Measuring a voltage of max 2,8v

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TrialAndError

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Hi,

I want to use the Teensy 3.1 module to measure a voltage of maximum 2,8 volts, but want to make sure that the Inputs are secured against overvoltage.
I have to Monitor the voltage output of a device that Returns distance Information as voltage. The voltage range that should be monitored is 0-2,8v.
A voltage over 2,8v should not usually occur, but the readings could be as high as 10v if something goes wrong.
All values above 2,8v can be read as 2,8v or any other voltage, but the voltage range up to 2,8v should be quite accurate (so internal reference voltage should be used).

Reading other forums I got Information on atmel uCs and something that I could come up with was the a 3:2 voltage divider used in conjunction with the internal 2,048 voltage reference clamped with a zener Diode and a series resistor to limit the current.
However I have no clue whether that would be a good solution for the teensy 3.1 module or what internal protection is present in the device.
Beside that I'm no electronics Genius, but a programmer, so I'm not quite sure of the schematic that would achieve that...

i would really appreciate if someone could Point me in the right direction...
 
In that case, its also possible that when your input is around say 100mV, then the actual input at the analog pin would be much much less. If the purpose of your project is to measure the voltage only, then that would really decrease your accuracy.
The best solution would be to have 3V zener diode diretly in parallel with the ADC pin to the ground. So if the voltage increases above 3V, zener diode will directly short that pin to ground. Otherwise it would simply act as open circuit.
 
Be careful -- while the MCU's pin can handle 10 mA, it just acts as a diode to the MCU's VDD. If the MCU doesn't consume 10 mA (e.g. is in reset, or a sleep mode), then the supply voltage will rise until something else limits it (the USB pins --5 V which is too high).

Best to use a zener also, but they (especially the low voltage ones) can leak at applied voltages lower than the clamping voltage.

Instead, while using the 10k, connect a 3.6 (or 3.3) V zener from the MCU's VDD to GND -- that will clamp the 10 mA safely, and you won't care about the leakage.


boarder2k7 said:
You should just be able to use a 1k resistor straight to the pin. Sensing voltage doesn't draw enough voltage to cause a dip across the resistor, and 10v at 1kohm is 10mA which I believe is within the current sink capabilities of the analog pins on the 3.1

Sourced this from Paul's response to my question here http://forum.pjrc.com/threads/24380-Reading-Automotive-Voltages?p=36889&viewfull=1#post36889 I think it should apply to you as well.

Good luck!
-B
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