Small change PF9800 smart electricity meter power circuit diagram

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The strong part of the instrument is relatively standardized. When the power supply is connected, a current transformer is connected in series and a voltage transformer is connected, and the signal to be measured is sampled and amplified. After A/D conversion, go to CPU processing. Therefore, we do not have to change the part of the power, and the weak part only changes the DC power supply for signal processing.

The DC power supply of the instrument is shown in the following figure (below) and cannot be adapted to the AC110V power supply.




So many digital tubes on the meter are driven by the +5v power supply, so this is a positive and negative dual power supply with a very asymmetric load. The market did not buy AC100V ~ 240V into the market for a while. DC +/- 5V dual output switching power supply, but in the electronic market only bought two AC adapters for South Korean Samsung notebook computers: AC 100-240V/1.8A input, DC 14V/3A output switching power supply.
The TDA2030 high-efficiency power amplifier is used as a complementary output to convert the unipolar power supply into a bipolar power supply with large output power and few peripheral devices (see the figure below).

TDA2030 is an audio power amplifier circuit produced by Telefunken. It has the characteristics of small size, large output power, harmonic distortion and small crossover distortion.

As can be seen from the above figure, the TDA2030 is connected in the form of a voltage follower. The two resistors at the positive input provide half of the supply voltage to le, and the output of the output is just half of the supply voltage, and the output is used as a virtual "ground". "This circuit is characterized by "virtual ground". When used, "virtual ground" must be completely isolated from the ground of the input power supply.

The purchased device is soldered according to the figure and then connected to the power supply test. It is found that the TDA2030 integrated block has a large starting current, and the steady state takes about 30 seconds. After steady state, its own no-load current is about 19mA, and the heat sink is hot. Although it can convert single power into dual power supply, its no-load current is so large. It is not ideal.

In order to complete the modification task, I have to continue to search the Internet for reference drawings. This time I found a lot of information in the online "electronic development community". The time base circuit 555 is connected to an unsteady circuit. The output frequency of the 3-pin is 20KHz and the duty ratio is 1. :1 square wave. When pin 3 is high, C4 is charged; when low, C3 is charged. Due to the existence of VD1 and VD2, C3 and C4 are only charged and not discharged in the circuit. The maximum charging value is EC, and the B terminal is grounded. At both ends of A and C, the dual power supply of +/-EC is obtained. The output current of this circuit exceeds 50mA, which does not meet our requirements.

However, it has been inspired: a combination of several solutions to form a new circuit, which consists of a resistor divider, a voltage follower and a parallel regulator. See the figure below for the specific schematic.



Principle analysis:

The circuit consists of R1 and R2 to form a voltage divider. The voltage divider value is taken out and sent to the non-inverting input terminal 5. The two-stage operational amplifier is connected to a voltage follower. The second-stage operational amplifier is combined into a voltage through VT1 and VT2 parallel adjustment tubes. The follower, the advantage of this processing is that the circuit always clamps Q' to 1/2Ec (ie, the zero position of the virtual ground). When the two loads are not the same, the adjustment tube should work. The voltage drop generated by the op amp output on R3 is used as the transmission bias voltage of VT1 and VT2, so that Q' is about 0.7V lower than the Q point. In the above figure connection, the feedback circuit can make the positive and negative power supplies basically symmetrical when the two loads are different.

This circuit is used in the instrument DC power supply scheme, the positive power supply load is large, the negative power supply load is small, because the upper arm load is large, the lower arm load is small, it will cause the Q' point position to be high, forcing VT2 to conduct, VT1 is cut off, thus making the upper arm Part of the load current is shunted by VT2, which causes the Q' point to decrease. When the Q' point reaches the equilibrium value, VT2 is cut off. In short, VT2 is constantly adjusting its work, so its radiator is higher than the VT1's radiator.

LM358 is a dual operational amplifier with a supply voltage of 3-30V.

D822 is an NPN power transistor. BVceo>=30V, ICm=3A

B772 is PNP power triode, BVceo>=-30V, cm=-3A. It can be seen from the parameters of the components: This circuit is suitable for input power supply below DC 30V, output power conversion circuit below plus or minus 15V, preferably +/- The conversion of 5V~+/~12V dual power supply is more reliable, and the output current control is about 1A.

Production points:

1. First put R1. The R2 voltage divider and the secondary voltage follower are soldered. The op amp inverting input 2 is temporarily connected to the Q point. The power-on test ensures that the commercially available op amp LM358 is good: the input voltage is DC 14V. The output voltage is +7V and a 7V. It can be seen from the figure that the power consumption of the circuit is zero, which proves that the LM358 is good.

2. Solder the parallel adjustment tube assembly as shown in the schematic diagram, and connect the inverting input terminal 2 of the operational amplifier to the Q' point. After the error is correct, the power-on test: the input voltage is DC 14V. The output voltage is +7V and -7V. It can be seen from the figure that the power consumption of the circuit is zero, which proves that the single-supply to dual-supply circuit is good.

3. Modified Samsung switching power supply: Remove the switching power supply molded case, adjust the trimmer potentiometer after power-on, adjust its single power output from DC 14V to DC 16V, and fix the trimmer potentiometer. Connect the self-made conversion board to the molded case, and finally plug in the power supply test. The reading should be DC +/-8V, indicating that the system works well.

4. Replace the original transformer on the two instruments with the installed converter, adjust the voltage regulator to 110V, and connect the load to load comparison test for one hour.

5. Change the large radiator: After 1 hour of load detection, it is found that the radiator of the lower arm adjustment tube is hot, which is not conducive to long-term work, and the radiator area must be increased. After changing the radiator for a long time, it is very stable for 8 hours of continuous testing. The American plug is configured, the socket is packaged and sent to the sales company in the United States, and the restructuring work is successfully completed.