Message 27294 in TECH.WW
From: G8MNY
Date: Wed, 05 Aug 26 08:28:00 Z
Newsgroups: TECH.WW
Subject: Adjusting 78xx/79xx Regulators
Message-ID: <1168_GB7CIP>
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Feb 05)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
There are 3 ways 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
slightly leaky zeners/diode to get the desired drop.
generally have the best stability!
RECTIFIER 16V === \_/ 0.5V === 0.1u
colour & 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.
BRIDGE 30V === Multi 10K === 0.1u
Thermal stability is 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.
NOTES
A/ 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. There are low drop out
types available (0.3V - 1V).
Volts
B/ The maximium input voltage must not exceed the regulator max (e.g. 40V)
othewise it may well go short circuit & put 40V on the Output!
C/ Many regulator types have Thermal shutdown & the current rating of 100mA,
0.5A or 1A may be limited by poor heatsinking in practice. The higher the head
room voltage the more power has to be dissipated.
D/ The metal tabs must be insulated as they are connected to the regulator
common on positive types, which is not ground! They are also the most negative
part of a regulator so for -ve types that means the -ve input!
E/ If there is a possibility that the O/P may be powered but the input not, a
diode must be put output to input across the regulator to protect it.
1N4001
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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