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G8MNY > TECH 21.07.03 00:28l 90 Lines 3944 Bytes #999 (0) @ WW
BID : 6473_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: Adjusting 78xx/79xx Regulators
Path: DB0FHN<DB0RGB<DB0MRW<OK0PPL<RZ6HXA<IK1ZNW<GB7CRV<GB7CIP
Sent: 030720/2028Z @:GB7CIP.#32.GBR.EU #:6473 [Caterham] $:6473_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
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 from the zener to the output could be
needed with sole leaky zeners/diode to get the desired drop.
in _______ out
+12V ------------------!_______!---------------- +8V
RAW DC ! 7805 !- - - R - -!
FROM >1000uF --- _!_ ---
BRIDGE 16V === /_\' --- 0.1u
RECTIFIER ! ! 3v !
0V -----------------------------------------------
Thermal stability fair, as it is dependent on Reg + Zener, 3 - 5V zeners
generally have the best stability!
in _______ out
+12V ------------------!_______!---------------- +5.5V
RAW DC ! 7805 _!_ !
FROM >1000uF --- \_/ ---
BRIDGE 16V === --- 0.5 --- 0.1u
RECTIFIER ! ! !
0V -----------------------------------------------
If the diode is an LED, the voltage increase could be 1 - 1.5V depending on
the 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.
in _______ out
+24V ------------------!_______!---------------- +15.5V BOOST
RAW DC ! 7805 _!_ ! +15.0V NORMAL
FROM >1000uF --- /_\' ---
BRIDGE 30V === _____! 10V --- 0.1u
RECTIFIER ! BOOST O _!_ !
! \o \_/ !
! ! --- 0.5V !
! ! ! !
0V -----------------------------------------------
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.
in _______ out
+22V ------------------!_______!----------------- +13.80V
RAW DC ! 7812 ! !
FROM >1000uF --- ! ---
BRIDGE 30V === Multi 10K --- 0.1u
RECTIFIER ! Turn ! !
0V -----------------------------------------------
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.
in _______ out
+24V ------------------!_______!---------------- +5V to +20V
RAW DC ! 7805 ! ! !
FROM >1000uF --- ! 330 ---
BRIDGE 30V === ! ! --- 0.1u
RECTIFIER ! ! 1K !
! ---->POT !
! ! !
! ! !
0V -----------------------------------------------
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.
/QSL
73 De John, G8MNY @ GB7CIP
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