Simple Designing 12V 5A Linear Power Supply

This is a 12V 5A linear power supply circuit. Also, how to find resistors onside. I like to use a series of 78xx regulator ICs. Because it is so easy with a few parts.

If we want 12V output. So, use 7812. But Maximum current at the output does not exceed 1A.

What can we do?

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Basic 12V positive regulator using 7812

First, look at work only one alone. In a circuit is a basic using 7812. It is so easy. When Vin comes into IC-7812. It will keep the fixed output voltage, 12V.

Then, I look at a datasheet Input voltage is 15V to 25V.

From my experiment, I use the lowest voltage level. It causes a low output current. But if using high voltage. It causes the output current to rise. But It is too hot.

So, I choose a medium voltage level is 20V.

Inside IC has many components. In general use, We do not need to understand those devices. We just use it by understanding its features and limitations.

Capacitor filter

Look at circuit again. How do both capacitors work?

Cin (input capacitor) — It is between Input pin and ground pin of IC. We need to use it when the distance between input and IC exceeds 5 cm.

Why?

It will temporarily store power from the input. The IC is able to pull energy to use immediately.

But…

Inputs and IC are close together. It is not necessary to use it. Because IC can get the energy immediately.

If necessary use it. we should choose size 0.33μF to 1μF. Most importantly choose a capacitor type that responds to high frequencies as well

Cout (output capacitor)—It improves transient response. Which does not affect the stability of the circuit

12V 1A power supply using 7812

Meet a nice 12V regulator. It is suitable for a mini load that uses current less than 0.8A. You can build it easily on a small PCB layout.

The PCB copper layout on 1.290 x 1.367 on real size printer.

The Components layout of 7812 regulator 12V

Keep reading: Small 5V regulator

5A Boost Current

Above we need 5A linear power supply. There are many ways to do it. Let me tell you this way. Using a transistor current booster is suitable. Because it is cheap and easy to build with normal components in a lot of local stores.

Look at in the circuit, many components have a different function as follows.

First, both transistors Q1 and Q2 act as increase current up to 5A.
Second, fuse resistors RS will protect the saturation of transistor Q1 and Q2.

If fuse resistors blow or have high resistances. Causing both transistors to not work.

The current that goes out to the load will have only the current of the IC regulator alone.

Third, resistor R1 sets a bias current to IC-7812. And set output current of Q1 not exceed its saturation current, too.

Then, meet the current.

Still, look at the circuit again.

The output current is a total Current Source, IT. Which it gets from mixing between IIC and IQ1.

IIC is the output current of IC-7812.
IQ1 is the output current of power transistor Q1.

Look back on the path of the electricity.

IIC gets the current from Vin through R1 into pin 1 to Pin 3 of 7812. IC1 keeps a fixed regulated voltage, 12V.
IQ1 comes from working together of both transistors Q1 and Q2.

See 3 steps in the current flows.

First, a base current of Q2 flows to its emitter. And, out of a base of Q1 through pin 1 of 7812 to ground as compete for a circuit.

The base bias current of Q2 also causes the base current of Q1 can flow.

So, resistance and voltage across emitter and collector of both Q1 and Q2 are low.

Second, there is a collector of both transistors. Because…

The output current of Q2 flows from Vin through an emitter to the collector. Via pin 1 of 7812 to complete cycle circuit to the ground.

Third, the output current of Q1 flows from Vin through a fuse resistor RS. With working of Q1, the current flow through pin 3 of IC1 to power to the load.

How to design 5A linear supply

We have details on the design of the components as shown above as follows.

Choosing transistors
1. Q1 Power PNP transistor on shape TO-3, should choose…
MJ29555—ICmax = 15A, CEvoltage = 60V, hFE = 20-70
2N3792—ICmax = 10A, CEvoltage = 80V, hFE = 50 min
BDX18—ICmax = 15A, CEvoltage = 60V, hFE = 60 min

2. Q2-PNP transistor on shape TO-66 should choose…
2N6049—ICmax = 10A, CEvoltage = 55V, hFE = 25 min
2N5956–ICmax = 6A, CEvoltage = 40V, hFE = 20 min
BDX78–ICmax = 8A, CEvoltage = 80V, hFE = 30 min

3. The output current is a total Current Source, called IT. Which it gets from mixing between IIC and IQ1.

IT = IIC + IQ1

4. RS- Fuse resistor is a ratio between 0.6V per IC of Q1(IQ1).

RS = 0.6V/IQ1

5. R1 always has resistance to more than 0Ω. But it must not exceed a ratio between (VbeQ1) 0.6V per bias current of 7812(IbiasIC1).

0 Ω ≤ R1 ≤ VbeQ1/ IbiasIC1
R1 ≤ VbeQ1/ IbiasIC1

Value on circuit

We know the value on the circuit below.

- Vout = 12V—voltage to load.
- IT = 5A—total output current to load.
- Vin = 20V—input voltage to this circuit
- IbiasIC1 = 12mA —the current to cause IC1 run

Transistor booster circuit design steps

Look at on the 12V 5A Linear Power supply circuit diagram.

There are component detail as follows.

1. I choose Q1 is 2N3792 and Q2 is 2N6049 respectively. Because they are so easy to buy and cheap.

2. The Q1 must power an output current up to 5A. Also, IQ1 is 5A.

So…

RS = 0.6V / IQ1
= 0.6V / 5A
= 1.2Ω

And the power of RS can find with…
PRS = IQ1² x RS

= 5A x 5A x 0.12Ω

= 3 watts

Thus, I choose resistor is 0.12Ω at 5W.

In normally, a regulator IC-7812 can power current to 1A max. But we set a bias current of 12mA. Or

IbiasIC1 = 12mA = 0.012A

We can find R1 with a formula.
R1 = VbeQ1/IbiasIC1
= 0.6V/0.012A
= 50Ω

Choosing a transformer

My friend ask me “Could you please tell me what the power of the transformers should be?”.
For example.

12V 1A Power supply. How many amperes should I use a transformer?

And, 12V 5A, transformer rate?

Normally, a transformer can supply a little more current than specified, such as

You need a 3A power supply. The minimum size of a transformer that should be used is a transformer of 3A. Because it can supply a minimum of approximately 3A. It may supply a maximum of 4A or 4.5A.

Of course, you may use a 4A transformer. It is good because the current can be power higher. But it will be bigger and expensive, is it worth it for your work?

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