Automatic control circuit composed of light emitting diode and fault handling

12 Feb 2020

After the light emitting diode is turned on, its forward resistance changes slightly with the current. The larger the forward current, the smaller the forward resistance. Using the characteristics between the forward current of the diode and the forward resistance, some automatic control circuits can be constructed.

1. Explanation of circuit analysis preparation knowledge
After the diode is conducting in the forward direction, its forward resistance is also related to the forward current flowing through the diode. Although the forward resistance of the diode after it is turned on is relatively small (compared to the reverse resistance), if the forward current is increased, the forward resistance after the diode is turned on will decrease further, that is, the larger the forward current, The smaller the resistance is, the larger it is.

The ALC circuit is used in the recorder and the recording card of the deck. During recording, the size and amplitude of the recording signal should be controlled. Understanding the following specific control requirements is helpful to analyze the diode VD1 automatic control circuit.

(1) When the amplitude of the recording signal is small, the amplitude of the recording signal is not controlled.

(2) When the amplitude of the recording signal reaches a certain level, the control of the amplitude of the recording signal is started, that is, the signal amplitude is attenuated. The circuit for controlling the amplitude of the recording signal is the ALC circuit.

(3) After the ALC circuit enters the control state, the larger the recording signal is required, the larger the attenuation of the signal is.

Light-Emitting-Diode

2.Analysis of Circuit Working Principles
(1) If there is no branch of VD1, all the recording signals output from the first-stage recording amplifier are added to the second-stage recording amplifier. However, with the VD1 branch, the recording signal output from the first-stage recording amplifier may flow to ground through C1 and the conducting VD1, forming a shunt attenuation of the recording signal.

(2) The second key of circuit analysis is the specific situation of the branch of VD1 to the ground shunt attenuation of the output signal of the first stage recording amplifier. The capacitor C1 in the branch is a capacitor with a large capacity, so C1 is a path for the recording signal, indicating that VD1 in this branch is a key component for shunting and attenuating the recording signal.

(3) One thing to understand from the analysis of the shunt branch circuit: if the signal output from the first-stage recording amplifier is shunted from the VD1 branch, the recording signal flowing into the second-stage recording amplifier is small, otherwise it is large.

(4) There are two cases of VD1 being on and off. When VD1 is off, there is no shunt effect on the recording signal, and when it is on, the recording signal is shunted.

(5) A resistor R1 is connected to the positive electrode of VD1, which gives VD1 a control voltage. Obviously, this voltage controls VD1 to be turned on or off. Therefore, the voltage sent by R1 is the key to analyze the on and off of VD1.

3.General analysis method of control circuit
(1) When there is no recording signal in the circuit, the DC control voltage is 0, the diode VD1 is turned off, and VD1 has no effect on the circuit operation. All signals output by the first-stage recording amplifier can be added to the second-stage recording amplifier.

(2) When the recording signal in the circuit is small, the DC control voltage is small and is not greater than the on-voltage of the diode VD1, so it is not enough to make the diode VD1 conductive. At this time, the diode VD1 also outputs a signal to the first-stage recording amplifier No shunting effect.

(3) When the recording signal in the circuit is relatively large, the DC control voltage Ui is large, and the diode VD1 is turned on. The larger the recording signal, the larger the DC control voltage Ui, the deeper the VD1 conduction, and the more internal resistance of VD1. small.

(4) After VD1 is turned on, the internal resistance of VD1 decreases, and a part of the recording signal output by the first-stage recording amplifier is shunted to the ground through the capacitor C1 and the turned-on diode VD1. The deeper the VD1 turns on, its internal The smaller the resistance, the larger the ground-to-ground flow rate of the output signal of the first-stage recording amplifier, to achieve automatic level control.

(5) The conduction degree of the diode VD1 is controlled by the DC control voltage, and the DC control voltage changes with the size of the recording signal in the circuit, so the change in the internal resistance of the diode VD1 is actually controlled by the size of the recording signal.

4. Fault detection method and circuit fault analysis
The best way to detect diode failure in this circuit is to perform a replacement check, because if the diode's performance is not good, it will affect the control effect of the circuit.
When the diode VD1 is open, there is no control effect. At this time, when the large signal is recorded, there will be undulating sound distortion. When the recording signal is small, the recording can be normal.
When the diode VD1 breaks down, there is no control effect. At this time, the recording sound is very small, because the recording signal is shunted to the ground by the breakdown diode VD1.

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