Variable Switching Power Supply

Engineeering Projects

Various type of power supply circuit from basic regulated power supply to advanced digital control power supply unit is already posted on bestengineeringprojects.com. Now here one more power supply circuit is posted and can be used to provide up to 300mA and voltage between 6V to 13V.

Circuit Description of Variable Switching Power Supply

The input AC is stepped down using a step-down transformer T1 to 18V AC which is further rectified by a bridge rectifier. The rectified output is filtered by 1000 capacitor C1. The Zener diode ZD1 is used to form the reference voltage and the output voltage is selected by potentiometer VR1.

Transistor T1 forms an emitter follower, and the voltage at the emitter of T1 is equal to the voltage at the varying point of VR1. Transistor T2 is used to sense the output voltage.

circuit diagram of variable switching power supply

Timer IC (IC1) is here configured as a monostable multivibrator (timer circuit) for the period of 0.2ms. The collector of PNP transistor T3  is connected to pin 2 of IC1  and is used to trigger. When the circuit is switched on, the voltage at T1  is equal to the voltage at the center point of VR1. If the voltage at the center point of VR1  is 8 volt, the emitter of T2  is also at 8 Volt and the base is at zero. When transistor T2  is in an off state no voltage is developed across R4  and transistor T3  continues in the off state. When transistor T2 conducts the voltage at the collector is developed and passes through resistor R4.

This turns transistor T3  on and voltage at pin 2 of IC1  becomes more than Vcc/2. This led output at pin 3 of IC1  goes high and transistor T5 starts conducting and goes into saturation, while capacitor C3 gets charged when the voltage across it reaches 5.6V. After 0.2 ms the output of IC1  goes low and the capacitor C3  discharges through the load.

When the voltage at pin 3 of IC1  decrease from 5.6V, transistor T2  goes off which further turn off transistor T3. This produces a negative trigger at pin 2 of IC1. Its output goes high and further turns on the transistor T4. This process is repeated continuously. The power loss is very low because transistor T4  only operates in saturation and cut-off mode.

When output load is increased, the capacitor C3  gets charged due to leakage current. To avoid this problem resistor R10  is used. To increase the current capacity of the circuit use a high power transistor instead of SK100 (T4) or add one SK100 to its parallel.

Here IC1  is used as a comparator and also provides some delay to output change. If the output ripple is high, decrease the value of C2  or omit it altogether.

BEP LAB NOTE: In BEP LAB we use AC128 instead of SK100 transistor and heat-sink to drive a stereo set.

Check out other power supply circuits posted on bestengineeringprojects.com

  1. Arduino Controlled Variable Power Supply
  2. Universal Digital Power Supply Circuit
  3. Stabilized Power Supply With Short-Circuit Indication
  4. Short Proof Variable Power Supply
  5. Adjustable Bipolar Voltage Regulator Circuit Using LM337
  6. Self Switching-off Power Supply
  7. Dual Polarity 5v from 9v Battery
  8. 3.7V to 5V, 5.3V and 6V Converter Circuit

PARTS LIST OF VARIABLE SWITCHING POWER SUPPLY

Resistor (all ¼-watt, ± 5% Carbon)

R1, R3  = 470 Ω

R2  = 330 Ω

R4,R5  = 100 KΩ

R6  = 82 KΩ

R7  = 1Ω, 1W

R8  = 4.7 KΩ

R9, R10  = 1 KΩ

VR1  = 10 KΩ Lin.

Capacitors

C1  = 1000 µF, 20V (Electrolytic Capacitor)

C2  = 2200 pF (Ceramic Disc)

C3  = 1000 µF, 15V (Electrolytic Capacitor)

C4  = 0.1 µF (Ceramic Disc)

Semiconductors

IC1  = NE555 (Timer IC)

T1, T2, T5  = BC148 (Silicon NPN Transistor)

T3  = BC558 (Silicon PNP Transistor)

T4  = SK100 (Silicon PNP Power Transistor)

ZD1  = 12.V zener diode

Bridge Rectifier

Miscellaneous

X1  = 230VAC primary to 18V AC, 500 mA secondary transformer

2 Thoughts to “Variable Switching Power Supply”

  1. roberto lorenzi
    May 8, 2022 at 7:19 pm

    Hi, I’m quite sure that Ic1 NE555 will die at power on:
    You wrote 18 volt transformer with a bridge rectifier with a 1000uF capacitor, it’s mean 18 – 0,7(bridge loss) * 1,41 =24,393V.
    That’s excedes the NE555 max supply voltage (18V), and I often observed that, upper 10V supply, NE555 n ot works well.
    Anyway, thank you for the ideas.
    R.Lorenzi

    Reply
    1. Engineeering Projects
      May 9, 2022 at 11:47 am

      Hi, Roberto,
      Let’s see the calculation of the bridge rectifier.
      Transformer output voltage Vs = 18V
      Vm = 1.414 x 18V = 25.45
      Vdc = (2xVm)/pi = 50.9/pi = 16.21
      i.e. The output voltage from bridge rectifier is around 16.2V
      And NE555 can withstand up to 18V max.

      Reply

Leave a Comment