JPS5841519B2 - Light emitting diode drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a driving circuit for a light emitting diode used as a digital display element of a digital device, and more particularly to a dynamic drive type light emitting diode driving circuit.

With the recent development of digital technology, for example, clocks, radio receivers, transceivers, etc. that digitally display numbers such as the time and reception frequency using a seven-segment display have been put into practical use.

There are two methods to display the 7 segment type: static drive method and dynamic drive (pulse lighting) method.

In general, the static drive method is considered to be better for displays with a small number of digits, such as one or two digits, and for displays with a large number of digits, for example, 2 or more digits, from the standpoint of saving parts etc. The dynamic drive method is said to be better.

In other words, the static drive method requires light emitting diode drive switches and their wiring for the number of digits x 8 (the number of light emitting diodes for 7 segments and 1 decimal point), whereas the dynamic drive method requires the number of digits - 8 (the number of light emitting diodes for 7 segments and 1 decimal point). This is because only 0 light emitting diodes are required.

In such a dynamic drive type 5 drive circuit, there is a segment switch that controls the light emission by sharing the same -7 segment l~ section of a plurality of light emitting diodes of each digit, and a character switch that switches and selects a segment for each digit. are connected in series to each segment, and the segment switches and character switches are opened in a time-sharing manner, so that the segment switches,
When the character switch is turned on, the current flowing through the light emitting diode changes in a pulsed manner, that is, in a step function manner, and there is a drawback that harmonic components of this pulsed signal may interfere with the reception of the receiver.

SUMMARY OF THE INVENTION An object of the present invention is to provide a light emitting diode driving circuit that can reduce reception interference signals generated from a read light emitting diode when the light emitting diode is driven.

The principle of the present invention will be explained below based on the conventional four-digit display dynamic drive force type drive circuit shown in FIG.

A well-known 7-segment display, as shown in FIG.
It is composed of seven segment light emitting diodes of "-g" and one light emitting diode h for displaying a decimal point.

In the drive circuit shown in Figure 2, 10 is a 4-digit human input signal terminal;
20 is a 7-segment decoder for the 4-digit signal supplied to the input terminal 10; 30 is a 7-segment 4-digit display that displays 4-digit numbers using a plurality of light emitting diodes a-h; 40
are the seven output terminals Va to v of the 7 segment decoder 20
h and the cathode of the light emitting diode row g of the 7-segment display 30. Controlled by 7 segment signals,
Control the light emitting diode a=g of the display 30.

50 is a digit signal input terminal, 60 is a digit selection circuit, and 70 is the four output terminals (■1 to ■4) of the digit selection circuit 60 and the display 3.
The figure shows a character drive circuit consisting of character switches (four switching transistors) 71, 72, 73, and 74 connected between the anodes of light emitting diodes a to h of 0, and the drive circuit is driven by the output signal of the digit selection circuit 60. and controls the digits of the display 30.

Character switch 71 of character drive circuit 70
．． 72°73.74 is switch 71-72-73-74
Switching is performed sequentially in the order of 71.

80 is a switch (switching transistor) 80h connected to the cathode side of the light emitting diode h of the display 30
This is a decimal point drive circuit consisting of

In such a circuit, the character switch 71 located at the anode of the one-digit light emitting diode a = h of the display 30 is turned on, and at the same time the light emitting diode a = h is turned on.
Segment switch 40a included in the cathode of
~40g, for example, switches 40b and 40e are ON
If so, the first digit of the display 30 will display "l +t."

Next, turn the 7-digit character switch 72 ONL, and turn on the segment switches 40a, 40b, and 40g.

When 40e and 40d are turned on, the second digit of the display 30 displays 2''.

In this way, one digit at a time, up to four digits, is displayed, and this is repeated to make the four digits appear as if they are standing still.

Next, Figure 3 shows the state of light emission from light emitting diodes a''-g.
This will be explained with reference to FIG.

FIG. 3 is a circuit diagram showing a part of the one-digit display circuit of FIG. 2.

In Figure 3, the character switching transistor 7
The emitter of 1 is connected to the power supply +VB through the current limiting resistor 71'.
The collector is connected to the anode of the light emitting diode a, and the base is connected to the output terminal v1 of the digit selection circuit 60.

The collector of the segment switching transistor 40a is connected to the cathode of the light emitting diode a, the emitter is directly grounded, and the base is connected to the output terminal Va of the seven segment decoder 20.

In such a circuit, when the output signal of the 7-segment decoder 20, that is, the voltage v1°Va shown in FIG.
A current L shown in Fig. 4 flows through the light emitting diode a.
emits light.

However, as is well known, in this dynamic type, since the segment switches 40a to 40g, character switches 71 to 74, etc. are controlled in a time-sharing manner, the current flowing through the light emitting diode a becomes pulsed, resulting in the above-mentioned drawback.

FIG. 5 shows a circuit according to an embodiment of the present invention which corrects such conventional drawbacks.

In FIG. 5, the emitter of the character switching transistor 71 is connected to the power supply +
Connected to VB.

A series circuit of a diode 122 and a resistor 123 is connected to both ends of the inductance 121 as shown.

The rest is the same as in FIG. 3, and the same parts as in FIG. 3 are given the same numbers and their explanations are omitted.

In such a circuit configuration, the transistor 71.40
When the same voltage as in the conventional circuit of FIG. 3, that is, the voltage V1°Va shown in FIG. flows, and its rising current waveform becomes as shown in Figure 6, and gradually increases with a time constant of .

Here, E...Voltage L of power supply +VB...
- Value R of inductance 121...Inductance 121. The diode is the conduction resistance of the transistor 71°40a.

Thereby, harmonic components at the rise of the current can be reduced.

Then, when the applied voltage stops, the transistor 71.40
a is blocked.

At this moment, the energy stored in the inductance 121 begins to be released, and a pulsed back electromotive force is generated in the inductance 121.

However, this back electromotive force is caused by the diode 122 and the resistor 123
It is absorbed and disappears.

FIG. 7 is a circuit diagram showing another embodiment of the present invention.
This is an example in which the character switching transistor 71 in the figure is omitted, and its operation and effect are the same as in FIG. 5, so detailed explanation thereof will be omitted.

FIG. 8 shows the relationship between the distance between the light emitting diode and the antenna of the receiver and the amount of noise when the light emitting diode drive circuit based on the conventional circuit shown in FIG. 3 and the circuit of the present invention shown in FIG. 5 are brought close to the AM receiver. It is a characteristic diagram showing a relationship.

In Fig. 8, 3 is the characteristic curve of the conventional circuit shown in Fig. 3;
5 is a characteristic curve obtained by the circuit of the present invention shown in FIG.

From this characteristic diagram, it can be seen that according to the present invention, noise can be significantly reduced compared to the conventional method, and unnecessary interference from the light emitting diode is also reduced.

[Brief explanation of the drawing]

Fig. 1 is an external view of a 7-segment display, Fig. 2 is a 7-segment display drive circuit diagram, Fig. 3 is a conventional light emitting diode drive circuit diagram, and Fig. 4 is the driving voltage and current of the circuit. Waveform diagram, Figure 5 is a light emitting diode drive circuit diagram showing one embodiment of the present invention, Figure 6 is a driving voltage and current waveform diagram of the circuit, and Figure 7 is a circuit diagram showing an application example of Figure 5. , FIG. 8 is a characteristic diagram for explaining the present invention in detail. 30... 7 segment display, 40... Segment drive circuit, 70... Character drive circuit, 4
0a to 40g, 71 to 74, 80h = i ranjisku, a
= h...Light emitting diode, 121...Inductance, 122...Diode, 123...Resistance.

Claims (1)

[Scope of Claims] 1. Consists of a segment switch means for driving one light emitting diode, and a power source connected to the light emitting diode, and the switch means repeats conduction and cutoff in a time division manner to drive the light emitting diode in a time division manner. In a dynamic drive force type light emitting diode drive circuit that drives
An inductance connected between the light emitting diode and the power supply and delaying the rise time of the light emitting diode and drive current flowing from the power supply to the light emitting diode when the segment switch means is conductive; A light emitting diode and drive circuit comprising a parallel circuit of diodes that consume the generated back electromotive force. 2. The light emitting diode drive circuit according to claim 1, wherein said switch means comprises a switching transistor. 3. The light emitting diode drive circuit according to claim 1, wherein the diode is formed by connecting a resistor in series.