Voltage Divider Problem - Page 1
Forum section: electronics / beginners

author: sonicj
author_rank: Frequent Contributor
timestamp: November 06, 2010, 09:27:51 am
content: i can't seem to wrap my head around how to do this....i am trying to measure total voltage of a string of six 48v batteries. the 48v pack should read about 60v tops fresh off the charger. the ADC input range is 0-5v. i have no problem getting the voltage down to 5v with say a 65v input (+5v for ADC safety) and it tracks fine all the way down to zero. the thing is, im not really interested in measuring anything below say 40v, because anything below around 46v has already damaged the batteries. so basically, im wasting most of my dynamic range of the ADC on voltage lower than i'll ever need to measure.is there anyway to utilize the entire dynamic range of the 10bit ADC for a "window" of voltage? say 40v-65v?looking forward to some expert insight! cheers!-sj

author: oPossum
author_rank: Super Contributor
timestamp: November 06, 2010, 09:40:24 am
content: Use an external low reference voltage of 3 volts and high reference voltage of 5 volts.

author: qno
author_rank: Frequent Contributor
timestamp: November 06, 2010, 09:43:29 am
content: The easiest way is to put a zener diode in series with your voltage divider.A zener of 39 Volt will start conducting arround 39 Volts.Be aware of some non linear behavior when the zener starts to conduct.Check the zener data for an appropriate current.You can also do this with an differential opamp. Set Vref on the - input to 3V.Set the gain to 0.2 and your 40-65 volt will be converted to 0.05 to 4.95 V adding the rail to rail limits of a good opamp.

author: Simon
author_rank: Unknown
timestamp: November 06, 2010, 10:30:34 am
content: you could use opamps to create an offset, or setup the ADC in the pic to use external lower and upper Vref's so you could set the lower ref at 3V and the upper at 5V, you will need to provide the reference voltages yourself

author: sonicj
author_rank: Frequent Contributor
timestamp: November 06, 2010, 11:20:48 pm
content: thanx for the answers! the vref seems like the easiest solution as i already have 3.3v on the board. 41.2v to 62.5 = 3.3v to 5v on the meter without any changes to my divider; nearly perfect! the thing is, i don't think the arduino will let me set 3.3v as the low and 5v as the high. the AVR datasheet leads me to believe its a either or thing.it sounds appealing, but im not sure i completely follow how to use the zener....i might give the opamp a go in a bit. i have a few i salvaged from a old cd player! cheers!-sj

author: Simon
author_rank: Unknown
timestamp: November 07, 2010, 12:40:12 am
content: you can set both top and bottom reffs with a pic

author: Bored@Work
author_rank: Super Contributor
timestamp: November 07, 2010, 10:06:00 am
content: The Zener will drift so much that you will give away many of your ADC bits. And getting a precision ref for such high voltages is difficult.Consider ICs like the LTC6802-1 instead.

author: sonicj
author_rank: Frequent Contributor
timestamp: November 07, 2010, 12:57:31 pm
content: that chip looks like its for the wrong chemistry. for the time being, im mostly interested in monitoring total string voltage only as wiring for individual cells becomes exponentially more complicated.-sj

author: scrat
author_rank: Frequent Contributor
timestamp: November 08, 2010, 04:17:11 pm
content: I'd just use opamps and a voltage reference to subtract offset and adjust gain such that the resulting signal will be inside 0-5 V range for an input of about 40-60V. For example: 13:1 divider (resistor) to achieve 5V from 65V, then subtract 3.3V and amplify the difference by 5/(5-3.3). The final result will be in the range 0-5V, if you use a rail-to-rail opamp with 5V supply.

author: Bored@Work
author_rank: Super Contributor
timestamp: November 08, 2010, 06:46:07 pm
content: Chemistry doesn't matter as long as you just measure. Use your engineering imagination, dude.

author: sonicj
author_rank: Frequent Contributor
timestamp: November 09, 2010, 04:38:37 am
content: Quote from: scrat on November 08, 2010, 04:17:11 pmi'll give this a shot. i actually just finished myfirst op-amp projectlast night!Quote from: BoredAtWork on November 08, 2010, 06:46:07 pmby chemistry, i meant in the context of voltage.i did... i imagined it would add many levels of complexity unnecessary for the task at hand. 7 wires would have to be pulled, loomed & terminated from the dash back to the string, or a second weatherproof enclosure added near the string, and 3 or 4 wires pulled from the dash. instead of battling with a single voltage divider problem, i would have 6 dividers to deal with as the max input for that chip is 5v per channel and my cells are 8v. overkill. i might try monitoring single cell voltages after i get past the basics.-sj

author: Bored@Work
author_rank: Super Contributor
timestamp: November 09, 2010, 07:21:14 am
content: Again, you didn't read the datasheet. But hey, I am out.

author: sonicj
author_rank: Frequent Contributor
timestamp: November 09, 2010, 09:38:04 am
content: i read enough of it to gather that it wasn't intended for use with 8v lead acid cells. if it can measure the sum without each cell of the series string connected, its not a feature they make blatantly obvious.-sj

author: sonicj
author_rank: Frequent Contributor
timestamp: November 10, 2010, 08:27:43 am
content: Quote from: scrat on November 08, 2010, 04:17:11 pmhmm.... would you consider atlc272appropriate for the task?i have a working circuit at the moment!voltage on the op-amp output starts climbing at around 40v input, but quickly tops out at around 50v in, 4.14v on the output. i think i need to adjust my gain resistors and take another look at the input voltage. almost there!-sj

author: scrat
author_rank: Frequent Contributor
timestamp: November 10, 2010, 10:37:06 am
content: This is not a rail-to-rail output opamp, at 5V supply its maximum output is typically 3.8V ("High-level output voltage" on the datasheet should be stating this). I'd use another one.