With the LM317 adjustable integrated regulator, the regulated power supply can output multiple voltages. If these different multi-output voltage values ​​can be used to indicate how good it is! In general, it is not cost effective to add a complex digital display circuit that assists in a simple power supply circuit. However, the power supply with digital display described in this paper does not need complex and expensive decoding circuits, driving circuits and digital tubes, but uses ordinary light-emitting diodes to form a 7-segment digital display, and replaces the decoding circuit with 2&TImes; 2 switches. Commonly used 3V and 6V two gears, so the entire power circuit is simple and practical, easy to make, may wish to try.
Circuit working principle
Voltage regulator circuit
Thanks to the LM317 adjustable regulator, the regulated power supply circuit is very simple, see Figure 1. The output voltage range of the LM317 is continuously adjustable from 1.25 to 37V. Pins 2 and 3 of the LM317 are input and output, respectively. 1 foot is the adjustment end. After the resistor R1 is connected between the pins 1 and 3, a reference voltage of 1.25 V can be obtained, and the reference voltage is constant. The value of R1 is generally 120 Ω to 240 Ω. If the 1 pin is grounded, the output voltage of the 3 pin is 1.25V. If you want to get an output voltage higher than 1.25V, just connect resistor R2 between pin 1 and ground (see Figure 2). Changing the value of R2 can change the output voltage of pin 3. The output voltage Vo of the LM317 is determined by the ratio of R2 to R1, that is, Vo=1.25 & TImes; (1+R2/R1). The size of R2 and R3 selected in Figure 1 is designed to output a voltage of 3V and 6V. When the switch Sa is turned to the left, R2 is grounded and outputs 3V. When Sa is turned to the right, R2 and R3 are connected in series and grounded, and output 6V. Here, the purpose of connecting R2 and R3 in series is to prevent the output voltage from increasing and preventing internal damage of the regulator when the contact of the switch Sa is poor. C1 is the filter capacitor. C2 is a vibration absorbing capacitor and functions as a stabilizing circuit. If the power supply is not a capacitive load, C2 can be disconnected.
2. Digital display circuit
A number can consist of 7 strokes, see Figure 3. If you replace the 7-segment stroke with a light-emitting diode, you only need to turn on the power of the LEDs in different segments to display the numbers. For example, the number 3 uses five pen segments a, b, c, d, and g; the number 6 uses six pen segments a, c, d, e, f, and g. Using a switch of 1 & TImes; 2 to control the b segment and the e and f segments, the conversion between the numbers 3 and 6 can be realized, as shown in the upper part of Fig. 1.
Component selection
The secondary voltage requirement of the power transformer is 12V to 18V. The four rectifier diodes are either 1N4001 or replaced by a bridge stack. There are many manufacturers of integrated voltage regulators, except for LM317, SW317 can be used. If the load is a few hundred milliamps, you need to add a heat sink. The seven LEDs use a flat square (such as 2 & TImes; 5) than the circular display is better. The color is chosen according to the preferences of everyone, usually the effect of red is better. The 3V, 6V voltage transfer switch uses a 2 × 2 small toggle switch, half of which is used for voltage conversion (Ka) and the other half is used for display conversion (Kb). There are no special requirements for the remaining RC components.
Production and debugging
When making, do not connect the three pins of the regulated power supply LM317, see Figure 4. After turning on the power, first measure the secondary output of the power transformer with 12V AC voltage output. After rectification and filtering, there should be a DC voltage of about 15V on the ground of the LM317. If the voltage switch is switched, the 3 pins of the LM317 should have 3V and 6V DC voltage outputs respectively. Otherwise check if the device is faulty and the component is intact. When selecting the 6V DC voltage output, connect a small 0.3A, 6V electric bead as a load, and the 6V voltage should not fall.
Figure 5 is a printed circuit board of a digital display circuit. It should be noted that the polarity of the LEDs in the digital indication section cannot be connected incorrectly, otherwise the numbers will be missing strokes. The 3V of the voltage part of the 2×2 switch should correspond to the 3V of the display part, otherwise the output voltage is 3V, but the indication is 6V. In order to enhance the display effect, a translucent paper or plexiglass can be added in front of the seven LEDs to cover the un-bright b-segment or e, f-segment, so that the digital display effect is more obvious.
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