hi all
I am using a CR1000+SDM-IO16 to measure the frequency-output of some TFC cup-anemometers. All anemometer signals are connected to the SDM-IO16 and everything works fine.
The amplitude of the sensor signal is equal to sensor supply(3V-42V) until a maximum of 15V.
Usually the supply is a 12V battery with logger, modem and the group of sensors each protected by a fuse. So the amplitude of the anemometer signals would be 12V.
Regarding input protection the manual of the SDM-IO16 says :
"To limit power dissipation and damage at higher voltages than 12V, an external series current limiting resistor is recommended."
As i understand, this mainly is of concern if using switches etc.
Nonetheless i am still not sure about the minimum/maximum voltage, that can be applied to an input channel without risking any damage.
What if using a regulated 15V supply, which brings signal amplitude to 15V?
When to use a a voltage divider/poti to bring a frequency signal level "down"?
Are there any differences regarding the pulse/control ports of the CR1000?
When to worry about the analog channels of the CR1000 (example : 7V applied to SE1/mV5000 vs SE1/mV250)?
Thanks.
artmann
* Last updated by: artmann on 3/24/2009 @ 4:54 PM *
I believe the problem is that the protection diodes will conduct if the input voltage exceeds the supply. When they do this, they add current to Vcc, and can push Vcc above its' nominal 12V, and disturb A/D readings, etc.
Adding a series resistor will keep the input current below the level where it will effect Vcc, while still allowing the digital inputs to read properly.
In the case of the IO16 the risk is that a sensor able to source high currents with cause excess power dissipation in the protection components of the IO16 risking damage and also wasting system power. The protection on the IO16 does not limit the current to a risk free level because the each channel can also function as an output able to source current so the design is a bit of a compromise.
I believe it says in the manual you can connect 12 V directly to the input although this is not recommended for long periods for the reasons mentioned about. You can connect even higher voltages if the sensor has some internal current limiting resistance (as anemometers will). Alternatively just add a cheap 10k series resistor to be on the safe side.
I would also recommend a similar series resistor when connecting to the control ports of the logger as they too do not have high value protection resistors as they also have an output capability.
There used to be some issues will the pulse count channels too, but they can now take the higher voltages with less risk, although an extra resistor will do no harm.
The analog channels have a different issue. With the CR800/1000/3000 you should avoid connecting >+/-8 V to the inputs for prolonged periods as the excess voltage can cause leakage in the front side multiplexer chips, which can lead to corruption of the analog measurments on other channels.
thx a lot.
greets artmann