A voltage stabilizer is an electronic gadget found in most of the circuit board of home electronics. A voltage stabilizer can be found in computers radios phone chargers and also DVD players. This important gadget comprises of diodes, capacitors, transistors, resistors and a step-down transformer. The work of a voltage stabilizer is to provide a steady current to the electronic equipment for it to function efficiently. A simple model of a voltage stabilizer looks like the diagram below. Symbols have their usual meaning.
In the above diagram, a step-down transformer reduces 240V of alternating current to around 10V of alternating current at the point XY. The current from the point XY is fed into the stabilizer. The ten 10V RMS AC sine wave from the full wave rectifier passes via a 1Ω resistor to limit the inrush current then through a 1000µF capacitor for smoothening. At point C, the direct current is not stable since it contains an AC ripple voltage dependent upon the load current. A 220Ω resistor is in series with an 8.2 Zener diode to provide an approximate voltage reference of 8.2V at point B, the base of TPI31 power transistor. This voltage reference is quite fundamental to the operation of stabilized power supplies. For our case, it determines the output voltage supply. The transistor operates in emitter-follower mode and the output voltage is at point E which is the emitter part of the transistor. The voltage here is 0.7V less than the voltage at point B, the base of the transistor. An 8.2 Zener diode is supposed to produce these kinds of results from 7.5 V power input from the rectifier. For the record the VCE should be sufficient enough to maintain the operation of the transistor. It is also vital to keep voltage more than 3V above the power supply’s output voltage. It is crucial also to put some heat sink on the transistor because the entire load current passes via the transistor. A light emitting diode should also be installed is series with a 1kΩ resistor to show that the power supply is active all the time. for our case scenario we are using 150Ω resistor instead. The transistor dissipates around 375mW of heat. This circuitry can be integrated to make a more complex gadget of three terminal voltage regulators.
Materials and method
The materials required are; step-down transformer, diodes, 1000µF capacitor, (1Ω, 220Ω, 150Ω) transistors, LED diode and TIP31 transistor, 240V AC source and a circuit board. C.R.O
Connect all the components in a circuit board using the diagram below
Label your finished gadget accordingly.
Apply some low out-put voltage from mains transformer across XY
Ensure that all link connectors’ area correctly attached to the circuit board. (NB) The LED should go green if all is connected well.
Set the multimeter to read AC current, and measure the voltage across X and Y.
Calculate the value of the input sine wave by multiplying the result displayed by 2½; Vpeak = VRMS × 2½
Set the multimeter to read DC and measure voltage at points A, B, C, and E all with respect to the point D. Record all the VXY(RMS)=.V, VXY(PEAK) = VXY(RMS) × 2½ =..V VAD =.V, VBD=..V, VCD=..V, VED=V
Compare the measured voltage at point at the calculated peak value of input sine wave (peak value). Ideally all these should be the same.
Measure the voltage at the base collector and the emitter part of the transistor and the 1Ω resistor and record the results accordingly as; VBE=.V, VVE=.V, VCB=V, and VAC=V.
Switch of power from the mains and disconnect the collector link. Set the multimeter to DC mode, switch the power back on again and measure the collector current IC, by connecting the multimeter with the expose connector link pins. Flip off the power, re-connect the collector link and this time remove the base link connection. Turn on the power and measure the base current IB. Record the values as per the findings. From the result find the current gain given that,
Current gain = hfe = IC ÷ IB
Switch off the mains and disconnect the capacitor link then reconnect and use the oscilloscope to view the power at point C with respect to point D.
Set the oscilloscope to the DC mode and use the vertical scale of 2V per division and horizontal scale of 4 or 5 ms per division.
Record the wave form by recording by taking a photograph of the screen or a USB disc drive. Measure the peak voltage.
Without changing the oscilloscope connection switch the power off and reconnect the capacitor link to restore the normal power operation and turn the power back again.
Record the wave form and the peak voltage.
Results and Analysis
VXY (RMS) = 9.92 V VXY (PEAK) = VXY (RMS) x √2= 14.03 V VAD= 11.32 V VBD=.8.29.V VCD=11.63 V VED = 7.66 V
VBE= 0.632 V VCE=3.96 V VCB=3.3V VAC= 70.9 V
Collector Current = IC = 56.51 mA Base Current = IB = 185.05×10-3 mA
Transistor Current Gain = hFE = IC/IB = 56.51 ÷ (185.05×10-3) = 305
The input voltage is slightly lower because the alternating current is expressed as shown below.
V=V cosine ø which is the real part of the current derived from the Euler’s Theorem which states
eiø = cos ø + isine ø where i is the imaginary part.
Hence for the alternating current we have
Veiø = V cos ø + iVsine ø
Ignoring the imaginary part we have the real part as = V cos ø
The wave form without the capacitor is as shown below.
The peak voltage is 7.70V
The wave form after connecting back the capacitor link is as shown below
The peak voltage is 5.00V
Engineering ECT224 Manual (2012). Low Voltage Stabilizer Power Supply. Pg 56 - 59