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One important parameter in embedded system designing is power consumption. This parameter is directlyrelated to the battery lifetime, if the system is to be powered from a battery. In order to determine the powerrating of your designed system, you need to know how much current the system draws from the source at agiven voltage. While working on my projects, I usually measure current by placing an external ammeter inseries with the currents return path. This is not always convenient to do, and so I thought of making a specialpower supply unit for my lab that would display both voltage and current information on a LCD screen while

prototyping my circuit. This way I can continuously monitor how much power my test circuit is drawing at aspecific operating voltage.

Multifunction bench power supply

This power supply unit provides fixed 5 V as well as a variable dc voltage ranging from 1.25 V to 9 V. A

PIC16F689 microcontroller is embedded into the power supply unit to measure the adjustable output voltagealong with the load current. Besides, this unit also has a built-in frequency counter to measure the frequency ofan external signal. The range is over 50 MHz. However, it has been tested up to 20.0 MHz, and works fine.There are still few I/O ports of PIC16F689 that are not used. So I am thinking about adding one or morefeatures (like capacitance meter) to it. But that would be on my second version.

Theory

The fixed 5 V power supplies are derived using LM7805 and LM7905 regulator ICs. A 12 V rectified outputfrom a center-tapped transformer provides the dc inputs to these regulator ICs. For the variable dc output, aLM350 IC is used. LM350 is an adjustable 3-terminal positive voltage regulator that is capable of supplying inexcess of 3A over a 1.2V to 33V output range. It requires only 2 external resistors to set the output voltage.

Since my dc input is only 12 V (rectified output from the transformer), the regulated output from the LM350 ICwould go up to 9 V (3 V less than the input, as specified in the datasheet). In order to obtain the full range ofoutput voltage (up to 33 V) from LM350, you need a transformer with a higher output voltage. I rarely usepower supply above 5 V in my projects, and therefore, a variable voltage source up to 9 V is good enough forme.

Next comes PIC16F689 microcontroller that measures the LM350 output voltage and current. You are right, Iam implementing voltage and current measurement only to the LM350 output. The principles of voltage andcurrent measurements using microcontroller were discussed in two of my posts: PIC-based digital voltmeterand How to measure dc current with a microcontroller. I am not going to repeat the details here. Both theparameters can be measured with built-in ADC channels of PIC16F689. Since the output voltage from LM350could go higher than 5 V (up to 9 V in this situation), it is not directly measurable from an ADC channel. Asimple voltage divider network using two resistors is incorporated to scale down the output voltage to the safe

ADC input range (0-5 V). On the other hand, the current to be measured must be first converted into voltageso that it could me measureable too with the ADC channels. The current to voltage conversion can be done by

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placing a small value resistance (shunt) in series with the current path. The voltage drop across the resistancewill then vary linearly with the current. I am using a 0.286 resistor (Rs) made by myself by coiling a 5 ft long22 AWG solid copper wire that has a plastic insulation on its outer surface (see the picture below).

Current sensing resistor

Since this resistance is so small in value, the voltage drop across wont be very high for small currents. Evenfor 1 A of current, the voltage drop across it would be only 0.286 V. To improve the resolution (and henceaccuracy) of current measurement, this voltage must be amplified before the AD conversion process. Anoperational amplifier with a suitable gain can do this task. The figure below shows how this whole techniqueworks.

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Block diagram for voltage and current measurements

An interesting thing to be noted here is that the output voltage across the load (Vo) is actually the measuredvoltage (Vm) minus the voltage drop across Rs (Vs). For small load current, the difference between Vm and Vowont be very much, but it could be significant for higher current. For 1 A of load current, the output voltageacross the load is 0.286 V less than the measured output voltage from LM350. This error will be corrected laterin the software part by subtracting Vs from Vm.

The frequency counter uses Timer0 module in counter mode to measure the frequency of an external signal. Asingle transistor preamplifier stage is used to boost the low amplitude signals before they are sent to theTimer0 module for counting. An easily available BF199 RF transistor is used for this purpose.

Circuit Diagram

As mentioned earlier, two linear regulator ICs (LM7805 and LM7905) are used to derive fixed 5 V supply. Acenter-tapped transformer output is rectified (using diodes) and filtered (using 4700 F capacitors) to obtainunregulated 12 V dc that serve as inputs to both the regulator ICs. The circuit diagram below shows all thecomponents and their connections required for the fixed power supply.

Circuit diagram for fixed +/- 5 V supply

The variable power supply unit uses an adjustable regulator IC (LM350). The output voltage can be controlled

through resistors R3 and Rp (see figure below). The expression for output voltage is, Vo 1.25 x (1+Rp/R3).This specific design allows the output voltage to vary from 1.25 to 9.0 V. R1 and R2 create a voltage dividernetwork to scale the output voltage within the safe limit (0-5 V) of the microcontrollers ADC channel. Anadditional zener diode at the output ensures that the input voltage at PICs ADC channel wont be greater than5.1 V. The scaled output is connected to AN0 channel (pin 19) of PIC16F689.

http://embedded-lab.com/blog/wp-content/uploads/2011/03/FixedPSupply.jpg

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Variable power supply with voltage and current measurement

The measurement of current is possible with a shunt resistor (Rs = 0.286 ) and an operational amplifiercircuit. Rs is placed on the return path of the current and therefore, the voltage drop across it is proportional tothe current drawn from the variable power source. This voltage is first amplified by a non-inverting Op-ampcircuit and then passed through another non-innverting buffer stage to lower the output impedance. The outputfrom the buffer goes to AN4 channel (pin 16) of PIC16F689 for A/D conversion. With R4 = 1.3 K and R5 = 12

K, the overall gain of Op-amp stage is 10.23. I am using LM324 IC for the Op-amp stage. LM324 does notprovided rail-to-rail swing; the maximum output is about 1.5 V less than the applied power supply voltage. Inthis case, LM324 is powered with 0-5 V, and therefore, the maximum output voltage could be about 3.5 V. Thismeans the maximum input voltage to the Op-amp stage would be 3.5/10.23 = 0.342 V. This gives themaximum range of the measurable current to be 0.324/0.286 = 1.2 A. Current values higher than this willsaturate the LM324 output. The range of the measurable current can be increased either by decreasing thegain of the Op-amp stage or by using a different Op-amp IC that provides rail-to-rail output swing (0-5 V).

The preamplifier stage for the frequency counter has an RF transistor (BF199) that is collector-feedbackbiased using a 10 K resistor. The 1 K resistor with diodes D1 and D2 at the input side clamp the input signallevels to 0.7 V. The 0.1 uF capacitor eliminates any dc component presented in the input signal. The

amplified output is derived from the collector with a 470 resistor in series and is sent to T0CKI pin ofPIC16F689. The Timer0 module counts the number of incoming pulses through the T0CKi pin to determine theincoming frequency.

http://embedded-lab.com/blog/wp-content/uploads/2011/03/VariableSupplyandOpamp.jpg

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Pre-amplifier stage for frequency counter

PIC16F689 runs with an external 20.0 MHz crystal. A 16 x 2 character LCD (HD44780 compatible) isinterfaced in 4-bit mode. Data were transferred through RB4-RB7 pins whereas the RS and E signals arecontrolled through RC7 and RC6 pins, respectively. A diode connected between RC1 and RA2 acts as a gatingenable for frequency counter. When RC1 is high, the T0CKI input is also held high, disabling the countingprocess. RC1 is set to low when the microcontroller has to count the external pulses from the preamplifierstage output. The 470 resistor at the output of the preamplifier stage is to prevent any possible short circuitat T0CKI pin due to the voltage conflict between pins RC1 and RA2.

http://embedded-lab.com/blog/wp-content/uploads/2011/03/FrequencyPreamp.jpg

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Microcontroller I/O port assignments

I constructed the complete circuit on a 180 mm x 128 mm general purpose prototyping circuit board which isshown below.

http://embedded-lab.com/blog/wp-content/uploads/2011/03/MicrocontrollerIOports.jpg

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The complete circuit soldered on a general purpose prototyping circuit board.

Front panel installation

A/D Conversion Math

The use of A/D conversion for voltage and current measurements require some math to re-convert the digitaloutputs back to the voltage and current for display. Lets work on displaying the voltage first. The analog inputvoltage (Va) at RA0/AN0 pin is related to the A/D conversion output (digital number, DN) as,

DN = Va * (1024/5) = 204.08 * Va (Vref = 5 V, ADC resolution is 10-bit)

or, Va = 0.0049 * DN

But Va = Vout * R2/(R1+R2), Vout is the actual output from LM350.

=> Va = 0.2418*Vout

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=> Vout = 4.136 * Va = 4.136 * 0.0049 * DN

=> Vout = 0.0203 * DN

This is the conversion equation for calculating the LM350 output voltage from the 10-bit A/D conversion result(DN) for channel AN0.

Similarly, for current measurement, the output of Op-amp (V0) is related to the AN4 channel A/D output as,

V0 = 0.0049 * DN (just as above)

But V0 is related to the load current (IL) through Rs as,

V0 = 10.23 * IL * Rs = 10.23 * 0.286 * IL = 2.926 * IL = 0.0049 * DN

=> IL = 0.001675 * DN (gives current from A/D conversion output)

As mentioned earlier, the output voltage is actually less than the calculated voltage (Vout) under loadcondition. This is because of the voltage drop across the shunt resistor Rs = 0.286 due to current I

L. The

adjusted output voltage should be, therefore,

Vout = 0.0203 * DN 0.286 x IL

Operation of Frequency Counter

The Timer0 module (no prescaler) is used as an external pulse counter whereas Timer1 module is used togenerate a precise 100 ms duration as counting interval. This means that the resolution of frequencymeasurement is 10 Hz. This resolution is valid for frequencies up to 99990 Hz. Beyond this, the Timer0module uses the prescaler (1:64) and the new resolution becomes 640 Hz or 0.64 KHz. As PIC16F689 isusing 20 MHz clock, wit h 1:64 prescaler value, it can count frequencies over 50 MHz. The operation offrequency counter is like this.

Initialize Timer1 registers with appropriate values for creating 100 ms delay.

Enable Timer0 as counter with prescaler 1:64. Count the pulses for 100 ms.

Frequency = 10 x No. of Pulses

If frequency < 99990, change prescaler to 1:1, re-calculate frequency, and display it (in Hz).Otherwise, display the frequency in KHz.

Software

I used MikroC compiler to develop the firmware. The compiler has built-in library routines for A/D conversion,LCD display, and Timer operation. This makes the firmware development easier and quicker. Both the sourcecode and HEX files can be downloaded from the link provided below. Configuration bits for PIC16F689 can beset using the project edit window in MikroC editor (shown below).

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Project Edit window in MikroC

Download complete project source and compiled HEX files

Testing

The finished project is tested for voltage, current, and frequency measurement. The displayed voltage is cross-checked with an external voltmeter, and the readings are found consistent. A 56 resistor is connected as loadto measure current. The output voltage is set to 5.2 V, and the displayed current is 90 mA. The ideal value ofcurrent should be 5.2/56 = 92.8 mA. This gives an error of about 3% in the measurement of current but the 56 resistor itself has tolerance of 5%. The testing frequency counter is done by applying the 20 MHz

microcontroller clock derived from OSC2/CLKOUT pin (3). The displayed value is 20.00128 MHz which isreally close.

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Testing with 20 MHz clock frequency from PIC16F689 circuit

Testing the voltage reading with an external voltmeter under no-load condition

Voltage and current under load condition (load is a 56 Ohm resistor with 5% tolerance).

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