Dimension of voltage divider

pom · Joined: 25 Nov 2004 Posts: 30 Location: Germany

Hello guys,

I work with a PIC12F683 and try to measure a voltage. It's very important that the circuit needs low energy.

My problem: For the voltage divider I chose 220 ohm and 390 ohm. Unfortunately I cannot remember how to calculate the current with this constellation, but connecting the circuit to power supply there is a current of about 8 mA although none of the ICs is working.

So I think I have to re-dimension the voltage divider. This is very hard, because it is build of SMD 0805 and not that easy to solder. So I would like to ask you, which dimension is better for the divider? I have heard that it can get to big so that the PIC cannot measure any longer. What would you try?

Thank you for your help, Martina

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

For the extremely lazy (like me...)

http://ourworld.compuserve.com/homepages/Bill_Bowden/r2.htm

John

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

If you really want to have a very low current wasted (e.g. in battery powered applications) I suggest much higher R values at first. (be sure to count with the input impedance of the A/D converter, if you want to measure a slow moving signal (like battery voltage) you can use very high R values and add a capacitor to make it low impedance.

As a second option I suggest to connect the low side of the divider to one of the PIC's port and drive it low only during the measurement (some time before it of course), and leave it floating (driven high by the measured voltage) all other time. (some calibration is required because the PIC' pin will not drive the low side to absolute zero voltage)

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

I love AN521. I have never used that technique in a salable product because I think other engineers doing a design review would have me committed to a rubber room...but I have used it in temporary fixtures.

For reasonable values the clamp diodes on the chip will provide protection. But on this BBS you can't be sure all the potential readers will the reasonable. Mr. pom was starting out with 220 & 390 ohm divider resistors which indicates to me that he is a novice. If he is using those resistors from a nominal 12V battery supply, at high battery voltage he could be dumping nearly 20mA into the clamp diode, which is the datasheet redline. I think that is just too close to trouble to be left floating about the internet for the unwary to read and copy.
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pom · Joined: 25 Nov 2004 Posts: 30 Location: Germany

Hello,

thanks for help. I just tried to use a divider of 390k and 220k in a temporary circuit. It was powered by a battery an seemed to work well. Unfortunately I couldn't change the voltage level and had to re-program the PIC several times to try how it reacts if battery voltage is above or below a fixed level.

So, it seems to be ok, but the battery powers the circuit nearly constant. What about an application with changing voltage level? In real I would like to measure the voltage level of a battery and of a solar cell. I guess it the solar cell changes voltage much quicker than a battery.

Can I take the suggestion of SherpaDoug of the other thread (link from ckielstra)? I hope that I will have the needed material tomorrow and can try it.

Best wishes, Martina (Ms. pom)