How to build a 1.25-20Vdc / 4A voltage stabilizer?

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Although simple at first glance, the application stands out for its originality, a simple adjustment, limiting the maximum current debilitated in the load, in a wide range. The voltage stabilizer shown is a wide application of the integrated circuit LM317.

Maximum permissible values ​​of LM317 at 25 ° C

  • Maximum input voltage (unstabilized) = 40V;
  • The minimum allowed difference between the values ​​of the input and output voltages = 5V;
  • Maximum output current = 0.5A.

Electronic diagram of the voltage stabilizer

Please note that, for example, when we want to obtain a stabilized voltage of 5V at the output, the value of the input voltage (unstabilized) must be at least 10V. To extend the value of the stabilized current up to 4A, the power transistor, model 2N3055, was used.

Of course like this must be mounted on a radiator suitable which can dissipate a thermal energy of the order of a 50 - 60W.

In order to limit the amount of radiated electricity in the form of heat, it is recommended that the voltage applied at the input be higher than that at the output within the limits of 5-10V. This condition is not mandatory. The current flow can be limited from the value of 2A, operating the slider of the semi-adjustable R1.

The assembly can also be used without the power transistor Q2. In this variant, the output voltage is collected between the terminals TP7 (-) and terminal TP4 (+). Thus, only the LM317 integrated stabilizer is used. WARNING! In the latter situation, the maximum allowable current is 0.5A.

Voltage stabilizer settings

By operating the semi-adjustable R5, the output voltage (terminals TP6 and TP7) can be adjusted between 1.25V and 25V provided that a voltage of 30V is applied at the input. As mentioned above, the maximum possible output voltage is equal to the input voltage from which we subtract the value 5.

Regardless of the position of the semi-adjustable slider R1, the output current is limited to the value of max. 4A, after which the output voltage starts to decrease.

Technical data of the project

  • Supply voltage, at input, min. 10V, max. 25Vdc;
  • Stabilized output voltage, adjustable: 1,25 - 20Vdc;
  • Limiting the maximum current charged in the load, in the range 2 - 4A;
  • Option for 0.5A (with LM317 (H)) or 1,5A (with LM317T) maximum output current, if the final power transistor is not used, 2N3055.

List of required components (with equivalents)

R1 = potentiometer / semi-adjustable 10k;
R2 = 10k;
R3 = 220 Ohms;
R4 = 68k;
R5 = potentiometer / semi-adjustable 5k;
R6 = 1k;
D1 = BYW29-200;
D2 = 1N4148 (or 1N4007);
D3, D4 = 1N4007;
D5 = blue or green LED;
C1 = 100 uF / 35V;
C2, C3 = 10uF / 35V;
Q1 = BC557;
U1 = LM317H for 1.5A or LM317T for 1.5A;
2 x double terminal contacts (for TP1-TP2 and TP6-TP7);
1 x triple terminal contact (for TP3-TP4-TP5).


Conex Club Magazine - no.5 - 2006


    1. I have a 30V / 15A source that has 7 transistors in the TO3 capsule connected in parallel and mounted on a radiator as big as the back of the source. It is industrially manufactured. At 15A, the transistors get quite hot.

      1. At 450W consumption, the transformer must output at least 500-550 W. If the transistors heat up, it means that you force it to the maximum. At 15A what consumption do you have?

        1. By consumption I mean current. For example, I say about a consumer that at 14V it consumes 15A. I make this specification because you asked me what consumption I have at 15A, and I don't understand what you mean. Now everyone understands how they got used to it. I only forged it on 15A once when I was charging a car battery, but the output voltage was not 30V. I just remember it warmed up nicely.

    2. At a 2N3055, at currents over two amps, the BE voltage starts to increase and stabilization begins to be affected. That is why several are used in parallel to decrease the resistance of the equivalent series. And as my colleague said, in series with equalization resistors. In addition, being a linear source, it has a variable yield. At low voltages and high currents the regulating element is really "stressed"!

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