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Before you can design an adjustable voltage regulator into your circuit, or do a redesign, you need to calculate the values for two resistors. This is not difficult in itself, but actually finding the right resistors may pose problems. Fortunately a trick is available to make it all much easier. With most adjustable voltage regulators like the LM317 and LM337, the input voltage has to be 1.2 to 1.25 volts above the desired output voltage. This is because the voltage at the ADJ (adjust) input is internally compared to a reference voltage with that value. The reference voltage always exists across R1.
Together with preset R2 it determines the current flowing through the ADJ pin, as follows: Vout = VREF [1+(R2/R1)]+I ADJ R2 If for the sake of convenience we ignore I ADJ, enter the reference voltage (1.2 V) and for R1 select a value of one thousand times that voltage (i.e., 1.2 k?) then the equation is simplified to: R2 = 1000 (Vout – 1.2) In practice, simply determine the voltage drop across R2 (output voltage minus reference voltage) and you get your resistance value directly in kilo-ohms. For example, for 5 V R2 becomes 5–1.2 = 3.8 k? which is easiest made by connecting 3.3k and 470R resistors in series. In the case of relatively low voltages, smaller resistor values are recommended. This is because sufficient current needs to flow to enable the voltage regulator to do its job. A simple solution is to choose, say, 120 ? for R1. R2 then becomes: R2 = 100 (Vout – 1.2)

In vehicles it is often required to have a powerful yet stabilized voltage that’s not affected in any way by fluctuations of the battery voltage. The circuit shown here does the job using discrete and inexpensive parts only. While its low cost is a definite advantage over just about any kind of regulator IC, on the downside we have a minimum voltage drop of 2 volts - in fact the output voltage can be set to any value between 1.8 V and about 10 V. Continuous loads up to 100 watts can be handled, while peak values of 140 W should not present problems.

The power stage consists of two parallel-connected 2N3055 transistors in TO-3 cases. Because of their high base current requirement, a driver transistor type BD241B is incorporated. The feedback voltage arrives at the inverting input of the regulator IC, a type 741 opamp. The level of the reference voltage at the inverting input is adjusted with potentiometer (or preset) P1. The circuit board, of which the layout is given here, accommodates all parts including the two 2N3055 power transistors. As a matter of course, they should be properly cooled.

Remember, the case of a 2N3055 is connected to the collector which is at battery-positive potential. If necessary the voltage regulator may be bypassed by an external switch connecting the battery + terminal with the output terminal. The switch, if used, should be capable of passing considerable currents - at relatively low output voltages (up to about 6 V) currents of up to 15 A (continuous) or 20 A (peak) may be expected. Although the output current is reduced to 10 A when the 10-V level is approached, it is better to be safe than sorry.

Resistors:

• R1,R2 = 0Ω1, 5W
• R3 = 1kΩ
• R4 = 100kΩ
• P1 = 10kΩ linear potentiometer

Capacitors:

• C1,C2,C3 = 100nF

Semiconductors:

• IC1 = 741CN
• T1 = BD241B
• T2,T3 = 2N3055 (TO-3 case)

Miscellaneous:

• K1,K2 = 2-way PCB terminal block, lead pitch 5mm
• S1 = switch, heavy-duty, 1 change-over contact (see text)