PIC16F873(A) based automatic voltage stabilizer Syed Tahmid Mahbub7 the user wants to recalibrate at any time. This is what I initially meant when I mentioned that the final product has no variable resistor in it.RELAY AND TRANSFORMER CONFIGURATION, AND SWITCHINGThe input switches between the 165V, 190V and 240V transformer tappings while the output switches between 240V and 215V tappings. The transformer is a simple autotransformer with the turns ratio 165V: 190V: 215V: 240V along with an auxiliary winding for powering the circuitry.

PIC16F873(A) based automatic voltage stabilizer Syed Tahmid Mahbub8REST OF CIRCUITThe microcontroller runs off of a 4MHz external crystal oscillator. An external crystal oscillator has been used since the PIC 16F873A lacks an internal oscillator, which would have been sufficient since there is no precise time-critical aspect to the automatic voltage stabilizer. The microcontroller is powered off of a regulated 5V DC supply. The autotransformer has a 12.5V auxiliary winding. The voltage at this winding will remain around 12.5V and not vary too much with the input voltage variation due to the switching of the relays and the output voltage regulation which acts to regulate the voltage across this winding too. This low voltage AC is rectified to DC using a bridge rectifier and then filtered with a bulk capacitance. You will also find that decoupling/bypass capacitors have also been used. This filtered DC is fed to the input of a 7805 linear voltage regulator. Since the current draw is not too high, a linear regulator such as the dirt-cheap ubiquitous 7805 is sufficient and no “fancy” switching regulator is required (I still do recommend switching regulators, cost permitting, especially with large current outputs and/or large input-output voltage differences). It is critical to use at least one decoupling capacitor (which should be placed as close to the microcontroller as possible) and you can see that it has been used. The regulated filtered DC voltage that is fed to the 7805 input is also used to power the relays. However, this voltage is not directly provided as the voltage is a tee-bit higher than what the 12V rated relays would probably “like”. Thus, the voltage is dropped by approximately 2.8V by passing this input voltage through four regular rectifier diodes in series. Each relay switching is controlled by the microcontroller. However, since the microcontroller cannot provide sufficient current to drive the relay coils, transistors are used to amplify the current and drive the relays from the required signals provided by the microcontrollers. The configuration is the simple common emitter mode. Each relay coil also has in parallel with it an anti-parallel diode that is used to “catch” or rather “bypass” the inductive kickback that occurs whenever the current flow through the relay coil is stopped, ie when the driving transistor is turned off. Now let’s move on to the seven segment display. As you may have already guessed (and it should be quite apparent, given that I’m using a 3-digit segment), the decimal points in the display are not used. Thus that leaves us with seven LED segments (conventionally referenced as segments A through G) that needed to be driven. Additionally, to minimize the number of pins required to drive the seven segment display, the three digits are turned on one after the other. However, this is done so quickly that to our eyes, it seems that the three digits are always turned on. I have chosen to use a 167Hz refresh rate –PIC16F873(A) based automatic voltage stabilizer Syed Tahmid Mahbub9 meaning that the entire display is refreshed 167 times a second – once every 6 milliseconds. Each digit is turned on, kept on for 2 milliseconds and then turned off before the next display is turned on and so on. Since the microcontroller output drive current is limited and we want optimum brightness (and thus drive current) of the seven segment display, seven transistors were used in the common collector (also known as emitter follower) mode to drive the seven LED segments in the display. Additionally, three transistors were used to provide or disconnect the supply to the individual digits, as required for continuous subsequent switching between the digits. Upon start-up, the microcontroller enters “delay mode”. This is when, for a specified amount of time (that is pre-programmed), there is no output. There is a switch that is used to select between short delay (default mode, when switch is open) and long delay (when switch is

closed/pressed). These delay times are pre-programmed, and I have chosen to use 2 seconds for the short delay and 3 minutes for the long delay. These, as far as I know, are the standard times present in the voltage stabilizers available in the market. The delays are set by simple software loops that do nothing – such delay functions are provided in the mikroC PRO for PIC library. There are three LEDs in the circuit that are used to provide visual feedback, besides that already provided by the seven segment display. These LEDs are used to indicate: 1.

When the delay mode is on 2.

When the microcontroller is operating in low-cut or high-cut mode 3.

When the microcontroller is operating in normal modePIC16F873(A) based automatic voltage stabilizer Syed Tahmid Mahbub10PICTURES (because nothing is complete without pictures): (The circuits on verroboard are from the test stage. You can see the PCB at the end.)

PIC16F873(A) based automatic voltage stabilizer Syed Tahmid Mahbub11