# Asking for simple LED driver circuit diagram

#### planetElectron

Joined May 5, 2022
17
Hi, I'm making a LED bulb and I want to ask for a constant current LED driver circuit diagram with good efficiency to drive 10 pieces parallel 5730 LEDs at 700mA,
power source is an 1800mAh 18650 battery (4.2V when fully charged),
LED forward voltage drop is around 3.28V,
a TP4056 (with battery protection) charger module is used.
I don't like series resistor driver because of dimming after some minutes, and I can accept shorter running time.

#### ericgibbs

Joined Jan 29, 2010
17,129
hi p,
According to the datasheet, the LED is rated at 180mA.
2.8v > 3.5Vfwd,
E

#### Attachments

• 1.8 MB Views: 4
Last edited:

#### BobTPH

Joined Jun 5, 2013
6,268
Putting LEDs in parallel defeats the purpose of a constant current driver. The current will split differently between the multiple LEDs. To drive all of them at the same current, they must be in series, or you must have 10 constant current drivers.

Is 700 mA the total or is that per LED?

If is is the total, you will get roughly two hours out of the battery. Is that sufficient. If it is per LED the battery cannot do this.

So, assuming it is total, i.e. 70 mA current each, you would put them all in series then use a boost converter to get the voltage up the the 32V needed to run them. There may be constant current drivers available that will output this voltage from a lithium battery. If not, you would want to use a boost converter chip.

#### ronsimpson

Joined Oct 7, 2019
2,596
The LED voltage is 2.8 to 3.5. If you are lucky your LEDs will all the about the same voltage but they might not. I is possible that you will have 2.9 and 3.4 LEDs in the same batch. That is why paralleling LEDs is not good.

The battery voltage is 4.2 to 3.0. Some LED drivers have a low voltage save the battery cutoff.

Because the battery voltage can be above or below the LED voltage, if all LEDs are paralleled, you need a boost/buck which is more complex.

I do not have time to design one for you but here is a place to start. This IC will work with that battery. It is boosting in this mode. One option is to have (5LEDs in series/parallel/5LEDs). In this case the boost voltage will be in the 15V range.
I used two current cense resistors. This helps balance the current between the two LED strings.

.

#### bassbindevil

Joined Jan 23, 2014
678
You could use the common AMC7135 constant-current driver, as used in zillions of flashlights. Two paralleled will give you 700 mA. They have a control pin that can be used to dim via PWM. There's also a 380 mA version:
https://www.fasttech.com/products/1007/10001705/1197600
Paralleling LEDs works fine unless you have a defective LED. Someone wrote on a stone tablet "Thou Shalt Not Parallel the Light Emitting Diode" but the truth is they have enough internal resistance that it isn't a problem.

#### ronsimpson

Joined Oct 7, 2019
2,596
but the truth is they have enough internal resistance that it isn't a problem.
No,
In the factory we parallel LEDs because we sort the LED to get bins all with the same voltage. Or you can buy them sorted. When you get LEDs without having them sorted bad things happen.

This LED is rated to have a forward voltage at 2.8/3.15/3.5 volts. Min/typical/max
Take a 2.8V and a 3.5V LED and parallel them at 3.25 volts. One LED will have 140mA and the other will have only 35mA.

What is the chance you will get a minimum and a maximum part in the same lot? Very small if you get them from Digikey.com and they come on the same real, made from the same die. But, if you get them from Ebay.com at a reduced price or from Allie at a reduced price, then the part is likely rejected LEDs. I have worked for two companies that got 20 million/month. If I get 3.0, 3.1 and 3.2V LEDs and the next company wants 3.3 and 3.4 LEDs and we take most of what is made, then what is left over, that no one wants gets pushed off on ebay. So you could get a 2.9 and a 3.5 in the same batch.

#### planetElectron

Joined May 5, 2022
17
In the factory we parallel LEDs because we sort the LED to get bins all with the same voltage. Or you can buy them sorted.
Yes, mine is a ready-made LED board for diy bulbs. If so, can I use a constant current driver? Here it is,
I bought it from this link:
https://a.aliexpress.com/_mKQROuk
(It is 5V board but I shorted/jumper all 20 ohms resistors because it is a bit much for my 3.7V battery)

And now the only option that I have is a single 18650 cell, 10 parallel Leds and a through hole type CC driver ic or some bjt/mosfet CC driver for the below reasons,
Buck-boost driver modules and smt/smd parts like LM3410 or AMC7135 are unavailable in my country and I'm unable to order online,
I'm unable to add more battery in series to increase supply voltage for some reasons,
I don't want to change to series my paralleled Leds and make the board ugly.

I wonder is there any through hole type(they are easy to find in my region) constant current driver ic with low voltage drop and current rating of above 700mA.

And for second I found a constant current driver circuit from
Here it is,
Info: That Smps module is 12V 1A supply,
Leds are 3.3V 1W.

I want to copy transistor driver design marked in blue, but I don't know which part numbers to use for lowest possible voltage drop/power consuming in my case, if possible I prefer mosfets because they consume less power than Bjt.

Here is another similar driver but for 100 watt Led:

Thanks a lot to @ronsimpson @bassbindevil and thanks a lot to all helps.

#### ericgibbs

Joined Jan 29, 2010
17,129
hi pE,
Do you have a photo shot of the Back of that LED panel.?
E

#### planetElectron

Joined May 5, 2022
17
Do you have a photo shot of the Back of that LED panel.?
Yes, I can take it, just a plane metal back,

here is front,

#### BobTPH

Joined Jun 5, 2013
6,268
Since they did not put the LEDs in parallel, they did not need the match them, so they presumably did not.

My advice: put the resistors back in the curcuit and power it with a USB power bank. This gives you all the charging and protection circuitry that you would need anyway, and you can probably get it cheaper than buying the battery separately.

#### ericgibbs

Joined Jan 29, 2010
17,129
hi pE,
What is this circuit PCB being used for, shown in this clip.?

E

#### dl324

Joined Mar 30, 2015
15,510
here is front,
The copper traces are painted over. Can you trace them and make a schematic?

Since there are 10 resistors and 10 LEDs, that makes me think each LED has its own current limiting or ballast resistor.

#### planetElectron

Joined May 5, 2022
17
Since they did not put the LEDs in parallel, they did not need the match them, so they presumably did not.

My advice: put the resistors back in the curcuit and power it with a USB power bank. This gives you all the charging and protection circuitry that you would need anyway, and you can probably get it cheaper than buying the battery separately.
I don't have or use power bank, and their prices are getting higher nowadays here. And one more thing, it is inconvenient to use with a power bank, but with the built-in battery the bulb is portable and I can hook it up everywhere I need.

#### planetElectron

Joined May 5, 2022
17

#### planetElectron

Joined May 5, 2022
17
Since there are 10 resistors and 10 LEDs, that makes me think each LED has its own current limiting or ballast resistor.
Yes, I already traced them, you are right, each LED has its own current limiting resistor, I don't think you will need a diagram. The board is originally designed for 5V input.

#### BobTPH

Joined Jun 5, 2013
6,268
How are you planning to charge your battery?

#### planetElectron

Joined May 5, 2022
17
How are you planning to charge your battery?
I use a TP4056 module.

Since they did not put the LEDs in parallel, they did not need the match them, so they presumably did not.
Now I just tested voltages, the voltage between two electrodes of each LED is the same, 3.18V.
Is that OK?
Battery voltage when switched off is 3.87V, two parallel 1.5 ohms resistors in series with Leds, and the protection circuit of TP4056 has a few internal resistance.

#### BobTPH

Joined Jun 5, 2013
6,268
They are all in parallel, so the voltage has to be the same. It is the current at that viltage that will differ.

#### eetech00

Joined Jun 8, 2013
3,418
I want to copy transistor driver design marked in blue, but I don't know which part numbers to use for lowest possible voltage drop/power consuming in my case, if possible I prefer mosfets because they consume less power than Bjt.
The circuit in blue is not a constant current driver, it is a current limiter circuit.