Simple reliable way to indicate supply rails are within +/-  10% - Page 1
Forum section: electronics / projects

author: Cicada
author_rank: Regular Contributor
timestamp: August 14, 2023, 08:35:42 am
content: HiOne can connect LEDs to the supply rails of equipment with multiple internal voltages to display on the front panel that the supplies are not dead.I would like to know if there is a simple reliable circuit that one can use to display that a supply rail is bigger than Vnom - (Vnom x 10%). For example that a 5V rail is bigger than 4.5V.Could this be done reliably with LEDs and say zener diodes?Would one always require one supply rail in a m,ulti supply rail system to be with in tollerance for such a circuit to work?ThanksCicada

author: nali
author_rank: Frequent Contributor
timestamp: August 14, 2023, 09:11:23 am
content: Probably easier just to use a dedicated supervisor / reset IC as they're common enough & cheap enough, rathe than messing about with discretes.E.g. quick look on Mouser APX803L40-45SA-7 4.5V open-drain £0.29 for 1-off.

author: Psi
author_rank: Super Contributor
timestamp: August 14, 2023, 09:15:25 am
content: Just to indicate it?Probably the most simple/easy way is an analog panel meter.Use a custom scale print inside the meter which has a shaded area showing the +/- 10% area in green and everywhere else in red.Easy to tell if it's in spec by looking at it and seeing the needle is in the green area.If you want LEDs, then probably easiest way is a LM3914in dot modewith resistors setup so 10% error happens at a specific LED transition point. You can even wire the top most and bottom most LEDs to an alarm or something.If using an LM3914 you may want to use an online LM3914 calculator to save time finding the correct config resistor values. They set the correct LED step voltages, which you would pick to match percentage errors of your input voltage.The chip itself is pretty simple.

author: David Aurora
author_rank: Frequent Contributor
timestamp: August 14, 2023, 09:24:13 am
content: Seems like it would be pretty easy to do with some window comparators and a voltage reference, lots of ways to skin this cat depending on how exactly you want the LEDs to indicate things

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: August 14, 2023, 12:03:13 pm
content: Hi Cicada,look for window-comperator.The TL431-datasheet (at least the T.I.-version)has this in its applicationnotes.Uses two TL431 and drives LEDs directly.Good luck

author: magic
author_rank: Super Contributor
timestamp: August 14, 2023, 01:22:20 pm
content: Or only one TL431 if you don't care about detecting and signaling overvoltage.The LED needs a series resistor to limit current and a parallel resistor must supply TL431 with 1mA before the LED lights up.

author: mawyatt
author_rank: Super Contributor
timestamp: August 14, 2023, 02:33:43 pm
content: The POR devices might be considered, like the Diodes Inc APX803S types as nali indicated. Some like the Onsemi NCP303 allow external delay setting via a cap.Best,

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 14, 2023, 02:55:06 pm
content: Quote from: David Aurora on August 14, 2023, 09:24:13 amYes, that is exactly what an LM3914 isAnd it also combines current limited LED drivers do you don't need a series resistor for each LEDUnfortunately its becoming obsolete and expensive. Building something similar from "more modern" parts is probably cheaper.

author: PwrElectronics
author_rank: Regular Contributor
timestamp: August 14, 2023, 06:24:19 pm
content: QuoteThat is what we do at my work. Window comparators OR'd together to 1 digital back to the system micro to alert of bad power. These are on a HV elevated and isolated board so the OR to one (from several rails) to save on expensive isolation parts crossing the boundary. Its good enough to know at least 1 is out of spec.

author: fourfathom
author_rank: Super Contributor
timestamp: August 14, 2023, 06:51:58 pm
content: For 10% accuracy, a transistor, three resistors and an LED should work. Two transistors set the base turn-on voltage, and the third limits the LED current.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 14, 2023, 07:29:39 pm
content: Of course you or someone else may be replying that they prefer a fully analog solution. But otherwise you can use a cheap MCU for monitoring power rails and get almost as much flexibility as you want for a couple $ or even much less.

author: David Aurora
author_rank: Frequent Contributor
timestamp: August 14, 2023, 11:03:13 pm
content: Quote from: Doctorandus_P on August 14, 2023, 02:55:06 pmHence my suggestion NOT being to use an LM3914It's no good if you can't get one (not just now, but 3 years from now)With a single voltage reference all your monitoring will have consistency, and with a handful of comparators/op amps/resistors you can easily scale and invert signals and adjust thresholds to taste. If all the rails you're monitoring are just +5V or something then an LM3914 style thing is definitely easiest, but if you want to monitor rails higher than the LM3914 supply, or negative rails, etc, you're going to have to do some stuff externally anyway to feed the IC a suitable signal. Same sort of thing with the MCU suggestion, though at least with that you have a lot more flexibility once signals are digitised such as running alarms and logging and so on.

author: Psi
author_rank: Super Contributor
timestamp: August 15, 2023, 08:03:33 am
content: Quote from: David Aurora on August 14, 2023, 11:03:13 pmI got the impression this was just a one-off project, but yeah if building something for production I agree.But for a one-off and where simple is key a LM3914 is great. but i still prefer the panel meter idea.

author: David Aurora
author_rank: Frequent Contributor
timestamp: August 15, 2023, 09:52:37 am
content: Quote from: Psi on August 15, 2023, 08:03:33 amI was assuming one off project too, but I still don't think the LM3914 would be the way I would go for the reasons I mentioned above. But yeah, just depends on the OP's exact needs I guess