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I have a small pile of LM7815 regulators. Can I take advantage of the .6V silicon voltage drop and add a few diodes between the ground pin and actual ground to raise the output voltage? I need to drop 24v from two sealed lead-acid batteries down to about 18 volts or so; regulation isn't critical and load is about 300mA.
Yes you can using diodes or a voltage divider from the output to ground the same way a LM317 regulator is configured.add a few diodes between the ground pin and actual ground to raise the output voltage
I have raised 78xx regulator's voltage many times with just a single resistor inserted in its ground lead. You will have to do this experimentally though as each chip will want a slightly different value. For a three volt jump, 470 ohms would be a good starting point. However, I am not sure how this method as compared to Steve's suggestion would affect CRITICAL regulation more. But it sounds like you don't need laboratory precision here.
Steve's suggestion is the right way. Connecting a resistor from out to 'gnd' pin swamps out the variation in Iq that occurs between units and with varying loads. 10 to 20 mA is recomended, so 1K from out to 'gnd' and 200 ohm from there to gircuit gnd would give 18V out.
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