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. 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, for example one from an Arduino or micro-controller, and a much higher voltage or mains current output signal. Obviously, tubing with a reflective inner would be more efficient than dark black tubing. The advantage of this home-made optocoupler is that tubing can be cut to any length you want and even bent around corners. An Led and a photo-transistor are inserted into a rigid plastic tube or encased in heat-shrinkable tubing as shown. Simple home made opto-couplers can be constructed by using individual components. There are many other kinds of source-sensor combinations, such as LED-photodiode, LED-LASER, lamp-photoresistor pairs, reflective and slotted optocouplers. 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. The four optocouplers are called the: Photo-transistor, Photo-darlington, Photo-SCR and Photo-triac as shown below. Optocouplers are available in four general types, each one having an infra-red LED source but with different photo-sensitive devices. Since there is no direct electrical connection between the input and output of an optocoupler, electrical isolation up to 10kV is achieved. 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. The photo-transistor can be used to switch current in the output circuit. When the current flowing through the LED is interrupted, the infra-red emitted light is cut-off, causing the photo-transistor to cease conducting. It could be linearized further with another opamp stage although there are diminishing returns because the isolation method with add at least a couple percentage points of distortion.The base connection of the photo-transistor can be left open (unconnected) for maximum sensitivity to the LEDs infra-red light energy or connected to ground via a suitable external high value resistor to control the switching sensitivity making it more stable and resistant to false triggering by external electrical noise or voltage transients. The linearity test compares the input to the recovered output. The variable PWM is nice because it can be easily sent to an FPGA/micro directly or just as easily filtered to recover an analog signal (the two pole RC shown simulates that). So the output of the opamp/comparator is a square wave that's suitable for transferring through any number of isolation options (not shown) including opto, digital isolator or pulse transformer. It uses just a single comparator or op amp. It's a variation on a relaxation oscillator set-up to operate in a psudo-regulated hysteretic mode where Vin equates to duty cycle out. I always have voltages to move across isolation barriers but often its a nice-to-have rather than hard requirement so I'm always searching for smaller/cheaper ways to do it so I can justify it in more places.Īnyway this appears to be a decent compromise of complexity for accuracy compared to say LTC6992. I'm thinking about this problem this morning.
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