POWER SUPPLY UNIT DESIGN AND IMPLEMENTATION

784 457 Smart Raysam

 DESIGN AND IMPLEMENTATION OF 3.3V , 5V, 12V AND VARIABLE REGULATOR POWER SUPPLY UNIT

Most of our components in embedded system mostly deals with 12V, 3.3V, or 5V  and a current of an approximate value of 2amps. In most cases one might design a circuit in which components used would be having a required voltage of 5Volts and 12Volts at the same time and at times even more voltage of different values would be required for the required circuit. This is where one would introduce a power supply unit. This unit is found in lost if not all electronics work it simply Is a unit that bears the responsibility of providing any voltage value required by the main circuit.

Today we would be designing a PSU that would source us a voltage of 3.3V, 5V,  12V, and one of a variable voltage between  1 – 10volts.

The components required are listed below;

  • ·         Diodes
  • ·         Transformer
  • ·      Polarized and non-polarized capacitors of 100µohm and 1000µohm respectively
  • ·         Terminal block
  • ·         Resistors of 220Ω
  • ·         Variable resistor
  • ·         Lm7812
  • ·         Lm7805
  • ·         Lm317
  • ·         3.3v converter]
  • ·         LED

One must take note that the diode used is a silicon diode hence there is a voltage drop of 0.7V and in our rectification, in every circle just 2 diodes allow voltage through them hence the total voltage loss is 1.4V this must be put into consideration later on while selecting our transformer.

We must still take note that the higher a transformer rating, the higher the current rating. Each voltage converter such as the lm7812 and lm7805 requires a minimum amount of +2v to be added to the voltage expected at the output terminal. i.e a 5v converter (lm7805) requires a minimum input voltage of about 7v and a 12 converter requires a minimum of 14v the 2v or more would act as the voltage dropped inside the converter. Our lm317 on the other hand requires an input voltage within the range of 1.2 – 15volts. The formular used to calculate the corresponding DCVoltage of an ACVoltage is given as follows;  Vdc  = Vac sqrt2

Hence a 9V transformer gives as 12.7 volts and when we remove our expected voltage drop on the diode we would be left with 11.3V.

A 12V transformer gives 16.97V and removing the expected voltage drop on the diode we would be left with 15.57V.

An 18V gives 25.6V and removing the expected voltage drop on the diode we would be left with 24V.

Selecting A Transformer

Like explained earlier on our converters need an extra voltage of at least 2V so if we are building a 12V supply, a 9V rated transformer cant be used because it can supply just 11.3V and we need 14V supply hence we would be left with only our 12V transformer and others above.

But if we are building a 12V supply we cant use a 12V transformer because the maximum voltage it gets from voltage source is 220v and a minimum of about 180v so at minimum voltage supply of 180 we can calibrate a corresponding voltage at the secondary side of the transformer which would be the needed secondary value (x) equals the low supplied voltage from source (180) multiplied by expected voltage at secondary if voltage were to be the normal 220v rated (12v) divided  by normal Volts rated (220).

                X = 180 x 12 ∕220

Replacing x with 180 we would have our voltage to be 9.8volts if converted to dc using our normal

Vdc  = Vac sqrt2 would become 12.4V which is lower than the needed 14 volts to supply our lm7812 voltage converter. Hence a 12rated transformer should not be used to power our 12V PSU.

A 24V supply can be used so also can an 18v transformer be sued but the 18v would be preferred to the 24V and this is the reason.

Following the earlier given calculations the18V corresponding dc value is 25.4V and the 24V corresponding dc value is 33.9V.

Removing voltage drop at diode the 18V trans would supply 24V and the 24V trans supplies 32.5V.

 Now removing the 12V that would be supplied out from the 12 converter we would see that the total voltage dropped in the converter is 12V in the 18V trans and 20.5V in the 24V trans(assuming  the source volts is 220) if we assume that the current flow is 1amps hence P = VI now for the 18volts trans the power dissipated in form of heat on the converter is 12W in the 18V trans and 20.5W in the 24V trans .

With normal sense the heat dissipated in the 24V trans is far too high compared to that of the 18V trans hence why an 18V trans would be preferred over a 24V.

It should be noted that the lm317 has an input voltage between the range of 4V – 25V.

Rectification and filtration  can be achieved following the circuit diagram that would come up later but the regulation can be achieved in two Ways either connected in parallel or in series.

The short coming of this system is that if the LM7812 gets burnt, the rest of the regulators would be disconnected and also the LM7805 load would be on the LM7812 while that of the 3.3V convt would be on the LM7805.

 The power loss in form of heat  on the LM7812 is 12W

The power loss in form of heat  on the LM7805 is 7W

The power loss in form of heat  on the LM317 is 1.7W

The power loss in form of heat  on the 3.3V convt  is 16.8 – 3W

This voltage loss is actually the advantage the linear form has over the parallel form of connection.

 

In this system the regulators each take their source from the 24V each hence if anyone gets faulty it does not affect the other. But in this case,

The power loss in form of heat  on the LM7812 is 12W

The power loss in form of heat  on the LM7812 is 19W

The power loss in form of heat  on the LM7812 is 12W

The power loss in form of heat  on the LM7812 is 20.7W

The power loss in form of heat  on the LM7812 is 12W

The power loss in form of heat  on the 3.3V convt  is 22.8 – 9W

From this its shown that the power dissipated in form of heat is much more than that in the linear form but to counter this short coming, one can use heat sink and fan to reduce the heat.

The capacitor placed immediately after the regulator is a non – polarized capacitor.

The main power supply  circuit itself can be built following the circuit diagram below:

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