Instructions for Use
– SCHMIDT
®
Flow Sensor SS 23.400 ATEX 3
Page 16
Switching outputs
The sensor is equipped with two current limited and short-circuit resistant
switching outputs with the following technical data:
Type:
Low side driver, open collector
Maximum switching voltage U
S,max
:
26.4 V
DC
Maximum switching current I
S,max
:
55 mA (typ. 50 mA)
Maximum off-state resistance R
Off
:
1.5
MΩ
12
Minimum load resistance R
L, min
:
Depending on switching voltage U
S
(see below)
Maximum load capacity C
L
:
Depending on switching current I
S
(see below)
Maximum cable length:
100 m (recommended)
Wiring:
The individual switching outputs can be used as follows:
Direct driving of a resistive or inductive load (e.g. LED or relays) with
a maximum current consumption of 55 mA.
Direct activation of digital inputs with integrated pull-up resistor R
L
(e.g.
PLC input).
Due to the internal measuring resistor, which is connected in parallel to
the transistor, the switching stage has a comparatively low off-state re-
sistance of 1.5 M
. This should be taken into account in case of a (high
resistance) pull-up resistor R
L
. For a digital evaluation, it is recommended
to choose a value of R
L
< 167 k
so as to achieve an active high level
(locked transistor) which is 10 % below switching voltage U
S
or higher.
Because of its open collector design, the switching voltage U
S
is independ-
ent of the operating voltage U
B
of the sensor. Thereby it does not behave
like an ideal switch (in particular in combination with the protective mech-
anism) but exhibits in conductive condition a drop voltage U
OC
with follow-
ing behaviour:
Well below the maximum current I
S,max
, the open circuit voltage U
OC
results from voltage drop via the emitter resistance R
E
plus saturation
voltage over the collector emitter path of the switching transistor:
𝑈
𝑂𝐶
≈ 47 Ω ∙ 𝐼
𝑆
+ 0.2 𝑉
12
Measuring resistor and switching transistor; additional leakage current of the TVS diode
connected in parallel (U
OC
≈ U
S,max
): < 100µA
