1
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

I have an AC wired fluorescent lamp (antique one,) and I am making it portable by removing the bulb inside it and wires.

I am going to replace it with LED which runs on DC power. I have bought a 3V yellow SMD LED. I have also bought two 1.5V AA pencil cells and connected it to the LED. But it ran for only 1.2 hours. I want it to last for max 6-8 hours. I have a solution like I can use a 9V rechargeable cell and drop volt to 3V and make it run more but it will make my lamp bulky and it will not look good.

I have tried with alkaline AA cells (Duracell, two 1.5V) and it ran for 2.5 hours max.

Is there anything I can do to overcome this issue?

This is the 3 watt SMD yellow LED I am using:

enter image description here

Mobile battery I am using:

enter image description here

The switch I am using:

enter image description here

So how to create that constant current LED driver circuit without LM IC's and just with Res, cap, and diodes I have seen Most of the people using LM317 IC and creating circuit from 12V power supply or 9V to LEDs but that's now power supply source is so...

\$\endgroup\$
  • 1
    \$\begingroup\$ Use C or D cells instead of AAA or AA like you tried? A 9V battery will have less capacity than the AA cells you already tried. Also, you should post a schematic diagram of your circuit. Your description sounds like you didn't do anything to control the current to the LEDs, which is bad. \$\endgroup\$ – JRE Nov 4 '19 at 11:55
  • \$\begingroup\$ I am running them right now from the mobile battery which is 3.8V and 2000 mAH. It's simple like you connect LEDs to power supply + to + and - to -. \$\endgroup\$ – Lucifer Nov 4 '19 at 14:30
  • 2
    \$\begingroup\$ Then either your LEDs are going to die soon, or else they aren't just LEDs. That's why I asked for a circuit diagram. Now I'm going to ask you for a link to the datasheet of the LED. What you describe is not the recommended way to power LEDs. \$\endgroup\$ – JRE Nov 4 '19 at 14:33
  • \$\begingroup\$ @JRE I have added what you have asked for and also added the components that I am using to create this. \$\endgroup\$ – Lucifer Nov 5 '19 at 5:17
  • \$\begingroup\$ As you said, due to this connection LED is heating very much. So I need to know how to create current control circuit for this. I have searched online mostly they were saying about using LM317 IC but it is used for 9V or 12V purpose and I don't have that right now with me. Can we create current control driver using basic components like Cap, RES etc? \$\endgroup\$ – Lucifer Nov 5 '19 at 9:24
1
\$\begingroup\$

The datasheet for your LED would have been better, but the photo and the hint that is rated for three watts helps to explain things.

A AA cell has an internal resistance of around 0.1 ohms at room temperature.

A yellow LED has a forward voltage of about 2.0V.

With two AA cells in series, you get about 3V with a series resistance of about 0.2 ohms.

The difference of the battery voltage and the forward voltage is 1V. Take that together with the battery internal resistance, and you'll find that you are delivering about 2 amperes to the LEDs.

You have the LEDs in parallel, so they are splitting it. That leaves 1 ampere for each LED. 1 ampere * 2 volts gives 2 watts for each LED. That's below the maximum power rating of 3 watts.

So, now we know why your LEDs survived. They were getting less than their rated power despite being connected directly to the batteries. In addition, the batteries can't deliver full power the whole time so the current (and therefore power) will drop over time - most likely with a big drop in the first few minutes and slowly fading until the battery voltage drops so far that the LEDs can't light up any more.

You appear to have gotten lucky in that the internal resistance of your mobile battery is about the same as the AA batteries.

Your only real option to make the light run longer is to use a bigger battery. You are already using (accidentally) a resistor to limit the current to your LEDs. You haven't mentioned the brightness, so I assume that it is bright enough and not too bright - if the light were too bright, then you could reduce the current to the LEDs and get a longer run time.

You could also reduce the current by using three AA batteries in series, and putting your LEDs in series as well. That would raise the forward voltage to 4V, and reduce the current to 1A or less. That also reduces the difference between the LED forward voltage (4V) and the battery voltage (4.5V) so that the LEDs will shut off sooner and leave more unused energy in the batteries.

If you go the route of a bigger battery, then you will need a current limiting device.

There are simple circuits on the internet for limiting current to an LED. You will need things beyond just simple resistors and capacitors, however. They all pretty much require the use on a couple of transistors and a couple of resistors.

Someone collected several examples on this "Instructables" page.

This example from the Instructables page shows the concept of a simple constant current regulator:

enter image description here

You would calculate R3 as 0.5/I - where I is the current you want to flow through your LEDs. Connect the LEDs in series, then power them from three or four D cells in series.

R1 is apparently not critical in that circuit. Probably anything from 1k to 10k ohms would do.

\$\endgroup\$
  • \$\begingroup\$ I had a heating issue with that LED powering directly from mobile battery and LEDs were in parallel so I connected resistors with them and now there is no heating issue and the brightness is also intact. I kept running LED for more than 1 hour but there was no heating and LED kept running good. I am also going to make the above circuit you said and try the way you told and will say here the conclusion. \$\endgroup\$ – Lucifer Nov 6 '19 at 6:49
0
\$\begingroup\$

You only have a limited number of choices. Bigger batteries would last for longer. Or you reduce the current, have a dimmer lamp, but the batteries last longer.

As JRE notes in a comment, you should have some form of current limiting. If you just connect the LED straight to the AA cells, the only limiting is the internal resistance of those cells. As soon as the cells start to fail, the LED gets dimmer. The better torches (flashlights) use an LED driver chip to keep the brightness constant even as the batteries run down.

\$\endgroup\$
0
\$\begingroup\$

LEDs are current-driven, not voltage-driven. Their I-V characteristic is quite sharp. Here's a sample from random LEDs. The I-V curve of your LED will have the same general shape but probably a different threshold voltage.

enter image description here

As you can see, once you have enough voltage to light it up, a small change in voltage will result in a large change in current. So driving it with a fixed voltage risks frying the LED if it is just a bit too high. Also brightness will dim as batteries drain, and the threshold voltage depends on LED and temperature.

So that's why LEDs should be driven with a constant current driver.

The simplest solution to your problem is: Purchase a constant current switching LED driver which outputs the current your LED needs. You can also use a dimmable one. If your LED has Vf around 3V, you'll probably end up with a driver for flashlights. Try to pick one which accepts an input voltage like 3.3V-6V or better 3V-12V.

Then, supply it with as many rechargeable AA batteries in series as you want, while respecting the driver's input voltage range.

Being a switching driver, increasing input voltage will decrease input current, so the battery life will increase with more batteries in series.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.