Adjusting 78xx/79xx Regulators
By G8MNY
There are 3 way to simply increase the regulator voltage.
(1) Add a series zener or diodes or a mixture in the common leg.
Most makes of regulator put about 50uA down the common leg, this may affect the zener/diode drop. A pull up R of a few K ohms from the zener to the output could be needed with sole leaky zeners/diode to get the desired drop.
Thermal stability fair, as it is dependent on Reg + Zener, 3 - 5V zeners generally have the best stability!
If the diode is an LED, the voltage increase could be 1 - 1.5V depending on the colour and current fed through it.
Thermal stability poor, as it is dependent on Reg + diode, the diode drop will alter quite a lot hot to cold.
Thermal stability poor as it is dependent on Reg + Zener + diodes
(2) Add a series R in the common leg. Most makes of regulator put about 50uA constant current down the common leg, so a small R can uplift the voltage.
Thermal stability good, as it is dependent on Reg + the constant current
through the R, but that will be regulator make dependent.
(3) Add a voltage offset ratio arm. The 330 & 1K pot give a zero to 4x max ratio arm, that magnifies the regulator voltage by up to 4 times, plus any voltage gained by adding resistance in series with the common leg.
Thermal stability good, as it is dependent on Reg/multiplier ratio.
N.B. For all 3 types the input must always exceed the O/P by at least 2V so that the regulator's drop out voltage is always exceeded. Also the max input voltage must not exceed the regulator max (eg 40V) The tabs must be insulated as they are connected to the regulator common, which is not ground!
/QSL
73 De John, G8MNY @ GB7CIP
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