In this project, we have designed a power supply to convert standard UK mains AC supply to a 12V DC supply.
The input voltage is 240V AC, which corresponds to a RMS value to 325V.
Emitter Resistance concept of Transistor Model
The transistor is used to stabilise the DC output voltage. This stabilisation is achieved by the use of an Emitter Resistance which provides the required amount of automatic biasing needed for a common emitter amplifier. This is the most used transistor configuration, which is also called common emitter transistor.
Here, a large amount of current flows into the base of the transistor. The voltage divider circuit thus formed, is responsible for the stability. The emitter resistance provides more stability. This addition helps transistor to control the base bias using negative feedback. The DC negative feedback provides stable biasing. The AC negative feedback signal transconductance and voltage gain specifications. Sometimes, emitter bypass capacitor is also attached in the circuit, in parallel with emitter resistance, to break the frequency response of the circuit at a designated cut-off frequency.
Here, the Emitter is not grounded, but kept at a small potential to provide the stability.
VB = VE + VBE
Emitter Resistance Current is given by,
IE = (VB – VBE)/RE
By following this design concept, we make sure that the desired value to current is not exceeded in the load resistance in our circuit.
Circuit Design and Simulation
We have designed the desired circuit by using the components specified and following the emitter resistance concept. A small resistance of 10 Ohm has been added after the AC supply to protect our circuit from surge and our transformer.
We see after simulations that diode breakdown voltages are never reached in the circuit, so diodes are protected.
As we need a voltage output of about 12V DC, we use a 20:1 step down transformer, as can been seen by the inductor value of the transformer windings. After four diode drops, its gives a voltage of about 13.5V DC.
The simulations under full load and no load conditions are depicted below with screenshots of the simulation on LTSpice.
Fig. Circuit designed on LTSpice
Calculation for the type of transformer
325V has to be brought down to 12V, with 4 diodes connected in the circuit, each with a drop of 0.7V. Keep a decent margin of about 1.5V above 12V required:
325/20 – 4 x 0.7 = 13.34V
Calculation for Emitter Resistance Concept on Full Load
RE = 500 Ohms
VBE = 1V
VB = 13.45V
Thus, IE = 0.025A
On no load condition, the current would be about 4A, or just a little less.
To be 4A current exact, the load should be 3.3625 Ohms.
The ripple (peak to peak) under maximum load = I/ (2fC) = 0.2V (as current decreases to a great amount in full load).
Simulation on full load:
From above simulation, we see that the design requirements have been met.
By this simulation, we see that about 1V difference occurs between full load and no load conditions.
The voltage surge is efficiently prevented by the capacitance.
Real Components for the Circuit
As we have incorporated the values of all the components to be exact as the standard components available, the real components are same as the components being used in the circuit. The list of components are:
1. AC Supply Socket
Available at the lab.
2. Resistors: 10 Ohm, 10k Ohm, 5 Ohm
They are standard value, can be bought from eBay at $0.5 each. Total cost: $1.5
3. Transformer (20:1)
Can be bought online from about $35 from Amazon.com. The rating of voltage and current are as per our requirements.
4. 4 diode (1N4148)
Can be bought from (https://www.mouser.in/ProductDetail/Fairchild-Semiconductor/1N4148/?qs=i4Fj9T%2FoRm8RMUhj5DeFQg%3D%3D) at $0.10 each. Total part cost: $0.50
5. Zener diode Can be bought from (https://www.mouser.in/ProductDetail/Micro-Commercial-Components-MCC/SMBJ5338B-TP/?qs=sGAEpiMZZMtQ8nqTKtFS%2fKiApRWFN7sOvQgw3Qc04zk%3d) at $0.80
6. Capacitors: 0.1uF and 5000uF
Can be bought from eBay at $0.50 each. Total part cost: $1
7. NPN transistor
Can be bought from (https://www.ebay.com/itm/100Pcs-2N3904-TO-92-NPN-General-Purpose-Transistor-/140846122431) at $1.56.
8. Heat sink
It is inbuilt for the transformer. For transistor, can be bought from (https://www.mouser.in/ProductDetail/Aavid-Thermalloy/577404B00000G/?qs=sGAEpiMZZMttgyDkZ5WiuqcEpFN0QqoQKaPGubZmWpo%3d) at $0.82.
Thus, the total cost of the project implementation is: $41.18 (approx.)
Power Dissipation in the transformer = 65W (maximum value)
Power Dissipation in the transistor = 48W (maximum value)
Thus, by using a proper heat sink, the temperature can be kept in control. The details about heat sink to be used has been attached with the list of the real components to be used. The heat sink used with the transformer is inbuilt, whereas those for the transistor should be externally mounted.
Result and Conclusion
The power supply has been designed successfully in LTSpice, which meet all our requirements, as seen from the simulations. The costing of the components used have been mentioned.
The emitter resistance concept to stabilise the output has been implemented successfully. It gives a negative feedback to provide a stable current.
An additional resistance has been added just after the voltage supply, before the transformer to prevent transformer from damage. This resistance have no negative effect on the circuit further.
The real components have been identified, and as the components implemented in the circuit are available as it is in the market, no circuit changes has to be done according to it in the simulation software LTSpice.
The price of real components have been mentioned (approximated value), and a total cost of the project if implemented on hardware, has been generated.
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