When we experiment a AC power. We always use power transformer as source but it cannot adjust voltage though it have a adjustable form but them too expensive. This project so may a good way of us.
This is AC variable power supply circuit that apply output of 0-30V at 3A by use electronic components are bridge diode as main and control voltage by potentiometer easily. If you need AC Power supply is cheaper this project is good way.
Main idea from bridge circuit.
We can adjust AC voltage with feature of Balanced bridge circuit as Figure 1. When we adjust a resistance between bride diode will can control the output voltage.
From this principle can use resistance between junction of transistor instead variable resistor. Then another transistor to control resistor between junction of the transistor. That resistance such adjustable by controlling to bias to transistor.
The working principle.
In Figure 1 a bridge circuit have feature that if we connects the variable resistor between bridge.
This resistor will control voltage at output. If low resistance will cause output current flow and voltage a lot. But if higher resistance cause current lower to least. So we use this principle for our project.
On working of the diode in practice. Be seen in forward bias When diode conducts current to a voltage drop across 0.7volts. Cause the loss at voltage in sinewaveform is lower than 0.7 volts. Diode still does not work And will lose again The voltage was decreased down to less than 0.7 volts. Diode stops working before making a sine wave at output have cross over distortion as Figure 2.
In Figure 1 will take the BD1-bridge diode to connects with ACV 36 volts at input. And then have variable resistor connected in as load of bridge diode circuit. Which next this potentiometer will be replaced with 2 transistors as described above.
A resisistor-R3 acts as full circuit. D6 and C1 are rectifier and filter voltage to provide power to IC1. Which it function like voltage comparator of rectifier and reference voltage at between 0-30 volts. Comparison results applied to control resistance. Which when replaced by transistors, the output of comparator circuit is used to control the bias of the transistor.
The working principle.
Figure 3 is a complete circuit of adjustable AC power supply. The BD1 is bridge diode and transistors Q2, Q3 are connected in darlington compound to control flowing current.
In controlling we will take output signal to compare with reference voltage to control both Q2, Q3 next step.
The working of circuit begin with R9 will acts as an inside load. To cause full circuit. D1 is rectifier to convert AC to DC and C1 is will smooth or filter voltage to steady direct current. The DC-voltage will be high voltage of 50 volts. R5 and ZD1 are connected to control DCV from 50volts lower to about 33 volts. Then sent to Q4-transistor which acts as controller this DCV in steady up. C2 is use as high frequency filter to ground.
After that will be sent to IC3-LM7812-DC voltage regulator 12 volts and have C3 as high pass filter to ground. It will also have some voltage is feedback to Q4-transistor to improve stability of the circuit.
This DC 12volts will be entered to IC1 and Q1-transistor. It also to a voltage divider set AV1-AV3 to as reference voltage fot compare voltage at output of IC1/2 at comparator circuit IC1/1.
D2 is rectifier AC to DC by have C4 is filter DC voltage that lower ripple. Then it through the voltage divider (R7 and R8). Which they will divide voltage drop across a Meter and use as voltage to compare with voltage at IC1/2.
The circuit as voltage indicator include VR4 and moving coil meter-M1 is an amps meter size 100 uA. VR4 will acts control current through M1.
The DC voltage from R8 will be entered to pin 3 of IC1/2 by IC1/2 is buffer. R3 acts as coupling a signal that send to comparator circuit between the reference voltage and output voltage output to IC1/2.
The comparator have R2 serves as set gain ratio of IC, output signal of IC1/2 will through R1 to bias to transistor Q1 to control brightness of LED inside optocoupler IC2 no. H11D2
The brightness of the LED will control the resistance of the photo-transistor in IC2. The light cause resistance of photo-transistor lower. Which it is connected to both transistor Q2 and Q3. When photo-transistor with reduced resistance. Will cause current to flow through itself Bias to conduct Q2, Q2 acts as a driver for Q3.
So make Q3 conducts currents, which means that resistance connected between BD1 reduced. The output voltage will reduced and when brightness of LED reduced will cause resistance of photo-transistor increase up, cause Q2 get bias lower. As a result, Q3 is biased less. That is, at which point the resistance increases, the output voltage will be higher. Concluded that when the LED light will stream through the bridge diode is very valuable. Make the output voltage is high, but if a little brighter LED output voltage will drop.
The components list.
Resistors ¼ watt +-5%
R9—————–100 ohm 10 W
VR1—–Variable Resistors 50K
VR2—–Potentiometer (B) 2K
VR3—–Variable Resistors 4.7K
C1————220uF 100V Electrolytic
C2————0.33uF 50V Mylar
C3————0.01uF 50V Mylar
C4————10uF 63V Electrolytic
BD1– Bridge Rectifier___220V 6 Amp_______ = 1 pcs.
D1, D2, D3———1N4001 ___1A 50V Diode__ = 3 pcs.
IC1——————-TLC272 ______________ = 1 pcs.
IC2——————-H11D2 _______________= 1 pcs.
IC3——————-LM7812 __12V regulator IC___= 1 pcs.
LED1—————-LED as you need. ___ = 1 pcs.
Q1——————–BC550B ________ = 1 pcs.
Q2——————–BC141 _________ = 1 pcs.
Q3, Q4—————BD241C ________ = 2 pcs.
ZD1—-Zener Diode__33V 1W _______= 1 pcs.
F1——-Fuse 1 Amp_______________ = 1 pcs.
F2——-Fuse 2 Amp_______________ = 1 pcs.
M1——Meter 100 uA______________ = 1 pcs.
T1——-Transformer Pri: 220 volts ; Sec: 36 volts 3 Amp___1 pcs.
How to builds
We need to provides all components as list above. Then make PCB as Figure 4 an actual-size of Single-sided Copper PCB layout. And Component layout for the PCB.
Note: This project have a lot of parts and high current, You need to have enough electronic experience.
Q1, Q3 and IC3 need to use heatsink. And importance Q4-BD241C must mounted large heatsink size.
Testing and tuning
Check the accuracy of the circuit again to make sure, before you experimental to enter input voltage. If a short circuit will damage the machine. Adjust VR2 is used to adjust the voltage to the lowest position. To connects AC 36 volts from transformer to input. Then use the meter to measure the voltage of the output switch. Pressure must not exceed 0.5 volts. If too much, to hurry off. And check the equipment again.
When sure that everything is normal, adjust to next step,VR2 were adjusted to increase or decrease the output voltage switch, to adjust minimum of 0.5 volts and a maximum of 30 volts.
Next, adjust the meter is used to switch the output voltage can be read correctly. To change the scale of the meter before 100 micro amps to 30 volts. Then start adjusting the meter by adjusting VR4 to the highest position, set the multimeter to measure AC voltage at the output.
Then turn it on, adjust VR1 to the minimum resistance, adjust VR1 to the top position. VR3 adjusts the output meter reads 30 volts without having to do the voltage shown on the meter of the machine.
Then adjust VR1 VR2 to adjust the minimum position potentiometer output voltage is 0 volts.
Then adjust VR2 to the top position again. The voltage reading is 30 volts. Then adjust VR4 for the meter reading on the meter is connected directly to the output. shall be available
We can apply the work to be associated with voltage switch it. Which may be used in place of a transformer Or used as a power source in the experimental circuit on AC power.
Caution is not the output short circuit. This will result in very high current flows through transistor transistor Q3 will make this waste.
When not in use, turn off the machine. Because within the 100 ohm load at all times to cycle to work. Therefore, the consumption of electric power to the load in vain.
We may be used to connect an external diode bridge another time. To change AC voltage to DC power. Gives us the power source for both AC and DC power to operate simultaneously.
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