Hello again,
A couple of days ago you guys helped me with out with a question regarding input protection. http://arduino.cc/forum/index.php/topic,58558.0.html
I have another question regarding a circuit utilising the Zener Diode Over-Votlage Protection.
The Question
I am using a Voltage Divider to measure the resistance of an external circuit (JP1) and I would like to add in Over-Voltage Protection, could you below circuit potentially work or am I completely off?
The plan is to use a +4V AnalogReference and have the Analog Input Voltage range between 0-4V.
Those two circuits do != the third one if that's the idea.
The 3rd circuit has no current limiting for the zener and no voltage divider, apart from that it's exactly the same
Do you still need the voltage divider? If so what's the expected max voltage because circuit #2 had a divider after the 4v so you will be reading a max of 1v.
I think this may be what you are trying todo? And maybe its way off. Sorry for the low quality.
J1 -- your jumper, im guessing the voltage to be read?
R1 -- Top resister of divider
R2 -- Lower resister in the divider
D1 -- Zener diode (5.1v) in this case, if your divider used reference of 0-5v would use a 5.6v zener
It is the same as your voltage divider circuit, (guessing that your 4v is coming out of the divider not in)(error in the schematic? your posted link uses a 12v into the divider), but is protected by adding a zener across the lower resister.
Sorry Guys, I haven't really explain myself very well. I am trying to put together an Arduino based Alarm System for my house. Currently I am only using non powered detectors which change their resistance based on their state (Sealed, Alarm and Tamper).
At the moment, I am measuring the detector resistance using a Voltage Divider to convert 5VDC into something lower.
In the illustration above, Vin is attached to the Arduino 5V, Vout is attached to an Arduino Analog Input, GND to GND and R1 would represent my detector. If you know the value of Vin, Vout and R2 then you can calculate the value of R1. Its probably a bit crude but it works perfectly.
I would now like to add in some powered detectors (PIR's, Glassbreak Detectors) which still change thier resistance based on their state but require power (12VDC). I am a little worried about the potential 12V the could be shorted to the Analog Input if the cable is damaged or cut, so i asked about Input Protection. http://arduino.cc/forum/index.php/topic,58558.0.html
What was recommended was the use of a 5.1V Zener Diode, however when approaching 5V, "current leaking through this diode causes a voltage drop across the inline resistor", so 5V would be read as ~4.6V. Setting the AnalogReference to 4VDC and using an input voltage of 4V would still allow me to utilise the full resolution of the ADC.
So the poorly written first question was meant to say...
How can i combine a Voltage Divider (with a 4VDC Vin and where R2 on the circuit above would represent JP1 on the circuit below, and JP1 would be connected to the detector) and a Zener Diode Protection Circuit?
I hope this post is easier to understand, thanks again guys.
Alex
