Re: Pc817 Application Note Pdf Free
Elbio Gottlieb
The application note explains that you need to keep the AC input current above 4mA (called Saturated Mode) so that the charge from the photodiode current will leak away slowly enough inside the device that pin 6 will remain a LOW output, even during the zero-crossing of the AC input voltage. See the below extract from the linked application note:
I recently had to do this and used two FOD817D opto-isolators. The inputs were connected opposite in parallel and the outputs directly in parallel. Those have a minimum guaranteed CTR of 3, which makes them much easier to use in this application. They are also cheap and available. Two of these can often cost less than one fancier opto, especially if you are already stocking them.
I'm designing a pcb right now and I have a question relating to two of the inputs. They are connected to two square wave inputs which will give me speed and rpm, but I cannot confirm whether the input signal will be 12v or 5v. I'm using a pc817 optocoupler to protect the arduino, but I'm not sure what size resistor to use on the input to the opto coupler. Can I size it to work over a range of potential inputs eg 1v to 15v, or would I need something like a zener to bleed off voltage above a certain level?
I'm designing a pcb right now and I have a question relating to two of the inputs. They are connected to two square wave inputs which will give me speed and rpm, but I cannot confirm whether the input signal will be 12v or 5v. I'm using a pc817 optocoupler to protect the arduino, but I'm not sure what size resistor to use on the input to the opto coupler. Can I size it to work over a range of potential inputs eg 1v to 15v, or would I need something like a zener to bleed off voltage above a certain level?
When I start to go much above 10 mA, the load starts to drop in voltage. I get somewhere between +/- 10 V and +/- 14 V depending on the load. The DC voltage on the feedback pin tracks my output voltage as expected. Also of note the voltage seems mostly stable even at these low voltages: 50 mV of ripple or so.
An opto-isolator connects input and output sides with a beam of light modulated by input current. It transforms useful input signal into light, sends it across the dielectric channel, captures light on the output side and transforms it back into electric signal. Unlike transformers, which pass energy in both directions[note 3] with very low losses, opto-isolators are unidirectional (see exceptions) and they cannot transmit power.[14] Typical opto-isolators can only modulate the flow of energy already present on the output side.[14] Unlike transformers, opto-isolators can pass DC or slow-moving signals and do not require matching impedances between input and output sides.[note 4] Both transformers and opto-isolators are effective in breaking ground loops, common in industrial and stage equipment, caused by high or noisy return currents in ground wires.[15]
The earliest opto-isolators, originally marketed as light cells, emerged in the 1960s. They employed miniature incandescent light bulbs as sources of light, and cadmium sulfide (CdS) or cadmium selenide (CdSe) photoresistors (also called light-dependent resistors, LDRs) as receivers. In applications where control linearity was not important, or where available current was too low for driving an incandescent bulb (as was the case in vacuum tube amplifiers), it was replaced with a neon lamp. These devices (or just their LDR component) were commonly named Vactrols, after a trademark of Vactec, Inc. The trademark has since been genericized,[note 8] but the original Vactrols are still being manufactured by PerkinElmer.[24][note 9]
Photoresistors used in opto-isolators rely on bulk effects in a uniform film of semiconductor; there are no p-n junctions.[28] Uniquely among photosensors, photoresistors are non-polar devices suited for either AC or DC circuits.[28] Their resistance drops in reverse proportion to the intensity of incoming light, from virtually infinity to a residual floor that may be as low as less than a hundred Ohms.[28] These properties made the original Vactrol a convenient and cheap automatic gain control and compressor for telephone networks. The photoresistors easily withstood voltages up to 400 volts,[28] which made them ideal for driving vacuum fluorescent displays. Other industrial applications included photocopiers, industrial automation, professional light measurement instruments and auto-exposure meters.[28] Most of these applications are now obsolete, but resistive opto-isolators retained a niche in audio, in particular guitar amplifier, markets.
The fastest opto-isolators employ PIN diodes in photoconductive mode. The response times of PIN diodes lie in the subnanosecond range; overall system speed is limited by delays in LED output and in biasing circuitry. To minimize these delays, fast digital opto-isolators contain their own LED drivers and output amplifiers optimized for speed. These devices are called full logic opto-isolators: their LEDs and sensors are fully encapsulated within a digital logic circuit.[34] The Hewlett-Packard 6N137/HPCL2601 family of devices equipped with internal output amplifiers was introduced in the late 1970s and attained 10 MBd data transfer speeds.[35] It remained an industry standard until the introduction of the 50 MBd Agilent Technologies[note 10] 7723/0723 family in 2002.[36] The 7723/0723 series opto-isolators contain CMOS LED drivers and a CMOS buffered amplifiers, which require two independent external power supplies of 5 V each.[37]
Phototransistors are inherently slower than photodiodes.[41] The earliest and the slowest but still common 4N35 opto-isolator, for example, has rise and fall times of 5 μs into a 100 Ohm load[42] and its bandwidth is limited at around 10 kilohertz - sufficient for applications like electroencephalography[6] or pulse-width motor control.[43] Devices like PC-900 or 6N138 recommended in the original 1983 Musical Instrument Digital Interface specification[44] allow digital data transfer speeds of tens of kiloBauds.[45] Phototransistors must be properly biased and loaded to achieve their maximum speeds, for example, the 4N28 operates at up to 50 kHz with optimum bias and less than 4 kHz without it.[46]
All opto-isolators described so far are uni-directional. Optical channel always works one way, from the source (LED) to the sensor. The sensors, be they photoresistors, photodiodes or phototransistors, cannot emit light.[note 11] But LEDs, like all semiconductor diodes,[note 12] are capable of detecting incoming light, which makes possible construction of a two-way opto-isolator from a pair of LEDs. The simplest bidirectional opto-isolator is merely a pair of LEDs placed face to face and held together with heat-shrink tubing. If necessary, the gap between two LEDs can be extended with a glass fiber insert.[48]
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