Needing help choosing a precision comparator - Page 1
Forum section: electronics / projects

author: Pentoad
author_rank: Regular Contributor
timestamp: October 05, 2019, 03:35:49 pm
content: Hello all,I am trying to design a module which is to be placed upon each cell in a lithium ion battery bank. The module is to dissipate excess energy to maintain the cell voltage at 4.1v. It needs to be accurate to within a millivolt. For the comparator, I was thinking of using an LM339 which has a typical input offset voltage of 1mv but this can be as high as 5 or 9mv. My voltage reference will be a TL431A.The bulb in the diagram will be a high power resistor. Will this setup give the accuracy I'm looking for? If so, I will use a 4 channel comparator and the other inputs will be used as over voltage and under voltage warnings which will be optocoupled to an external management unit.

author: jhpadjustable
author_rank: Frequent Contributor
timestamp: October 06, 2019, 02:17:50 am
content: Any particular reason you aren't using a proper battery protection IC? Ablic (formerly Seiko Semiconductor) offers a wide variety of battery pack functions, some with balancing functionality built in.

author: Marco
author_rank: Super Contributor
timestamp: October 06, 2019, 03:26:57 pm
content: The TL431 is a three pin device with an integrated opamp, you don't need a comparator just an external transistor.

author: Pentoad
author_rank: Regular Contributor
timestamp: October 06, 2019, 06:45:47 pm
content: Quote from: jhpadjustable on October 06, 2019, 02:17:50 amJust because I want to make something simple and accurate for a college project with full voltage adjustability.

author: Pentoad
author_rank: Regular Contributor
timestamp: October 06, 2019, 06:55:56 pm
content: So I found a circuit here which would accomplish that without using an external comparator. Where about could I place a potentiometer to adjust the voltage it triggers at to 4.1?

author: SeanB
author_rank: Super Contributor
timestamp: October 06, 2019, 07:03:00 pm
content: Replace R1 with probably around 10k in series with a 10k 10 turn preset resistor. Preferably a real Bourns one, not a fleabay fake, as at least the genuine ones have an actual spec for temperature coefficient, and they are not as susceptible to having the track go open circuit with time, or go noisy.

author: Zero999
author_rank: Super Contributor
timestamp: October 06, 2019, 10:13:58 pm
content: There's nothing wrong with using a MOSFET. It just needs to be able to pass sufficient current with a low gate voltage, such as the NDP6020P.https://cdn.sparkfun.com/datasheets/Components/General%20IC/NDP6020P.pdfA feedback resistor is required to introduce some hysteresis and prevent oscillation.Good luck with 1mV accuracy with the LM431. Even the best grade has a tolerance of 0.5% and a much greater drift than 1mV.

author: Pentoad
author_rank: Regular Contributor
timestamp: October 07, 2019, 07:31:06 pm
content: Thanks for that, hugely appreciated. If this setup does not give the desired accuracy, I will try again with a LM239DT 4 channel comparator with a 2.048v LM4040 as the voltage reference. I'm just wanting to create a balancing system which will also give out overvoltage and undervoltage warnings via optocouplers. It will be very basic yet it will have all the necessary safety features a battery will need.