Photocoupler Application Circuit-EEWORLD

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The photocoupler introduced here is a device composed of a light-emitting diode and a phototransistor. The light-emitting diode converts the electrical signal of the input side into light with the same changing rules, while the phototransistor converts the light back into an electrical signal with the same changing rules. Therefore, light plays the role of a medium. Since the photocoupler has strong anti-interference ability, it is easy to complete level matching and transfer, and is not restricted by whether the signal source is grounded. Therefore, its application is becoming more and more extensive.

1. Multivibrator circuit composed of photoelectric coupler
The multivibrator circuit composed of photoelectric coupler is shown in Figure 1.
When Figure 1 (a) is just connected to the power supply Ec, since UF increases with the charging of C, until UF≈1 volt, the light-emitting diode reaches saturation, and then the transistor is also saturated, and the output Uo≈Ec. After
the transistor is saturated, C discharges (discharges from two paths, C→F→E1→Er and C→RF→+Ec→Re), uo decreases, and the diode is cut off after C discharges to a certain extent, and after the transistor removes all the stored charge, it is also cut off, and uo is zero. After the transistor is cut off, the power supply Ec charges C again, repeating the above process, and the peak output waveform shown in the figure is obtained, and its period is (when RF》Re):
T=C(RF+Re)In2
Figure 1 (b) is another form of circuit with the same principle.

Figure 1: Multivibrator using optocoupler

2. Bistable circuit composed of photoelectric coupler
The bistable circuit composed of photoelectric coupler is shown in Figure 2.
The steady state of the circuit after the power is turned on is that BG is cut off and the output is high potential. Under the action of the trigger positive pulse, ib increases to make BG enter the amplification state, forming ib↑→if↑→ib↑↑, and as a result, BG is cut off. This circuit has higher anti-interference ability than the ordinary trigger sequence. If the limit current Ic of BG is set to 6 mA, then R2= is taken as:
R2≥(13-1)/(6×10)=24 ohms
The current limiting resistor R1 can be calculated according to the following formula
R1≥(E-IbmRce2min)/Ibm
In the formula: Ibm is the maximum base current of the transistor, and Rce2min is the minimum resistance value between the collector and emitter of the photosensitive transistor.

Figure 2: Bistable circuit using photoelectric coupling

3. Shaping circuit composed of photocoupler

Table 1		Shaping circuit composed of photocoupler
	Photocoupler-Transistor Shaping Circuit	Photoelectric coupling-fixed component shaping	Inverting shaping	Quick plastic surgery
Circuit
illustrate	This is a Schmidt shaping circuit because no matter whether the input is a distorted square wave, a sine wave or a sawtooth wave, a square wave is obtained at the output.	The output of the photocoupler is connected to a NAND gate for shaping	The output end of the photocoupler is connected to two stages of NAND gates to form an inverting shaper.	Two transistors are connected behind the output end of the photocoupler to form a common-phase shaping circuit.

4. Chopping circuit composed of photoelectric coupler
The chopping circuit composed of photoelectric coupler is shown in Table 2

Table 2		Chopper circuit composed of photocoupler
	Direct chopping circuit	Isolated chopper circuit (I)	Isolated chopper circuit (II)
Circuit
illustrate	The measured voltage of output Ei is sent to the encoder for encoding measurement after chopping sampling. When point A is at low potential and point B is at high potential, GD1 is turned on, GD2 is turned off, and the measured voltage Ei is directly sent to the output end. On the contrary, when point A is at high potential and point B is at low potential, GD1 is turned off, GD2 is turned on, C is discharged through GD2, and the output end returns to zero. It has higher accuracy than ordinary transistor or field effect tube choppers	When the chopping pulse is input, BG is turned on, then GD is turned on, ui on the input side is transmitted to the output side, and uo is proportional to ui but opposite in phase. Conversely, when the chopping pulse is zero, GD is cut off, uo is a high level, which is higher in accuracy than the ordinary modulator isolated by a transformer, because the transformer voltage cannot be too large, causing the output pulse wave top to be uneven.	Two GD1 and GD2 are used. GD1 is used as a switch. When the chopping pulse is input, GD1 is turned on, and ui is transmitted in reverse phase to the output side of GD2, isolating uo from the input ui and the chopping pulse.

Reference address：Photocoupler Application Circuit

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