Using a Fixed Regulator as an Adjustable Regulator

Fixed regulators such as the L7805 are easy to find and generally quite cheap. Adjustable regulators (even the LM317T) can be more pricey, or harder to get a hold of.

It is possible to use a fixed regulator, such as the L7805, to produce more than 5V (but not less, unless you can invent a negative resistor!). All you need to do is pretend the ground pin is the adjust pin, and insert an appropriate resistor.

Unfortunately the datasheet tends not to give you the specific values of the internal resistors, and depending on the regulator, this is likely to change. However, determining this yourself is easy.

Upon experimentation, it's possible to take an L7805 (more specifically here, an ST L7805CV, serial CCDV2V6), and find the following relationship. A number of different resistors, all around the 100's ohm range, are put between the ground pin (pseudo-'adjust') and actual ground.

Vo = 5.064 + 0.0034 R

Thus, the following resistors give the following output voltages:

• 5V - 0 ohm (connect ground pin to ground, as per normal)
• 6V - 270 ohm (use a 500 ohm trimpot)
• 7.5V - 720 ohm (use a 1k ohm pot)
• 9V - 1200 ohm (use a 2k pot)
• 10V - 1500 ohm (use a 2k pot)
• 12V - 2040 ohm (use a 5k pot)
• 15V - 2900 ohm (use a 5k pot)
• 24V - 5600 ohm (use a 10k pot)

Note that by using a fixed value rectifier in this way, you're moving outside the territory of the device's specifications and datasheet. Thus you do so at your own risk, as the design of any given regulator may not take into account usage in this manner. It is possible you may cause damage to your regulator, and/or equipment connected to it. Please experiment in a controlled manner first, checking performance over a range of current loads, before considering this application in your circuit design. The author takes no responsibility for any damage caused.

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