Digital power supply with large graphic display

With all the abundance of ready-made amateur radio equipment in stores, most often you want not to buy a ready-made device, but to assemble it yourself. Therefore, we want to present a digital power supply for self-assembly, if your professional level, of course, allows you to take on such a complex device. This is a power supply with analog stabilization and digital control. Power supply parameters:

• output voltage 0-25 V in steps of 0.01 V
• current 0-5 A in steps of 0.01 A.
• The old but still quite effective LM723 chip was used as the basis for the analog part. This is almost standard inclusion. A higher quality diagram in PDF format is available at the link .
  

  Analog part of the block

  The problem with the standard m / s LM723 inclusion is that the alternating current works in limited areas, relative to the reference voltage of ~ 7.15V. Using a simple procedure, we get rid of this problem by applying a negative voltage of -5 V to the V pin, which will reduce the ground potential. According to the datasheet, the voltage measured between NI- and the ground potential V- cannot be higher than 8 V. So, the output voltage of the power supply goes to IN through a 1:10 resistor divider, so the maximum value of 25 V at the output of the power supply is 2.5 V at the NI input, and this together gives 7.5 V relative to V-, i.e. the condition is <8 V.

  The LM 723 chip has a built-in error amplifier (NI+ and NI-) and strives to ensure that the voltages on both lines are the same. So with the MCP 4922 12-bit DAC, the voltage is equal to the specified output voltage in a ratio of 1:10, so for 10.00 V it is 1.00 V, for 25.00 V it is 2.50 V. That is, the circuit ensures that IN has the same voltage as on IN+, and thus we have an output voltage 10 times higher than that set by the DAC. The voltage from IN is also input to the MCP 3202 12-bit converter and is used to measure the voltage on the LCD.

  The problem that the LM723 has in this circuit is the rapid disappearance of the -5.0 V voltage used to power some of the LM723 blocks and allow the output voltage to be regulated from 0 V. This causes the supply voltage to appear momentarily at full output. voltage, which is unacceptable. To eliminate this, an optocoupler was used in the gate circuit of the transistor driving the power transistors. The anode of the optocoupler is connected through a resistor to GND, and the cathode to -5V, and when the negative voltage disappears, the base of the control transistor is pulled up to GND and the power transistors are not overloaded. The solution is very efficient and works without problems.

  The current measurement is monitored as a voltage drop across the 0R1 resistor on the GND side, which is applied to the MCP617 op-amp with a low input voltage, which is amplified by 5 times, so for 5 A, the voltage is 2.50 V, after which it is applied to the second element, the amplifier works as a comparator that compares the reference voltage of the DAC (in the range of 0-2.50V), and the output is fed to the input of the LM723 CL and through a resistor to GND. These are the inputs of the current limiting circuit in the circuit of the microcircuit. Applying a voltage above 2.5V to this pin will activate the current limiting transistor contained in the LM723 structure. The output of the comparator is also fed to the gate of the transistor, which switches the diode signaling the current limit, and to the transistor to the output of the micro controller.

  The voltage output from the amplifier is also input to the MCP 3202 12-bit converter and is used to measure the current on the LCD.

• The power control circuit was implemented on two 2SC5200 power transistors connected in parallel, controlled by a BD139 transistor, with appropriate emitter resistors.The -5V negative voltage for the LM723 is generated by the so-called ICL7660 m/s charge pump with +5V positive from the buck converter in the LM2576. The +5V potential is also used to power other digital circuits, including the DAC and ADC.

  The choice of MCP4922 and MCP3202 was not accidental, these chips must introduce an external reference voltage. This voltage was generated in the TL431 and is 4.096 V. This voltage makes it possible to obtain voltage values ​​without divisions on 12-bit DAC and ADC converters, since the resolution for 12 bits is just 4096.

  The power supply also has the ability to switch transformer taps or operate without switching. This means that a 24V or 2 x 12V transformer can be used to power the PSU. Depending on the transformer used. In this case, the 24 V transformer should be connected to the connector described on the board as ~ 24 V, and remove the jumper marked
  12/24V 2. Transformer 2×12V – one tap must be connected to the described ~0-12V connector, and the other similarly to the other. The jumper interrupts or disables the signal from the DAC, so if the jumper is removed, the input voltage at the comparator is 0V, the relay remains in one position regardless of the voltage setting, in the case of a 24V transformer. However, due to the possible very large voltage difference at the input and output of the transistors, a large power loss in the form of heat will occur.

  This power supply used a 200VA 2x12V and 2x8A transformer.

  The power supply has thermal protection in the form of measuring the temperature of the radiator using the DS18B 20 sensor and, depending on the temperature, turns the fan on or off. Despite this, the power supply has double protection, if the heat sink reaches a temperature above 60 degrees Celsius despite the operation of the fan, the PSU remains off until the fan cut-off temperature is reached. The fan control circuit is assembled on IRL 540.

• Digital part of the power supply

  the control is based (and here I will definitely let my favorite colleagues down) on the At mega 32 P. The At mega runs in a standard configuration with 16 MHz crystal.

  Inputs:

  • MCP3202 – voltage and current measurement
  • rotary encoder with button (encoder)
  • start/stop button
  • temperature measurement on DS18B20
  • current limit measurement

  Outputs:

  • MCP4922 – voltage and current limit setting
  • fan control output

  The DAC and ADC sensors are connected using the SPI bus. The power supply board has connectors for the encoder, Rx, tX and En signals for potential control via the RS485 bus. In this case, Rx and Tx are used to control the Next ion display. I2C bus for EW, other LCD support is also allowed.

  The screen used was a Nextion NX4827T043_011 touch display with a resolution of 4.3″ 480×270.

  The pins on the power supply board allow you to use other LCD’s, such as the standard 20×4 I2C or others, there are a few other pins available, so you can also use, for example, LCD after SPI. The software for another type of LCD naturally needs to be adjusted.

  Power supply operation

  The power supply is simple, and control using an encoder is implemented here. After pressing the encoder, the LCD flashes the screen voltage setting, and when the encoder is rotated, the value changes, the next press is the current setting, similar to the voltage. Another press on the encoder turns off the setting mode.The encoder has an automatic coarse and fine mode of operation, with faster rotation the values ​​increase faster, while slow values ​​can also be finely set.The screen has an oscilloscope mode. After entering this function with the button, you can see the voltage and current wave forms on the graphs, display the set voltage and current values, and change the settings in this mode using the encoder as described above.

• There is also a settings button on the main screen. This allows you to set the maximum voltage of the power supply to 1-25 V, and the current limit to 0.01-5 A. In addition, you can set the temperature for switching on and off the fan. If the switch-off temperature is set higher than the switch-on temperature, the program will automatically change the switch-off temperature according to the hysteresis principle.
• The coefficient value is used to calculate the current voltage using a polynomial of the third degree. It does not affect the correct value of the output voltage of the power supply, but is used to correct the non-linearity of the ADC converter, unfortunately they are not perfectly linear.

• Starting the power supply

  1. After soldering and before installing all the ICs, turn on the power supply and measure the voltage at the Vcc and GND pins, it should be almost perfect +5V.
  2. Measure the voltage between GND and pin 7 of the LM723 – there should be a negative voltage between -4.5 and -5V.
  3. The VREF multi-turn potentiate must be set to the 4.096V reference voltage 8 U4 or pin 11 or 13 U7.
  4. Now turn off the power and discharge the main capacitors, then plug in all the ICs, in the case of the Atmega, pre-program it of course.
  5. Turn on the power, the program will automatically calibrate, then set the output voltage with the sensor to 5V, and turn on by measuring the voltage at its output, change the output voltage with the VSET multi-turn potentiometer to 5V.
  6. Turn off the power, hold the encoder button and turn on, keep the button pressed until the calibration screen appears.
  7. Setting the current limit is limited by the connection of the load, the voltage and current settings must exceed the current consumed by it, for example, a 10 Ohm 5 W resistor – voltage 5V, current limit 1 A, a multimeter connected in series with the load must be connected in current measurement mode, and when By turning the ISET potentiometer, the current measured by the multimeter should match what is displayed on the screen. Then lower the current limit value and check on the multimeter and screen that the values ​​are correct.