2019-05-05 Using a 7805 For An Adjustable Regulator

from FB group Building Transistor Radios

Willie Barnett
When you understand how the 3 terminal regulators work, you can replace the 1N4148 and the almost impossible to get 1N34 with just a pair of resistors.

The 7805 or 78L05 regulator always has a fixed voltage of 5V between the O and G pins. So you turn this into a constant current source by putting a resistor, 1k, between O and G. Now you have a constant 5/1000 amp going to ground. There is a very small .05 mA coming out of the chip itself, which is only 1 percent of the 5 mA through the 1k resistor.

So if you want to increase the output voltage by 2V to 7V output, you put another resistor in series with the ground, just like the adjustable LM317. To add 2V, you calculate this resistance as 2V / .005A, which is 400 ohms. So use the closest value, 390 ohms. Your output will then be 7 volts. Or you can use a trimmer for adjusting to any voltage.

Another way would be to put a red LED in place of the resistor to ground. This should give about 1.8 to 2V drop. But the LED has a temperature coefficient that gives a lower voltage as the temperature increases. The resistor is more stable and so is the voltage.

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