# Beginner - Simple LED Circuit Help

#### Pedro243

Joined Oct 18, 2020
6
Hi all,

I am a beginner with electronics but I thought I would give this simple project a go.

A few years ago I made a simple LED circuit for my tool bench in the garage. I used 2 small LED light strips (50x8mm strips) connected to a 4x AA battery box. They work great but use the batteries quicker than I would like.

I have 2 powerbanks which I thought I could use so I changed the wiring from the battery box to an old male USB plug.

When I plug them into the powerbank, the LEDs just flicker. I think they are drawing too much from the powerbanks and the banks just shut off thinking there is a short.

Does anyone have any ideas of how I could regulate the volatage/current?

#### LesJones

Joined Jan 8, 2017
3,497
To reduce the power consumption of the lights requires reducing the light output. (Or waiting until someone invents a more efficient method to convert electrical power into light.) The power banks are probably designed to shutdown if they are overloaded. You need to start by measuring how much current the lights require and selecting a power source that can provide that current. You also need to calculate the amp hour rating required to power the lights for the time you require them to operate. A car battery may be a suitable solution with the two strips connected in series. (Or use switch mode step down converter to give 6 volts output for the lights from the 12 volts input from a car battery.

Les.

#### BobTPH

Joined Jun 5, 2013
3,664
How much current do the LEDs take? Or, if you don’t that, how long di the AAs last?

Also, what is the rating of the powerbank? If it is any but the smallest of them, it should have far more capacity than the AAs.

Bob

#### AnalogKid

Joined Aug 1, 2013
9,355
What information do you have about the LED strips and the power banks? Manufacturer, part number, voltage and current requirements on a label, product page on a vendor website - ?

ak

#### Tonyr1084

Joined Sep 24, 2015
6,267
A standard Energizer AA battery, like all AA batteries, has 1.5 volts. With the constant voltage determining the watt-hours, specification becomes a matter of obtaining the Energizer specifications in milliamp hours and performing a conversion. Data sheets for Energizer's technical information can be searched on their website. For their AA battery, it looks to have a capacity of 2800 milliamps hours, or 4.2 watt-hours.

All AAA batteries, a smaller size battery than AA, are 1.5 volts. Energizer's AAA battery has a capacity of 1250 milliamp hours, or 1.87 watt-hours, giving the AAA battery considerably less capacity than a AA battery.

[Source]

You can read the entire article if you want a greater understanding of power (a.k.a "Watts")

IF your batteries are in series then 4 AA batteries will give you a combined total voltage of approximately 6V at 2800 mAH (or 2.8 amp hours). If they're in parallel then you have a voltage of approximately 1.5V and (2.8 x 4 =) 11200 mAH (or 11.2 amp hours). 1.5 volts is typically insufficient for lighting LED's so unless there's some boost electronics in the battery box, LED's won't light on just 1.5 volts (typically). (I say "typically" because there are sometimes exceptions that I may not be aware of).

So if you want a power supply then you need something that can provide sufficient voltage and current. Since LED's are current driven devices choosing a voltage is the easy part. To light any of MY "SuperBright" LED's they typically don't drop more than 3Vf (forward voltage). Whatever supply you choose it must be higher than the forward voltage in order to work. THEN there's the need for current control. So assume you have a 12 volt source with more than enough amp hour ratings. Take that 12 volts and subtract the Vf. In my case, that would be 9V. LED's, as mentioned, are current driven components. Mine don't like more than 30mA (0.03 amps). So I need to limit their current using a resistor. 9V ÷ (I'll use 20 mA) 0.02 = 450 ohms. A 450Ω resistor is a specialty item. Use of a (common) 470Ω resistor will result in (9V ÷ 470Ω =) 19mA. VERY close to the 20 mA target and plenty bright. But the power source needs to be capable of that much amperage. Since I'm speaking about a single LED, it doesn't equate to the light strip you're using. You said you're using LED strips. Without us knowing what amperage or wattage they need it's only a best guess as to how big a supply you need.

However, given that you now know AA batteries are good for 2800mA (2.8 amps for one hour), knowing how long your LED's stay lit at sufficient brightness it shouldn't be hard to extrapolate how many amps you're drawing. For instance, if your strips are lasting for just two hours then 2.8 ÷ 2 = 1.4 amps. Again, WE'RE GUESSING. Nevertheless, you should now be able to determine how many amps your lamps are drawing.

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#### dl324

Joined Mar 30, 2015
13,122
Does anyone have any ideas of how I could regulate the volatage/current?
You need something that can supply sufficient current at the correct voltage. 4 AA batteries will have a nominal voltage of 6V. A USB power brick will provide 5V nominally.

#### Pedro243

Joined Oct 18, 2020
6
Hi All,

Thanks for the replies so far.

I have removed the LED strips from a light similar to this: https://www.amazon.com/Promier-Products-TV207805-Switch-Light/dp/B01A7RDIMM

From what i can find and measure, the strips are these: https://www.aliexpress.com/i/1000008482992.html?spm=2114.12057483.0.0.395324bbAdJKQo

The power band i wanted to use is 2500mah, but i also have a 4400mah.

I will dig the multimeter out in the morning and see if i can get a reading on the current draw.

#### MisterBill2

Joined Jan 23, 2018
9,285
A simple and effective means of limiting the current when you have a constant voltage and a constant load is a resistor in series. It is not efficient but it works well and is effective. Since the strips contain more than one LED and since they worked OK on a set of 4 AA batteries, , we can guess that a six volt supply will be a good start. You could start out with a 100 ohm resistor in series and a six volt supply and observe the brightness as compared to what you want. If they are not bright enough try an 82 ohm resistor, if too bright add a 47 ohm resistor in series with the 100 ohm one. The reason for the values I gave is that they are stabdard and most likely to be available.

#### dl324

Joined Mar 30, 2015
13,122
I will dig the multimeter out in the morning and see if i can get a reading on the current draw.
Meter resistance can affect the circuit and give you a false (lower) current reading.

BTW. Welcome to AAC!

#### BobTPH

Joined Jun 5, 2013
3,664
If you are correct about the LEDs you have, then you are over driving with your 4AAs or a 5V supply. The Amazon product says it uses 3AAs, which is 4.5V, the AliBaba says 3.7V for the ones you linked to.

Bob

#### Dodgydave

Joined Jun 22, 2012
10,050
Those LEDs are 3.7 V @ 300 mA max current, so you will need series resistors for higher voltages.

#### Audioguru again

Joined Oct 21, 2019
3,533
There is a big difference between a cheap AA "super heavy duty" battery from China and a Western AA alkaline battery.
Your LED strip looks like one long LED. Our LED strips are a long ribbon with many tiny LEDs spread out along it.
Your (Chinese?) power bank is a little 2500mAh. My American power bank is 16000mAh.

Did you notice the typo? AliExpress has such poor English that they rate the "angle of the beam" at 120 dollars.
They also say the LEDs are 1.5W each but 3V x 300mA= only 0.9W.

#### Pedro243

Joined Oct 18, 2020
6
Could i try a dc to dc buck voltage converter?

#### Audioguru again

Joined Oct 21, 2019
3,533
A buck voltage converter reduces a voltage.
You do not need to reduce the voltage since the LEDs limit the voltage themselves.
Instead you need to limit the current, the absolute maximum current is 300mA for each LED if the LED is cooled properly. Maybe you should limit the current to 200mA for each LED.
What voltage source will you use?

#### Pedro243

Joined Oct 18, 2020
6
I will probably use the 4400mah power bank.

If I am running 2x LEDs at 300mA each (less than 300mA With my multimeter), shouldn't that be well under the 1A rated from a USB power bank? What would case the flickering then?

#### MisterBill2

Joined Jan 23, 2018
9,285
LED current is very much dependent on the applied voltage. Once they start to light up just a very small increase in voltage gives a very large boost in current. A lot like other diodes.

#### Audioguru again

Joined Oct 21, 2019
3,533
Maybe your LEDs flicker because their "300mA" rating is wrong or because you did not cool them properly. Maybe the "1A" rating of your power bank is wrong or it is not fully charged.

The LEDs are 3V to 3.7V. If you cool them properly they survive 300mA. Without cooling they might survive 200mA but AliExpress knows nothing about it. You might find an English Epistar datasheet about it.
A power bank produces 5V to power the charger circuit in a phone and might limit the current to 1A which will blow up your LEDs unless you limit the current.

6V - 3 2V=2V. 2V/200mA= 10 ohms. With 200mA in a 10 ohms resistor it heats with 0.4W so a 1.2W resistor will get very hot. If the LED is actually 3.7V then the current is (5V - 3.7V)/10 ohms= 130mA.

#### Pedro243

Joined Oct 18, 2020
6
I took the circuit to a local electronics store today to see what they thought. They mentioned that some powerbanks use a duty cycle to aid in cooling. He tested my powerbank on a multimeter and found it had a frequency of 2hz, about the rate it was flickering. I tried one of the powerbanks they had on display and it worked perfectly.

I purchased another powerbank and went home. One of the wires had broken off the solder so i decided to replace the wires and resolder all joints. To my surprise, when i tried it after the re-work, the powerbank which originally flickered worked fine. Maybe there was a dodgey connection in the original wiring...

#### MisterBill2

Joined Jan 23, 2018
9,285
Certainly a poor connection can cause all kinds of problems. That is why thumping on a piece of equipment can be a very useful diagnostic tool. Not super-hard hits, just slaps on the side of the case. At one service place I was familiar with, that was step number three when an item was being checked to verify a customer complaint. A weak solder connection can work quite well for a while and then crack and cause problems.

#### Tonyr1084

Joined Sep 24, 2015
6,267
In my lifetime I've had two TV's that required a thump on the side to get the horizontal circuitry to work correctly. Early on in my life the first TV, I didn't know anything about electronics so I sold the TV "As Is". The second TV was donated to me, I was much older and much more experienced with electronics. Disassembled the TV and let it sit for a few days to discharge any capacitance that might be hanging around. Then took a wire and a long screwdriver, tied the wire to the chassis and to the screwdriver, then wedged the screwdriver up under the rubber cap on the picture tube (CRT) and heard a snap. After that I was confident to go ahead and reflow the solder joints on all the horizontal circuitry using a cheap Radio Shack iron. The TV worked perfectly.

It was donated to me because it had that problem AND BECAUSE IT WAS HAUNTED! Yes, the donator believed it was haunted. Said it would play for a short while then turn itself off. Upon further diagnostics I found the sleep timer was set for 15 minutes. Turn it on and in 15 minutes it would turn itself off. Simply shut the timer off and used that TV for many years. Donated it when I bought my first 55 inch flat screen LED TV. Sony Bravia. Still have it. Occasionally it has picture issues that if you give it a thump will clear up - or if you let it sufficiently warm up it will self clear. Definitely a bad connection somewhere, but I haven't had the time or the desire to chase after it. The TV lives in the family room where we seldom watch TV. When we have movie night with friends over, and the fireplace going - I'll turn the TV on a half hour before they arrive. By then it's working nicely.

Bad connections can be very hard to track down. Also intermittent components that fail after warming up. Those can be tracked down with "Freeze Spray", though I've never used it. Don't even know if it's still a diagnostic tool. But there you have it. You found your gremlin and have sleighed it. Congrats on a successful repair.