VorTek SonoPro S36 Series, Инструкция по эксплуатации, Страница: 37 / 107

Series S36 Instruction Manual Chapter 2 Installation
800120 2-25
Frequency Output Connections
The frequency output is used for a remote counter of the flow rate. It can be
scaled to output a 1 to 10 kHz signal proportional to mass or volume flow,
temperature, or density.
The frequency output is an optocoupler (phototransistor) that implements a
normally-open switch. It is galvanically isolated from the meter electronics
and power supply. The frequency output requires a power supply and pullup
resistor (current-limiting resistor). Be mindful of the polarity; it is important.
The optocoupler is safe up to 40V and 40mA. However, it is recommended
to choose a pullup resistor value so that the current is approximately 10mA.
The choice of supply voltage and pullup resistor affect achievable output fre-
quency and signal dynamics. If the resistor is too large (current is too small)
then the optocoupler cannot switch fast enough and the maximum frequency
is reduced. (In extreme cases, the achievable frequency can be less than
1kHz). If the resistor is too small (current is too high) then the optocoupler
becomes deeply saturated and the logic low level is raised significantly above
zero volts. (Example: Using a 24V supply and a 600 ohm resistor results in a
switch current of 40mA. With this setup, the low level was measured to be
about 8.3V, not 0V as desired). Another concern with small resistors (high
current) is the power rating of the resistor.
There is a sweet spot compromise between these two extremes: 10mA.
Switching at 10kHz is achievable while keeping logic low levels reasonably
close to 0V. To determine what resistor value is needed, divide the supply
voltage by 10mA (R = V/I).
Here are recommended values for common voltages:
•
5V supply → use 500 ohms (499 or 470 ohms is okay)
•
12V supply → use 1.2kohms (1kohms is okay)
•
24V supply → use 2.4kohms
It is up to the user to verify correct operation with their setup.
Freq. Out -
Freq. Out +
Freq. Out voltage = +V
Select resistor so that current
through Freq . Out <= 40 mA
Figure 2-33. Isolated Frequency Output Using External Power Supply