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Model 6259B 10V 50A supply, the full-load regulation
measured at the load would be about 120 millivolts as com¬
pared to the 1.2 millivolt regulation that could be measured
at the supply's output terminals. Thus even relatively short
load leads can cause a considerable degradation of the sup¬
ply's performance. The remote sensing connections shown
in Figure 3-3 improve the voltage regulation at the load by
monitoring the voltage there instead of at the supply's
output terminals. (The advantages of remote sensing
apply only during constant voltage operation.)
3-29
As can be seen in Figure 3-3, remote sensing
involves removing the +S and —S jumpers from the output
terminals, connecting the load leads normally, and using
a separate pair of wires to connect the +S and —S sensing
terminals to the load. The following paragraphs discuss
some precautions that should be observed when making
a remote sensing installation.
NOTE
The +S jumper is the one that links the +S
terminal to the + OUT terminal when the
supply's terminals are strapped for normal
operation as shown in Figure 3-2. The -S
jumper is the one that links the -S terminal
to the -OUT terminal.
3-30
The load leads should be of the heaviest practicable
wire gauge, at least heavy enough to limit the voltage drop
in each lead to 0.5 volt. This limitation is dictated by the
adverse effect that a greater load lead voltage drop has on
bias voltages within the supply when remote sensing is used.
Twisting the load leads may help to minimize noise pick-up.
While there are practical limitations on the distance that
can separate a power supply from its load when using remote
sensing, it isn't possible to define these limits precisely due
to a variety of factors that are unique to each particular
installation.
3-31
Since the sensing leads carry only a few milliamps,
the wires used for sensing can be much lighter than the
load leads (22 AWG is generally adequate), but they should
be a shielded, twisted pair to minimize the pickup of
external noise. Any noise picked up on the sensing leads
will appear at the supply's output. The shield should be
grounded at one end only and should not be used as one
of the sensing conductors. The sensing leads should be
connected as close to the load as possible.
3-32
The sensing leads are part of the supply's program¬
ming circuit, so they should be connected in such a way as
to make it unlikely that they might inadvertently become
open circuited. If the sensing leads were to open during
operation, the output voltage would tend to rise. Although
the increase would be limited by protective resistors R108
and R1 09, damage to the supply or to the load might occur
if the loss of sensing were accompanied by a load transient.
For this reason no switch, relay, or connector contacts
should be included in the remote sensing path.
-CAUTION--
When using remote voltage sensing, it is possible
to damage the supply by disconnecting a load
lead while the sensing lead is still connected and
the supply is energized. If a load lead becomes
disconnected, current flows through internal
protection resistors R108 and R109, the sensing
leads, and the load and may burn out the resistors.
Additional factors could compound the damage
caused by an opened load lead. If the output of
the supply is connected to an inductive load or a
battery *, or is connected in parallel with another
supply, then opening a load lead would allow
current from the external source to flow through
the sensing leads and damage the supply's input
circuits. If the crowbar fires, the damage could
even be greater.
For these reasons, if there is any risk of an opened
load circuit while remote sensing is used, 1/16-amp
fuses should be installed in both sensing leads.
Fuses in the sensing leads will not affect the
performance of the supply and should protect
against costly damage.
*
Remote sensing is not recommended when charging or
discharging a battery. See paragraphs 3 87 and 3-91.
3-33
Another factor to be considered when making a
remote sensing installation is the inductance of the long
load leads. Although dc and low frequency performance
are improved by remote sensing, the higher inductance of
longer leads does impair transient response and could affect
3-4
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