Optoelectronic relay. Scheme, description

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Optoelectronic relay. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Optoelectronic relays are electronic switches controlled via an optical channel. Their main advantages are galvanic isolation between the control circuit and the switching element, as well as the absence of mechanical contacts. Radiating diodes are used in the control circuit of optoelectronic relays, and photothyristors, phototransistors or field effect transistors are used as a switching element. In the latter case, photodiodes operating in the voltage generation mode are used to control transistors.

Since optoelectronic relays are not always available, and sometimes the industry does not produce such devices with the necessary parameters, their analogs on discrete elements are of interest to radio amateurs. Such an analogue can be made on the basis of powerful switching field-effect transistors from International Rectifier ("Powerful switching field-effect transistors from International Rectifier" in Radio, 2001, No. 5, p. 45) and IR emitting diodes, using the property of their reversibility. The scheme of the optoelectronic relay and its inclusion for controlling the load in the 220 V network is shown in fig. 1.

optoelectronic relay

Very low static signal power is needed to drive a high switching FET. To open the transistor indicated in the diagram, it is enough to apply a control voltage to its gate in the range from 4,5 to 10 V. In this case, the resistance of its channel will decrease to 0,85 ohms. The voltage required to open the transistor is generated by IR emitting diodes BL1 - BL5, which operate in the photodiode mode. Emitting diodes B11-B15 are placed exactly opposite the photodiodes BL1 - BL5. Radiating diodes and resistor R1 form the control circuit. When current flows through the control circuit, IR radiation hits the photodiodes, the generated voltage is applied to the gate of the field-effect transistor and it opens. Thus, to connect the load to the network, it is necessary to apply voltage to the control circuit.

The number of photodiodes depends on the gate voltage at which the FET turns on. Since, when illuminated, a voltage of 0,9 ... 1 V appears on each photodiode, at least five such diodes must be connected in series. In the control circuit at a current of 20 ... 50 mA, the voltage drop across each emitting diode is 1,1 ... 1,2 V, therefore, for five diodes, the control voltage must be greater than 6 V. Depending on its value and the required current through diodes calculate the resistance of resistor R1:

R1=(Uy-NUd)/Id,

where Uy - control voltage; Ud - voltage across the diode; N is the number of diodes; Id - current emitting diode.

If it is necessary to reduce the control voltage, then it is permissible to connect radiating diodes in the control circuit in parallel, but for each of them you should choose your own current-limiting resistor.

Most of the parts are mounted on a printed circuit board made of one-sided foil fiberglass on the side of the printed conductors. The board drawing is shown in fig. 2. The diodes are placed exactly opposite each other with a gap of about 1 mm and, after adjustment, are glued to the board. From above, the diodes are covered with an opaque screen of insulating material. The transistor is soldered to the board, and the soldering area is filled with epoxy glue.

optoelectronic relay

It is permissible to use any medium-power IR emitting diodes in the device, which must be checked for operability in generator mode beforehand. Using other field effect transistors, you can get a relay with the required parameters. For example, if you install an IRLR2905 transistor, which has an opening voltage of 2,5 V, the number of photodiodes connected in series can be reduced. In this case, the maximum relay current is 30.. .40 A, but the switching voltage should not exceed 55 V. Depending on the load power, the transistor may have to be placed on the heat sink. Diode bridge VD1 must provide the required load current.

Author: I. Nechaev, Kursk

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