Low cost, low power battery led indication cirucit - Page 1
Forum section: electronics / beginners

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: February 20, 2020, 06:30:19 am
content: 1. I have a battery, a charger & a load as in ckt attached.2. Have to glow led when charging happens. While charging current decreases while battery getting full.Have conencted a small resistor in battery path whenever current will flow across it from charger to battrey, there will be drop across the resistor, which appears across opamp input. Since opamp is in open loop configuration even a small drop will saturate the opanmp & will glow led.3. Will it work as intended? Havent tried in actual yet.4. Most of time bothe charger and battery will be connected to load, once in a month mains may go then system shifts to battery.Probem may arrise:1. There is small vid diff due to offsets, so opamp may be saturating even if no current is flowing, so how to avoid this situation2. Any other low cost solution based on voltage measurements or current measurement circuit will work only?3. Low cost is must have, low power is better if available.4. Any other better way to do that with/without cost constraints.

author: VEGETA
author_rank: Super Contributor
timestamp: February 20, 2020, 09:43:45 am
content: You have to see your opamp capabilities, like it may not be able to drive 2.2k ohm connected to an LED. Maybe 100 ohms is enough.In your circuit, the LED will always be on (assuming op-amp specs are good) until the battery is fully charged or until voltage difference is beyond the capabilities of the op-amp.

author: mikerj
author_rank: Super Contributor
timestamp: February 20, 2020, 11:55:04 am
content: No, it won't work. You are attempting to use the input offset voltage of an op-amp as a voltage reference, but the offset voltage is not a well defined or controlled parameter; it can vary in both magnitude and polarity across parts and over temperature. If you build enough devices a few may work well enough over a limited temperature range, some will never switch off the LED or may switch it off prematurely. Even if you hand picked a device with a suitable offset at room temperature, without any hysteresis the LED will flicker as the current approaches the switching point.Additionally for switching applications rather than linear operations, you should be looking at comparators rather than op-amps.

author: Jwillis
author_rank: Super Contributor
timestamp: February 20, 2020, 11:51:09 pm
content: Simple low cost , low voltage indicator for 12V.Can be adapted for any voltage by changing diode values.With this circuit the LED remains off when the voltage is above approximately 11V and turns on below approximately 11V.Use across your battery terminals .If you want it go turn on at around 8V use 5.1 Zeners.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 21, 2020, 03:29:22 am
content: Here's a circuit that can be made with jellybean parts and will, in theory, detect when a small current is flowing into the battery. The P-channel MOSFET can be pretty much any part.The gain can be adjusted by changing the values of R1, R2, and R3. As shown it has a gain of 510 which should illuminate the LED when about 5mA of current is flowing into the battery.This seems to work in the simulator but I can't really say how well it would work in real life. It's something you would want to prove-out on a breadboard, obviously.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 21, 2020, 09:31:20 am
content: Two opamps (with LM431) and mosfet is too much for to be called "simple circuit". Try to use just one opamp as comparator. There is no need in mosfet at all.

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: February 21, 2020, 10:03:59 am
content: Attached is circuit.1. Now sense moved to negative side, this will remove +ve rail common mode range of opamp, LM339 opamp has gnd in common mode range.2. At +ve input of opamp, a small divider of 1K & 1M, is created so that voltage at +ve side is always lesser than of -ve side of opamp.

author: mikerj
author_rank: Super Contributor
timestamp: February 21, 2020, 11:37:16 am
content: Still no hysteresis.Input offset voltage still determines switching point.Potential divider doesn't add an offset, it just scales the input i.e. at zero current there is no offsetLED is connected back to front.At what current do you want the LED to switch off?

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2020, 12:22:51 pm
content: How about a PNP BJT?EDIT:R1 might be too low for 1A, try 1R, if the LED doesn't light.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 21, 2020, 10:12:01 pm
content: Quote from: Vovk_Z on February 21, 2020, 09:31:20 amYeah, the more I play around with it, the more I dislike it. The op amp actually has to have a much lower offset voltage than the part shown. The model had an offset of ~50uV but the TL081 can be as bad as 6mV.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 22, 2020, 08:22:35 am
content: I played around with my circuit in the simulator some more. I changed the amp and swapped the TL431 with a high-beta NPN transistor.I tried to do as Vovk_Z suggested but didn't have any luck. My feeling is that you would need a fairly exotic op amp to do that. Either that or make the sense resistor much bigger. I completely nulled-out the offset voltage on a OPA197 (the amp used in my circuit) and the results were a good deal worse than my circuit.The configuration used in my circuit (with the MOSFET) was designed for high-side sensing by people that are much smarter than myself.Edit: Had a misprint in the attached image. The offset voltages in the diagram were listed as mV but are uV.