Instructions for Use
– SCHMIDT
®
Flow Sensor SS 20.500
Page 23
In general, AGND should be selected as measuring refer-
ence potential for signal outputs.
Figure 4-2
Depending on the value of the load resistance R
L
the electronics
switches automatically between its operation as voltage interface
(mode: U) or current interface (mode: I), hence,
the designation „Auto-
U/I“. The switching threshold is within the range of 500 to 550
(for
details refer to chapter 5
Signalling
). However, a low resistance value
in voltage mode may cause significant voltage losses via line re-
sistances R
W,S
, which can lead to measurement errors.
For voltage mode it is recommended to use a measuring
resistance of at least 10 k
.
The maximum load capacity is 10 nF.
Short-circuit mode
In case of a short circuit against the positive rail of the operating DC
voltage (+U
B
) resp. against both rails during the positive AC half-cycle
the signal output is switched off. Due to internal measuring re-
sistances it is possible that a current of max. 15 mA flows into the
sensor output (referred to AGND).
In case of a short-circuit against the negative rail (GND) at DC supply
or against both rails during a negative AC half-cycle the output
switches to current mode (R
L
is calculated for 0
) and provides the
required signal current.
If the signal output is connected to +U
B
via a resistance or to one of
the rails for AC voltage, the R
L
value is calculated incorrectly which
leads to false measurement values or cyclic switching of the signal
modes with the frequency of the AC voltage.
!
V
w
N
R
L
T
M
AGND
GND
/ U~
+
-
+U / U~
B
A
A
R
L
V
R
W,B
R
W,S
R
W,S
R
W,B
C
L
!
