Optocoupler PC817 working principle and very simple test. Optocoupler PC817 working principle and very simple test 817 optocoupler input voltage

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Sometimes there is such a malfunction, with seemingly serviceable elements of the power supply, turning on the TV causes an explosion of a microcircuit in the power supply of the TV (or transistor), and the exact reason cannot be established. In this case, you should pay attention to the optocoupler.

I will not describe all optocouplers, I will only touch PC817, its datasheet and verification methodology.

Optocoupler PC817 it is quite common and it is not a problem to buy it, and the price is not high. Of course, there should always be several optocouplers in stock, just in case.

The PC817 optocoupler consists of an LED and a phototransistor. The opening of the phototransistor depends on the illumination of the LED.

If the required optocoupler is not available, then you can install another one, for this check the datasheet of the optocouplers you have for the coincidence of the pins with the datasheet PC817 and the main parameters of the input voltage (LED), current and voltage of the transistor. Use literature or the Internet. I give the RS817 analogs in the table

Checking with an ohmmeter is an approximate check and boils down to checking the diode (resistance about 1.5 ohms) and the transistor (does not ring), see the datasheet, that is, if you can see with the ohmmeter that the optocoupler is faulty, then it is faulty. If no defect is found, this does not mean that the optocoupler is working properly.

A 100% guarantee cannot be given by checking the operability of an optocoupler using small circuits. You can easily find them on the Internet. Here is one of them.

Using this circuit, you can check two types of optocouplers, switching occurs using the switch S1. It is possible and even easier

The illumination of LEDs D1 and LED1 will indicate the operability of the optocoupler. When connecting, check the datasheet.

Failure of an optocoupler is quite rare, although it happens, for example, in Sharps after a thunderstorm, can be called a typical defect.

Description, characteristics, Datasheet and methods for checking optocouplers using the example of PC817.

Continuing the topic "Popular radio components for repairing switching power supplies", we will analyze one more detail - an optocoupler (optocoupler) PC817. It consists of an LED and a phototransistor. They are not electrically connected with each other, due to which, on the basis of PC817 it is possible to realize galvanic isolation of two parts of the circuit - for example, with high voltage and with low voltage. The opening of the phototransistor depends on the illumination of the LED. I will analyze how this happens in more detail in the next article, where, in experiments, by feeding signals from the generator and analyzing it with an oscilloscope, you can understand a more accurate picture of the optocoupler's operation.

In other articles I will talk about non-standard use the optocoupler is the first in the role, and in the second. And using these circuit solutions I will assemble a very simple optocoupler tester. Which does not need any expensive and rare devices, but only a few cheap radio components.

The detail is neither rare nor expensive. But a lot depends on it. It is used in almost every running (I do not mean any kind of exclusive) pulse POWER SUPPLY and acts as a feedback and most often in conjunction with the very popular TL431 radio component

For those readers who find it easier to perceive information by ear, we recommend watching the video at the very bottom of the page.

Optocoupler (Optocoupler) PC817

Brief characteristics:

Compact body:

• lead pitch - 2.54 mm;
• between rows - 7.62 mm.

The manufacturer of PC817 is Sharp, there are other manufacturers of electronic components that produce analogs, for example:

• Siemens - SFH618
• Toshiba - TLP521-1
• NEC - PC2501-1
• LITEON - LTV817
• Cosmo - KP1010

In addition to the single PC817 optocoupler, other options are available:

• PC827 - dual;
• PC837 - built-in;
• PC847 is quadruple.

Optocoupler check

For quick check optocouplers, I have carried out several test experiments. First on the breadboard.

Breadboard option

As a result, we managed to get a very simple circuit for testing PC817 and other similar optocouplers.

The first variant of the scheme

I rejected the first option for the reason that he inverted the marking of the transistor from n-p-n to p-n-p

Therefore, to avoid confusion, I changed the scheme to the following;

The second variant of the scheme

The second option worked correctly, but it was inconvenient to unsolder the standard socket

under the microcircuit

SCS-8 socket

The third variant of the scheme

The most successful

Uf is the voltage across the LED at which the phototransistor starts to open.

in my version, Uf = 1.12 Volts.

The result is a very simple structure like this:

View from above

Bottom view

As you can see from the photo, the part is not deployed by key.

Using which you can very quickly check the item. During my repair practice, of course, not often, but I came across non-working optocouplers and before I had to bother checking a part when sometimes I got stumped during a complex repair.

The final version is very simple.

An optocoupler turns out to be a very useful feedback device. Usually you can find an optocoupler in circuits with signal transmission between parts of a circuit with different voltages, in switching power supplies, when the output voltage becomes higher than normal, the optocoupler LED starts to glow, while opening the phototransistor, which in turn covers the power transistor of the primary winding.

In general, this device appeared a long time ago, then instead of LEDs, incandescent lamps were used, the power consumed by them is high, the light output is small, and the frequency with which it can be used is extremely low, since the filament comes into operation slowly, and it does not go out instantly. Now there is a large assortment of optocouplers with varying degrees of integration, with a closed or open optical channel, with many types of photodetectors and a light source, but we are interested in the most common PC817 in discrete design.

The maximum input current is 0.05 A, the maximum impulse can reach 1 A, the voltage is typical 1.2 V. Reverse voltage max 6 V, and power dissipation up to 70 mW. In a phototransistor, the collector current can reach 50 mA, the collector power is 0.15 W, the collector-emitter voltage is 35 V, the emitter-collector voltage is 6 V. Below simple circuit to test the health of your instance.