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You are here: Home / Mini Projects / Low-Dropout 5V, 12V Regulator Circuits using Transistors

Low-Dropout 5V, 12V Regulator Circuits using Transistors

Last Updated on July 2, 2020 by Swagatam 19 Comments

The transistorized low-dropout voltage regulator circuit ideas explained in the following article can be used for getting stabilized output voltages right from 3 V and above, such as 5 V, 8 V, 9 V, 12 V, etc with an extremely low dropout of 0.1 V.

For example, if you make the proposed 5 V LDO circuit, it will continue to produce an output of a constant 5 V even if the input supply is as low as 5.1 V

Better than the 78XX Regulators

For the standard 7805 regulator we find that they compulsorily need a minimum of 7 V to produce a precise 5 V output, and so on. Meaning the dropout level is 2 V which looks very high and undesirable for many applications.

The LDO concepts explained below can be considered better than the popular 78XX regulators like 7805, 7812 etc since they do not require the input supply to be 2 V higher than the intended output level, rather can work with outputs within 2% of the input.

In fact, for all linear regulators such as the 78XX or LM317, 338 etc the input supply must be 2 to 3 V higher than the interned stabilized output.

Designing 5 V Low-Dropout Regulator

5 V LDO circuit
NOTE: PLEASE ADD A 1K RESISTOR BETWEEN Q1 BASE AND Q2 COLLECTOR

The figure above shows a simple low-dropout 5 V stabilized voltage regulator design that will give you a proper 5 V stabilized even when the input supply has dropped to less than 5.2 V.

The working of the regulator is actually very simple, Q1 and Q2 form a simple high gain common-emitter power switch, which allows the voltage to pass from the input to the output with a low dropout.

Q3 in association with the zener diode and R2 work like a basic feedback network which regulates the output to the value equivalent to the zener diode value (approximately).

This also implies that by changing the zener voltage value, the output voltage could be changed accordingly, as desired. This is an added advantage of the design since it enables the user to customize even the non-standard output values which are not available from the fixed 78XX ICs

Designing a 12 V Low-Dropout Regulator

12 V LDO circuit
NOTE: PLEASE ADD A 1K RESISTOR BETWEEN Q1 BASE AND Q2 COLLECTOR

As explained in the previous section, merely changing the zener values change the output to the required stabilized level. In the above 12 v LDO circuit, we have replaced the zener diode with a 12 V zener diode to get a 12 V regulated output through inputs of 12.3 V to 20 V.

Current Specifications.

The current output from these LDO designs will depend on the value of R1, and the current handling capacity of Q1, Q2. The indicated value of R1 will allow a maximum of 200 mA, which can be increased to higher amps by appropriately lowering the value of R1.

To ensure optimal performance, make sure that Q1, and Q2 are specified with high hFE, at least 50. Also, along with Q1 transistor, Q2 also must be a power transistor, as it might also get a bit hot in the process.

Short Circuit Protection

One apparent drawback of the explained low drop circuits is the lack of short circuit protection, which is normally a standard built-in feature in most normal fixed regulators.

Nevertheless, the feature can be added by including a current limiting stage using Q4 and Rx as shown below:

NOTE: PLEASE ADD A 1K RESISTOR BETWEEN Q1 BASE AND Q2 COLLECTOR

When the current increases beyond the predetermined limit, the voltage drop across Rx becomes sufficiently high to turn ON Q4, which begins grounding the Q2 base. This causes Q1, Q2 conduction to become highly restricted, and the output voltage shuts down, until of course the current draw is restored to the normal level.

Low-Drop Transistor Regulator with Soft Start

This high gain voltage regulator using just a couple of transistors includes qualities better than those of the widely used multiple emitter-follower variants.

The circuit had been tried in a 30 watt stereo amplifier that strictly demanded a highly regulated supply and also an output voltage which could climb slowly and gradually through zero volts to maximum, whenever the circuit was initially powered up.

This soft-start plan (around 2 seconds) for the power amplifiers helped the 2000 uF output capacitors to charge without triggering too much collector current within the output transistors.

Normal regulator output impedance is 0.1 ohm. Output voltage is found by solving the equation by:

VO = VZ - VBE1.

The rise time of the output voltage is evaluated by calculating through the formula:

T = RB.C1(1 -Vz/V ).

A number of digital devices call for a preset switch on sequence for their power supplies. By establishing proper RB/C1 values, the rise time of the circuit's output could be fixed to deliver this sequence or delay interval




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About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!

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  1. Search Related Posts for Commenting

  2. Waggoner Kranmer says

    Hi, thanks for this schematics.
    I’m wondering, can I use this circuit to charge a battery safely or if the power supply turn off the reverse current provide by the battery will burn out the circuit?

    Reply
    • Swagatam says

      Hi, you can use this circuit to charge your battery safely, the reverse voltage from the battery will not affect the transistor since due to the absence of the base bias the BJT will be turned OFF which will not allow the reverse voltage to get though the transistor.

      Reply
  3. Mp says

    Hi mr swag..
    Thanks for reply and suggesting this circuit, so by replacing few componenets specialy R2 1K5 with pot 10K i can get variable output starting from 0.1volt..awesome..
    Thanks & keep inspiring.

    Reply
    • Swagatam says

      You are welcome Mp, yes as per the working specifications of the circuit, it should be able to do the job for you!

      Reply
  4. Christian says

    Hello and thank you for the diagram
    If I want to use the 12V LDO with 10A, I replace Q1 by a TIP36 and do I also have to change the value of R1 ? And for the Rx value, it is 0.6/10 = 0.06Ohm and 0.6×10= 6W, right?
    Could a 5W resistor do it?
    Thanks

    Reply
    • Swagatam says

      Yes the R1 value will depend on the value of the resistor which must be included at the Q1 base.
      Q1 base resistor can be calculated using the following formula:

      R = (Supply – 0.7) x hFe / 10
      Once this is found, using Ohms law we have to determine the current that may flow through this resistor, and then calculate R1 using again the same formula as above, but with the figure 10 replaced with the figure obtained from the Ohm’s law.

      The Rx value and wattage calculations are correct

      Reply
      • Christian says

        Thank you very much

        Reply
  5. Klaas Nel says

    I want to build the 12 V Low-Dropout Regulator for the CPE of the fibre network. The transformer rated 12V 1A. So I want a current of 1.2 to 1.5A but cannot test the amps or calculate the value of the resistor.

    The power is from my solar system and the volts vary between 12.3 and 14.

    Regards

    Klaas Nel

    Reply
    • Swagatam says

      You can try any of the circuits explained above for your application, all will provide you with the intended LDO results

      Reply
  6. Pratap says

    Hello sir, Can this circuit be powered & used with solar panel to charge mobile safely.
    Solar panel spec. is
    3.5 watts peak power Output.
    Power Current: 390 ma (Max)
    Power Voltage : 9V (max)
    Open Circuit Voltage : 10.2 V

    Thanks in advance.

    Reply
    • Swagatam says

      Hello Pratap, yes you can use the circuit for your solar application safely, but make sure to confirm the results with a multimeter and a dummy load before connecting a mobile.

      Reply
  7. Carlos Alfredo says

    Good Morning
    Grateful for the response and attention

    Reply
    • Swagatam says

      Thank you, appreciate your feedback!

      Reply
  8. Carlos Alfredo says

    good afternoon
    I’m looking for a circuit like that with discrete components, don’t you have a similar circuit with a mosfet?
    grateful for the attention

    Reply
    • Swagatam says

      Hi, You can try replacing Q1 with a MOSFET and check the results.

      Reply
  9. Jack says

    Please clarify: To me, a 1K resistor is 1000 ohms. A 1.5K is 1,500 ohms. What is the value notation of
    1K5 ohms, 4K7 ohms and similar notations for capacitors, diodes, etc. as used on some of your schematics. Thanks, Jack

    Reply
    • Swagatam says

      1K5 = 1.5 K, 4K7 = 4.7 K and so on.

      Reply
      • Jack Shubert says

        Rather embarrassing question for me, being an electronic tech. for many years, never run in to that before; something you made up? Thanks much for your reply. Jack

        Reply
        • Swagatam says

          No Problem, appreciate your feedback!

          Reply


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