Keithley 237

+
SECTION2
Operation
Source-Measure
Considerations
2.26 CURRENT CLAMP
The Model236/237 /238 has a current clamp
<kuMP)
cir-
cuit for Source
V
Measure
I.
The purpose of the
!CLAMP
is to
prevent current from exceeding the programmed current
compliance limit value
when
an external power source is
connected to the output.
An
example configuration showing the
!CLAMP
is pro-
vided
by
Figure
2-61.
The Model 236/237/238 is
pro-
grammed to source
+SV
with a current compliance of
lOrnA. The external circuit connected to the output places
a 25Vbattery
in
series with the
+SV
programmed output.
The result is a circuit that would normally provide cur-
rent well in excess of the programmed compliance limit
of lOrnA.
However, the
!CLAMP
will
enable at
lOrnA
and
limit current to that value. The
!cLAMP
protects the Source
Measure
Unit
and
external circuitry from excessive cur-
rent.
+
When programmed to Source I Measure
V,
the Model
236/237/238 does not incorporate a current clamp. Thus,
current that exceeds the programmed current source
value is possible
when
an
external power source
or
an en-
ergy storage device is applied to the output.
An
example
of this is shown in Figure
2-62.
The external current
source, formed
by
the
25V
battery
and
lk!l
resistor, in
se-
ries with the
output
of the Model236 /237/238
will
result
with a current far exceeding the programmed
lOrnA.
Also, voltage measurements
(VMETER)
by
the Model
236/237/238 may be erroneous.
Caution
must
be exercised when using Source I Measure
V to perform tests
on
circuits or energy storage devices
(such as batteries, capacitors
and
inductors) that apply
power to the
output
of the Model236 /237/238. The user
may need to provide protection against excessive current
and
voltage surges that could damage the Model
236/237/238
or
external circuitry. Figure 2-63
and
Figure 2-64 summarizes some basic techniques that can
be used to limit current
and
voltage.
2-99