Papabravo said:I did not misread the original post. Zener diodes are very poor clamping devices because the knee of a particular device can very over a wide range. The solution I am proposing will scale any possible input within some range to a value which makes the A/D converter happy. I use this technique all the time in our industrial control products. I know it works -- trust me on this one.
For example if you have a 0-10V analog input and the A/D converter has a 3.0V reference, then a voltage follower with a Vcc of 15 Volts is used so the output will not be close to the positive rail. The follower feeds a divider with a ratio of 0.3. and the A/D converter is happy as a clam.
philba said:note also that zeners aren't terribly accurate. IIRC its around 10% which means it will clamp between 3.87 and 4.73
out of curriosity, why would you want to clamp at 4.3? can't you just do that in software - read the ADC and if the value is above the value for 4.3, simply set it to the value for 4.3.
I don't believe the ADC on the stamp is limited to 4.3...
Papabravo said:If you say you are only interested in the lower half of the range and the sensor you want is not available then you have to work with what you have.
It would need to be a Schottky diode, or the PIC pin would always conduct all the current. Even with a Schottky, it's somewhat uncertain what current will go through the external diode and which goes through the PIC's shunt diode.dknguyen said:Now that Oznog reminded me of the voltage clamping diodes the PIC uses, you could just connect a diode between the ADC input and Vdd. It should allow for higher current clamping than just the internal PIC diodes. THe clamping voltage is Vdd+Vforward_diode, so you have to make sure the diode forward voltage remains within PIC limits.
RedCarzRFaster said:Is a resistor a good way to lower voltage? It is to limit the voltage coming off a 0-5v car sensor and my current design is a stamp, digital potentiometer, and a to d converter to sense voltage, the stamp raises resistance as voltage raises past the "clamp" mark and then lowers resistance if voltage drops.
etc.....
The problem is my digital pot hasn't arrived yet but I am wondering if this is the correct way to go about it.
philba said:this seems like an awful lot of hassle for something pretty simple. the sensor will never output above 5V. thus, no damage will occur. use a voltage reference to reduce the range if you want so you can measure more accurately. You could use different references for the different ranges you want to measure.
I would also pay very close attention to noise which is pretty common in an automotive environment. this will reduce your accuracy. at some point the noise will make more accurate readings impossible.
A thorough understanding of electro magnetic coupling mechanisms, along with the requisite diganostic skills and sophisticated instrumentation.RedCarzRFaster said:What do you recommend for noise reduction?
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