Issue 144 July 2002
An 80C31-Controlled Power Supply

---

Circuit Description

The brain of the circuit is the popular 80C31 microcontroller, which is the version of the 80C51 without ROM. The 80C31 is a widely available (it’s produced by several manufacturers) and high-quality microcontroller for embedded design considering its instruction set and price.

The 80C31 has 128 bytes of RAM, two external interrupt pins, two timer/counters, and serial ports (see Figure 1). It has 32 I/O pins, but doesn’t have ROM, so some of the ports (namely ports 0 and 2) are used for the address bus and data bus. A few of the pins of port 3 are also used to interface to other devices like ADCs and DACs. The pins serve as read and write signals for proper bus operation like the read cycle and write cycle. So, after interfacing the ROM, ADCs, and DACs, you’re left with 14 I/O pins.

(Click here to enlarge)

Figure 1—The 80C31 has 128 bytes of RAM, two external interrupt pins, two timer/counters, and serial ports.

Port 1 is used to interface and read the keypad. Some of the pins in port 3 are used for the Set Voltage and Set Current buttons, while others are used to control the relay and analog switches to read the voltage and current to be fed to the ADC.

U2 74HC373 is a transparent latch (see Figure 2). It’s used to obtain the address because the address bus and data bus are multiplexed on port 0 to conserve pins coming out of the IC. The address is obtained after the address latch enable, or ALE, is asserted. U3 is an 8 KB × 8 UV EPROM (27C64) that stores the program that sets the voltage and current and reads the ADC. I chose the 27C64 EPROM because it’s inexpensive, available, and the firmware will fit in it. You can actually use larger EPROMs like the 27C512, but they’re a waste of space, money, and time because you’ll have to wire the added address pins. The 4K × 8 EPROM can also be used but it’s hard to obtain.

(Click here to enlarge)

Figure 2—In the digital portion of the power supply, U2 74HC373 is a transparent latch used to obtain the address.

U4 ADC0820 is a successive approximation analog-to-digital converter that converts an analog input voltage to an 8-bit equivalent value. It’s fast and provides the necessary handshaking pins to interface to the three-bus architecture without glue logic. U4 ADC0820 also converts the analog voltage to an 8-bit equivalent value.

U7 CD4053 is a triple one-of-two switch. It is used to select between the voltage and current. Wondering why I said current? Using a high-side current detector, the current flowing through a sense resistor is converted into a suitable voltage to be read by the ADC.

U5 and U6 AD7524 are digital-to-analog converters. They convert a digital value to an equivalent analog current. They’re chosen because they provide the hardware needed to interface to the system bus. In addition, they’re fast and behave like RAM memory.

U9 LM358 and the adjustable resistors form a pair of current-to-voltage converters. U8 74HC138 is an address decoder that’s used to access the ADC, DAC, and LCD. The MOVX memory space is divided into 8 parts of 8 KB each. I used this scheme so that you can also insert an 8-KB RAM just in case it’s needed. Actually, the MOVX instruction generates the read and write signals whenever it’s executed, so you can read and write to external devices connected to the bus.

LCD1 is an LCD module based on the popular HD44780. It’s interfaced to the three-bus system using four NAND gates. Because the LCD is slow, the busy flag is read after an operation to see if it is busy or not. Failing to poll the busy flag will result in erratic operation or no display at all. In order to interface the LCD properly, you’ll often need two signals at one time to perform an operation. Namely, the decoded signal like a chip select signal and the read signal for a read operation. The two signals must be turned on at the same time for a proper read operation. The same principle applies to the write cycle.

C9 and R4 form the RC time delay for proper reset. The specifications require several clock cycles during which the reset is high until the clock generator is stable. D2 serves to discharge the capacitor after the power is removed. C5 to C14 serve as an immediate source of voltage because the supply rail drops as a result of the internal switching of the transistors. The voltage drop is caused by the inductances in the wire or traces of the PCB. And as you know, the current cannot change immediately when there is an inductor present in the circuit.

Rectifier diodes D2, D3, D5, and D7 1N5400 convert alternating current to pulsating DC (see Figure 3). C1 filters the pulsating DC to smooth DC. C7, C8, D17, and D18 are configured as a charge pump that serves as a source for the negative voltage needed by the high-side current detector.

(Click here to enlarge)

Figure 3—When looking at the analog portion of the power supply, you notice that the rectifier diodes convert alternating current to pulsating DC.

Zener diode D1, R1, R6, and Q1 provide a constant current source for the thermal sensor. Q8 serves as a thermal sensor and limits the base current to the series pass transistor after it reaches a certain temperature. The collector current increases as the junction temperature increases even if the base current is constant. This works in theory, but it has not been tested because I don’t have a themocouple themometer. I did use, however, a 4² × 4² heavy-duty heatsink.

D12, D13, C4, and C5 form a split supply power source that’s used to power the logic circuit, ADC reference, and the negative supply for the DAC reference. U1 78L05 and U3 79L05 serve as pre-regulators for the voltage references. U2 7805 is the voltage regulator for the logic circuit. Note that the 1-A variety is used because this also powered the EPROM emulator used during development of the firmware, which consumed a great amount of electricity.

C2, C3, and C6 are filter capacitors used to make the regulators more stable. R5, R27, and D4 LM336-2.5 form the ADC voltage reference. The variable resistor R27 trims the voltage to 2.55 V. R20 and D14 LM336-2.5 form the voltage reference needed by the DAC ICs. R26 is used to bias the Darlington transistor, which is a series pass element comprised of Q5 and Q6, which form the Darlington transistor. The series pass element acts as a variable resistor to change the output of the power supply.

R10, R21, R22, R23, R24, and U7 MC4741 act as a high-side current detector. Essentially, it’s a subtractor that gives a voltage in proportion to the ratio of the resistor and voltage difference. The current moving across R10 produces a voltage drop that’s detected by the subtractor, and then it’s amplified according to the ratio of the resistances. A 2.5-A current produces a 2.5-V potential. This potential is compared to the voltage generated by the DAC. The comparator U5 LM358 controls the series pass element in order to regulate the current output of the power supply. R11 and R15 are a sampling element of the output voltage. The two resistors form a voltage divider and scale the voltage by 10. Thus, a 23-V potential becomes 2.3 V. The comparator U5 LM358 compares this potential to the voltage generated by the DAC and in addition to controlling the series pass element, which regulates the output voltage of the power supply.

D15 and D16 isolate the two op-amps. D8, D9, D10, and D11 protect the analog switch and ADC when there is an error and the sampled voltage or current rises above 5 V or drops below zero. C10 and C11 serve as filter caps to smooth the sampled voltages. R14 and R18 limit the current from the sampling points. R28, R29, D19, and D20 act as a regulator to limit the voltage going to U7 MC4741. If the circuit is unloaded, the B+ potential can reach as high as 31 V. And if this reaches U7, it will suffer electrical overstress, or EOS.

The firmware, which can be downloaded from the Circuit Cellar web site, is written in assembly language using a Metalink assembler. One thing to note is why the reference is 2.55 V. The reason is that the ADC and DAC values go from zero to 255. Therefore, if you use 2.55 V you won’t need to compute the values needed to obtain a desirable voltage or the reading obtained by the ADC. With this scheme, you can simply read the ADC and move the decimal point. For example, a 100 reading from the ADC is equivalent to a 10-V potential. And with an 8-bit value, the resolution of the voltage is 100 mV and the resolution of the current is 10 mA. To program a 1-A current you can send 100 to the current DAC. To program 1 V, simply send 10 to the voltage DAC.