LED Charging Circuit

Thread Starter

mattone1

Joined May 10, 2021
1
Hi guys. I as hoping somebody might be able to help me with an issue I have been experiencing. My goal is to an imlpement an LED into my circuit such that it will get brighter as I charge the 12V power supply. Please help. Thanks.
 

MrChips

Joined Oct 2, 2009
24,207
Off the top of my head, I would try a 1kΩ resistor, LED, and a 12V zener diode.

+V----VVVV----|>|----Z<|----GND
-------1kΩ---LED0--ZENER---
 

LowQCab

Joined Nov 6, 2012
1,099
Sometimes judging the relative brightness of an LED is hard to do.
You may want a Red & Green, "Charging", and "Charging-Complete" Indicator.

If You supply a Schematic, and all the details of your Circuit,
you'll probably get a variety of different ideas for your Indicator.
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kiroma

Joined Apr 30, 2014
49
As @LowQCab mentioned, if you are open minded about it, you can change your design.
I'd recommend the LM3914, as you can select how many LEDs you want your bar graph to have.
I don't know if you have electronic background or how deep it is, so I leave the wikipedia image for this IC.
1627426017720.png
You can select it to have, for example, 4 LEDs and adjust the reference to light up the last one at the full charged voltage.

As a reminder, I have to tell you that brightness is misleading because our pupils adjust the incoming light to a certain level. Therefore you won't be able to tell exactly how charged the battery is, even if you apply exponentially more light.
 

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Ian0

Joined Aug 7, 2020
3,245
If you're charging lead-acid or lithium, then a voltage indication is not that helpful. Quite a lot of the charging happens in the "absorption charge" phase, where the voltage is constant.
A current meter would be more helpful. As the absorption charge progresses the current reduces.
If something that can be read at a distance in bright light is important, then a plain old analogue meter does the job!
Therefore you won't be able to tell exactly how charged the battery is, even if you apply exponentially more light.
How about two LEDs? One which gets dimmer and one which gets brighter as the charge progresses?
 

kiroma

Joined Apr 30, 2014
49
How about two LEDs? One which gets dimmer and one which gets brighter as the charge progresses?
The problem with LEDs is that it's either too translucid to let the ambient light reflect to you, or (the most common is that) the LED is that it's a bit opaque, so you don't see well "how dark" it is.
Remember when you barely light up a LED in a bright ambient, you can't see if it's completely off or barely on.

The thing is that when you sweep ambient brightness, you can't see well what's the absolute brightness of an LED (or with 2, one for the dark part). Thus, I'd saturate or don't light it up at all, which is a bar graph.
I think that's one of the reasons why braking lights (of vehicles) are only on or off (also for simplicity and cost-effectiveness)...
 

kiroma

Joined Apr 30, 2014
49
Just thought about that with only 1 LED you can change it's frequency of blinking to let you know how charged the battery is. So maybe 40 Hz could be fully charged and 1 Hz could be fully discharged. It's a suggestion.
 

Ian0

Joined Aug 7, 2020
3,245
I think that's one of the reasons why braking lights (of vehicles) are only on or off (also for simplicity and cost-effectiveness)...
More likely due to the technology available when brake-lights were invented - either a pressure switch on the brake pipes or a microswitch on the pedal.
 

Ian0

Joined Aug 7, 2020
3,245
Just thought about that with only 1 LED you can change it's frequency of blinking to let you know how charged the battery is. So maybe 40 Hz could be fully charged and 1 Hz could be fully discharged. It's a suggestion.
Same problem - when it's flashing quickly (above 10Hz) is it possible to tell just how quickly?
You could use the two LEDs in a bicolour LED - say red and green - then the problem becomes how to tell exactly how yellow it is.
 

kiroma

Joined Apr 30, 2014
49
Same problem - when it's flashing quickly (above 10Hz) is it possible to tell just how quickly?
You could use the two LEDs in a bicolour LED - say red and green - then the problem becomes how to tell exactly how yellow it is.
I can tell coarsely few hertz like 1 to 7, and then I can tell the difference between 10, 20, 30 and 40 Hz. With 2 minutes of training you can do it too.
But the problem with analogic things is that the humans aren't very good with (analogic) measures.
 

dl324

Joined Mar 30, 2015
13,122
Welcome to AAC!

Since you didn't bound solution space, I'd go with PWM using the battery voltage to vary duty cycle.
 

kiroma

Joined Apr 30, 2014
49
Just remembered that there is the same problem with that green LED that is placed deep inside the cars battery. You have to make a tube with your hand to look to it when the surrounding of your vision is nearly dark. And that works, so I changed my mind about it, @mattone1. Just make an exponential amplifier with an OP AMP and a rectifier diode, and adjust its gain. There you have it.
 

Tonyr1084

Joined Sep 24, 2015
6,265
My experience with battery chargers and charge indicators is that when current is flowing into the battery an incandescent light bulb will glow brightly. Once the battery is fully charged the light has diminished to the point where it is no longer illuminated. I know that's the opposite of what you want, but like others are saying, judging brightness is relative to so many surrounding conditions.

As an emergency lighting technician I used to service them back in the early 80's. A 12 volt lead acid battery was held at 13.8 volts as a float charge. After a power outage the battery would be depleted to some extent. When power came back on the current would win o flow. Keyoard failure! oa o.
 

Tonyr1084

Joined Sep 24, 2015
6,265
Before my keyboard failed I made this statement:
When power came back on the current would win o flow. Keyoard failure! oa o.
What the intent was to say was:
When power came back on the current would begin to flow.
Was going to add:
With the higher current flowing the filament of the lamp would glow brightly. In that case you'd know the battery was charging. When the light went out you'd assume the battery was fully charged back to 13.8 volts.

OK, I'm now using a wireless keyboard. On my wired keyboard the 5, T, G (and) B would not work. Also some of the scrolling arrows failed as well. You saw it as it happened. One moment it was working fine the next it was dropping some characters. Lucky me. Wonder what breaks next.
 
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