[SOLVED] DC Volt measurement using microcontroller Issue

Hi All,

I am planning to measure 70V DC using microcontroller but after 30 I am getting >1% output tolerance, Please help me to solve this problem.

Used resistors all are 0.1% tolerance.

Circuit is given below

The zenner diode is certainly starting conduction, and its reverse current is affecting the linearity of the resistive divider. As an initial test, just remove the diode and measure again. By the way, I presume this diode isn't too necessary, due most microcontrollers have built in antiparallel diodes from input to both GND and VCC, ensuring the ultimate protection.

If you don't fully trust the internal protection of your micro-controller, you may use two diodes in series, as illustrated here.



The trick is to use low-leakage diode(s), one I have utilized is that SMD dual diode in the schematics

ted2 said:

If you don't fully trust the internal protection of your micro-controller, you may use two diodes in series, as illustrated here

Click to expand...

It is always useful to provide additional protection to the main device, but on this case in particular, any incremental voltage on 70v external terminals would produce a current limited to something about 2.1mA accross uC input, but on PIC family the inner clamp diodes supports forward currents up to ±20mA.

andre_teprom said:

It is always useful to provide additional protection to the main device, but on this case in particular, any incremental voltage on 70v external terminals would produce a current limited to something about 2.1mA accross uC input, but on PIC family the inner clamp diodes supports forward currents up to ±20mA.

Click to expand...

Hi,

I am going to use LPC1768 microcontroller

My readings are attached below with zener regulator and without zener diode. My question is if any spark or vollt above 90v means ? how can i Protect my controller. my ADC is pin will support upto 3.3V only

With Zener diode readings



Without zener diode readings



Also I have one more doubt. Zener has to conduct after it's avalanche region, In this case 3.3V my it's making trouble below 3.3V. Please explain me thanks in advance.

Zener has to conduct after it's avalanche region

Click to expand...

If you see the merit figure of the zenner diode, can note that bellow its avalanche, there is a very small current flowing which means a kind of 'resistive' behaviour on this region.

My question is if any spark or vollt above 90v means ?

Click to expand...

Concerning to the protection scheme, this mainly depends on the class of protection you want to achieve. For just 90v and nothing more, at firs sight, even the inner diode could suffice.

My readings are attached below with zener regulator and without zener diode.

Click to expand...

The 2nd table did not compute the "difference" value, as you did in 1st table.
Would be great to have both tables fulfilled, just to evince the impact of the zenner diode there.

The zener voltage is always rated at a specific current, usually 10 mA.

But it will start conducting and drawing small amounts of current far below that voltage. That is called the zener "knee".

It is better if you employ the double diode solution that Ted2 showed in post #3