This is a shortened translation from Russian of my article published in Радио (Radio) magazine, no. 8 (2010) pp. 21-23. The voltmeter is designed for controlling DC voltages at the output of a dual bipolar power supply. It not a universal instrument, since its input resistance does not exceed 40K, which might be too low for some measurements. However, it is perfectly fine for power supplies and does not noticeably load their output. The range of input voltages is 0 to 24V for positive supply and -24V to 0 for negative one.
The voltages are sampled every 300 msec and displayed on a 9-digit LED display. Only 8 digits are used, the middle digit slot is always off and serves a a separator between negative (on the left part of display) and positive (on the right one) readings. The unit provides a 0.01V resolution. For small negative voltages (not exceeding 10V in absolute value) a leading minus sign is displayed. For positive voltages a leading zero is suppressed.
| Schematic | Protoboard | |
The voltages are sampled by an external 18-bit dual channel delta-sigma ADC MCP3422 manufactured by Microchip. Using built-in ADC in PIC microcontrollers is inappropriate because they only provide a 10-bit resolution, hence only 210 = 1024 codes. In our case at least 24·100 = 2400 codes are needed, which implies using at least a 12-bit ADC. The MCP3422 has a built-in 2.048V voltage reference, so the input voltages must be scaled for sampling. The scaling factor for positive voltages is 24V/2V = 12, which is provided by the resistive voltage divider R3-R5. The ADC is configured for a 14-bit resolution, out of which only 13 bits are used because the voltage at CHi+ input is supposed to be not smaller than the one at CHi- input for i=1,2. Therefore, the resulting resolution of input voltages is 12·2.048 / (213 - 1) ≈ 0.003V which is even more than needed.
Negative input voltages are scaled by voltage divider R1, R2, and R6. They are converted into positive voltages (required for the ADC) by attaching the upper end of R6 to a 2.048 voltage reference DA1. The scaling factor in this case should be (24V + 2.048V) / 2V ≈ 13. The ADC communicates with microcontroller via the I2C interface. Since ADC is a slave device, no pull-up resistor at the clock pin is needed.
The LED display is controlled by MAX7221 (MAX7219 is used on the proto-board), which refreshes the display at about 1kHz frequency. The data is forwarded to the LED controller via the SPI interface. Resistor R10 determines the LED brightness. The minimum value of this resistor is 10K according to the data-sheet.
Moving the LED driving and ADC function outside of microcontroller allows to use a very simple device with no built-in ADC. The code size is about 430 words, so the smallest (and cheapest) PIC12F508 with 8 pins would be sufficient. Note that using external ADC does not increase the total number of pins and the area occupied by ADC+PIC on the PCB.
| Top view | Bottom view | |
The voltmeter is intended to be powered directly from the rectifier filter of the power supply. In my case this voltage is about 20V while the circuit needs 5V. This voltage is provided by a DC-DC converter on DA2. The current drawn by voltmeter from the converter is 71 mA and the total current drawn from 19.8 V is 24 mA. Hence, the converter's efficiency in this case is 5·71 / 24·19.8 ≈ 74.7%. If a linear voltage regulator like 7805 would be used, its efficiency would be only about 5/20 = 25% and it would then dissipate (19.8 - 5)· 71 ≈ 1W of heat. This makes it necessary to use a heat sink, while in our case no heat sink is needed. The converter components are just slightly warm and it can deliver up to 400mA without a noticeable heating of its components. The used inductor L1 is TDK SLF7045T-101MR50-PF. Trimpots R2 and R4 are Vishay SM4A102. It is desirable to use 1% resistors for R1, R3, R5, and R6, since otherwise tuning the voltmeter with 1K trimpots might be impossible. Alternatively, one could use trimpots with larger resistance.
| Power supply | The guts | |
The voltmeter is embedded into a bipolar power supply (see the above photos) based on LM317/LM337 and assembled in a 7x5x3" project enclosure from Radioshack. The schematic is standard and matches the one in a simple power supply. The only difference is the usage of two pots controlling the output voltage. A 5K resistor is connected in series with a 500 Ohm resistor for fine tuning. This way it is real to adjust the voltage with a 0.01V accuracy.
Last modified:Mon, Jan 23, 2023.