current is great enough.
 

1. What is an optocoupler circuit?

An optocoupler circuit is a device that is used to transfer electrical signals between two isolated circuits using light. It consists of a light-emitting diode (LED) on one side and a phototransistor or photodiode on the other side, separated by a transparent barrier. When the LED is turned on, it emits light which is detected by the phototransistor or photodiode, allowing the transfer of signals without any physical connection between the two circuits.

2. How does an optocoupler circuit work?

The optocoupler circuit works based on the principle of photoelectric effect. When the LED is turned on, it emits light which is converted into an electrical signal by the phototransistor or photodiode. This signal is then transmitted to the receiving circuit on the other side, allowing for isolation between the two circuits. The amount of light emitted by the LED can be controlled by varying the input voltage, thus controlling the output signal received by the receiving circuit.

3. What are the advantages of using an optocoupler circuit?

There are several advantages of using an optocoupler circuit, including:- Electrical isolation between two circuits, preventing any interference or damage.- High voltage isolation, making it suitable for use in high voltage applications.- Low power consumption.- Fast response time.- Low cost and readily available.- Can be used for both digital and analog signals.

4. In what applications are optocoupler circuits commonly used?

Optocoupler circuits are commonly used in applications where electrical isolation is required, such as:- Power supplies and voltage regulators.- Motor control circuits.- Switching power supplies.- Signal isolation in electronic devices.- Audio amplifiers.- Industrial control systems.

5. How do I choose the right optocoupler for my circuit?

When choosing an optocoupler for your circuit, some important factors to consider include:- Voltage and current requirements of the input and output circuits.- Speed and response time needed.- Isolation voltage required.- Temperature range.- Type of input signal (digital or analog).- Cost and availability.- Package type and size.It is important to carefully consider these factors and select an optocoupler that meets the specific needs of your circuit.

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