How to isolate between high & low voltage using optocoupler
Devices used in the control of high voltage / high power equipment must have good electrical isolation between the high voltage output and low voltage input. Relying on a layer of silicon oxide a few atoms thick to provide the necessary insulation in such conditions is not a real option.
If a failure occurs (and is more common in high performance circuits), the consequences can be catastrophic, not only for circuit components but also for the users of such devices. What we need is a physical separation (i.e. there is no electrical contact between inputs and outputs). Fortunately, there are readily available solutions to this problem. Many high power circuits today are controlled by low voltage and low current circuits such as microprocessors. Optical electronics such as opto-triacs, opto-thyristors, and solid state relays are used to separate the low power and high power circuits.
The terms optocoupler and optoisolator are often used interchangeably, but there is a small difference between the two. The differentiating factor is the expected voltage difference between input and output. An optocoupler is used to transmit analog or digital information between circuits whilst maintaining a dielectric at a potential of up to 5,000 volts. An optical isolator is used to transmit analog or digital information between circuits where the voltage difference is greater than 5,000 volts.
Opto-triac(Isolating High voltage AC loads)
Figure 1: opto-triac circuit
As shown in Figure 1, the circuit can be used to control and isolate high voltage AC loads. This circuit is built around triac, opto-triac and a few discrete components. The microcontroller is connected to the anode of the optocoupler. Working of the circuit is simple. When the microcontroller pin is high, the opto-triac conducts and sends a pulse to the gate of the triac which turns ON the load. The pulse is positive in the positive half cycle of the ac signal and is negative in the negative half cycle of the ac signal. To turn off the load, the microcontroller pin is held low and the load will turn off in the next zero crossing cycle of the ac signal. The above circuit cannot be used to control DC loads.
MOSFET Solid state Relays (Isolating High voltage DC as well as AC loads)
A typical circuit of the MOSFET Solid state relays (SSR) is shown below in figure 2. This circuit can be used to separate high voltage AC and DC loads. A current of 20 mA through the LED is sufficient to activate the MOSFETs that replace the mechanical relay contacts. LED light (infrared) falls on the photovoltaic module, which contains several photodiodes. Since a photodiode only generates a very low voltage, the diodes in the Photovoltaic module are arranged in series / parallel in order to generate enough voltage to power the MOSFETs.
Figure 2: MOSFET SSR
When the MOSFETs are turned OFF, the current does not flow through either of the MOSFETs. It can be seen that during the positive half of the AC load voltage, the top diode is reverse biased, similarly during the negative half of the AC load voltage, the bottom diode is reverse biased.
When the MOSFETs are turned ON, the positive half of the AC load voltage passes current flow through the top MOSFET and bottom diode, whilst at the half of the AC load negative voltage, the current flows through bottom MOSFETs and top diode.
Figure 3: MOSFET current flow
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