Electrolytic capacitor question - Page 1
Forum section: electronics / beginners

author: rahulb
author_rank: Contributor
timestamp: April 25, 2025, 04:13:06 pm
content: Hi Everyone,I am just testing a circuit where a led is turned on slowly using a capacitor. you can check the schematic below:The circuit on breadboard is working fine.But, my question is , can the capacitor hold the charge for 24 hours? I want to keep led glowing continuously for 24 hours.Ideally, it should but what I am reading in books and web that capacitors in practical cannot hold charge for long. specially electrolytic ones discharge in few hours due to leakage.What type of capacitor I can use which can hold charge for at least 24 hours?thanks

author: TimFox
author_rank: Super Contributor
timestamp: April 25, 2025, 04:25:21 pm
content: If the LED is "glowing" for a long time, it is conducting current away from the capacitor.However, if your battery is still connected and has not discharged, the current (after charging the capacitor) through the LED is 0.3 mA, or (12 - 2)V/(34 k\$\Omega\$).Will that give you enough light output?

author: bdunham7
author_rank: Super Contributor
timestamp: April 25, 2025, 04:41:15 pm
content: Quote from: rahulb on April 25, 2025, 04:13:06 pmBy "hold the charge" are you referring to a situation where you disconnect the battery or leave it connected? If you disconnect the battery, then the capacitor will quickly discharge through the LED until the voltage drops below the point where the LED conducts, then the capacitor will slowly self-discharge through internal leakage which may take anywhere from several minutes to several years.If you are referring to the case where the battery remains connected, then there's no issue and the circuit will work for as long as the battery has charge. The capacitor can remain charged forever as long as there is voltage applied. Any small amount of loss due to leakage will be replaced from the battery, these currents will be in the microamp range or even less.

author: rahulb
author_rank: Contributor
timestamp: April 25, 2025, 04:50:32 pm
content: Quote from: TimFox on April 25, 2025, 04:25:21 pmHi,I think it is more than enough.thanks

author: rahulb
author_rank: Contributor
timestamp: April 25, 2025, 04:56:36 pm
content: Quote from: bdunham7 on April 25, 2025, 04:41:15 pmhi,the battery will remain connected.Also, one question. I asked about this circuit locally in my area. they were saying, keeping the capacitor charged for a long time is a bad design.Is this true?

author: bdunham7
author_rank: Super Contributor
timestamp: April 25, 2025, 05:11:36 pm
content: Quote from: rahulb on April 25, 2025, 04:56:36 pmNo, that isn't correct. Many circuits, including most that use larger electrolytics like yours, keep the capacitors charged all the time. Almost every electronic device that you plug into mains power will have electrolytic capacitors that are being used this way.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 27, 2025, 05:52:49 am
content: Keep a capacitor there if battery is still prrsent is bad idea because it's doesn't make much sence. The best accumulator of energy is a chemical element but not a capacitor.

author: bdunham7
author_rank: Super Contributor
timestamp: April 27, 2025, 05:59:22 am
content: Quote from: Vovk_Z on April 27, 2025, 05:52:49 amThat's some spherical reasoning!The OP said the capacitor was to make the LED turn on slowly, after that it serves no purpose but does no harm.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 27, 2025, 06:56:39 am
content: Yeah, if the idea is to power the LED from the battery, and just use the capacitor to slow down the initial turn-on, then the circuit works fine for that. The capacitor does no harm during extended runtime, and doesn't leak (self-consume) any significant enough amount to reduce battery lifetime compared to what the LED already consumes anyway.

author: CaptDon
author_rank: Super Contributor
timestamp: April 27, 2025, 11:11:36 pm
content: Your logic or sense of how things work is nonsense. If the capacitor is discharged below the threshold voltage of the LED and you apply the battery voltage then the LED 'turn on' will be delayed. After that the LED remains lit and the capacitor has no real influence. IF your question is "Will the LED remain lit for 24 hours after removing the battery the answer is NO!!! It may remain lit for maybe a few seconds based on the size of your capacitance. Given your circuit values the capacitor will only charge slightly above the LED conduction voltage. Your circuit shows several flaws depending on what you really want the circuit to do. My question to you is Why wouldn't the LED remain lit if the battery remains connected?

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 28, 2025, 12:27:20 am
content: Quote from: CaptDon on April 27, 2025, 11:11:36 pmThe OP already said:"hi,the battery will remain connected.Also, one question. I asked about this circuit locally in my area. they were saying, keeping the capacitor charged for a long time is a bad design.Is this true?"He keeps the battery connected so if the battery doesn't run out it will keep the LED on for a long time.

author: CaptDon
author_rank: Super Contributor
timestamp: April 28, 2025, 12:54:35 pm
content: Most likely scenario for a modern capacitor left connected to voltage in this circuit with no ripple current would be a decrease in capacitance along with increased E.S.R. and eventually looking like two leads leading to an open circuit or a high resistance value leakage resistance.

author: rahulb
author_rank: Contributor
timestamp: April 28, 2025, 05:08:08 pm
content: Quote from: CaptDon on April 27, 2025, 11:11:36 pmhi,It is an experimental circuit. If capacitor remain charged for 24 hours, Later I want to add an opamp instead of Led which only gives output when the voltage in capacitor is above the threshold voltage.The problem is that the capacitor's voltage must be above the threshold voltage for at least 24 hours to keep the opamp output on. The circuit depends upon the capacitor charging and needs a time delay.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 28, 2025, 05:36:41 pm
content: Quote from: rahulb on April 28, 2025, 05:08:08 pmIt all depends on your battery. If the battery can maintain the voltage then the capacitor voltage won't drop.

author: Zero999
author_rank: Super Contributor
timestamp: April 29, 2025, 09:55:14 am
content: Note that R2 doesn't do much, as the voltage drop across it is very small, because R1 is a much higher value.

author: bdunham7
author_rank: Super Contributor
timestamp: April 29, 2025, 01:12:00 pm
content: Quote from: rahulb on April 28, 2025, 05:08:08 pmIf I understand you correctly, you want a circuit that turns on after a 24 hour delay. In other words you apply battery power and the capacitor slowly charges until a threshold is reached 24 hours later. Is that what you're after?If so, that's going to be very difficult to do with an electrolytic capacitor like that. The currents involved will be very low and the normal leakage of the capacitor is fairly unpredictable at such low levels.

author: SteveThackery
author_rank: Super Contributor
timestamp: April 29, 2025, 02:44:13 pm
content: Quote from: bdunham7 on April 29, 2025, 01:12:00 pmActually I read it the opposite way round:Quote from: rahulb on April 25, 2025, 04:13:06 pmThis seems to imply a capacitor holding enough charge to keep the LED lit for 24 hours (which I think would be highly impractical). But in a later post the OP says the battery will remain connected.This is all very confusing - we need a much clearer description of exactly what is required. At the moment we are all just guessing.