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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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