What is wrong with my led circuit?

Thread Starter

khazan

Joined Nov 23, 2022
6
I designed a simple circuit for LEDs with 5V 1A.
This is the schematic, all resistances are 8.2Ω.
But when everything is ready and power 5v 1A at the input. I catch it is only 0.3A on my multimeter. It should be (5.0-3.3)/8.2*4 = 0.83A. Why is there such a big deviation?

Screenshot 2022-11-23 at 15.48.00.png
 

Ya’akov

Joined Jan 27, 2019
6,856
Welcome to AAC.

How did you arrive at 8.2Ω for the current limiting resistor?
Are you sure your 1A supply can actually provide 1A?
 

WBahn

Joined Mar 31, 2012
27,898
First off, all of your LEDs are backwards, so I'm surprised you are even getting that much current.

Having said that, if these are blue LEDs intended for forward currents in the 100 mA range, it may be possible that you are seeing 40 mA reverse currents as some of these LEDs have quite low reverse breakdown voltages.

Second, it's not a good idea to run LEDs in parallel, and that's particularly true for higher power devices (unless they are in the same package and specifically intended to be operated that way). You risk inducing thermal runaway if you do.
 

LowQCab

Joined Nov 6, 2012
2,646
Use 1 Resistor for each LED to insure equal Currents in each LED.

Are You running all 8 LEDs on the same 1-Amp Power-Supply ?

What is the Continuous-Current-Rating of each individual LED ?
.
.
.
 

bertus

Joined Apr 5, 2008
22,120
Hello,

As said, the leds are reversed biased.
Do you have a datasheet of the leds?
As said, use a resistor for each led.
Can the powersupply really give 1 Amp?
You would need some overhead, a powersupply of 1.5 Amp could do.

Bertus
 

Thread Starter

khazan

Joined Nov 23, 2022
6
Use 1 Resistor for each LED to insure equal Currents in each LED.

Are You running all 8 LEDs on the same 1-Amp Power-Supply ?

What is the Continuous-Current-Rating of each individual LED ?
.
.
.
Thank you.
I used 1 resistor for 2 LEDs in parallel. Is this not correct?
I used 1 power bank with 5V 1A for all LEDs.
The LED is 5730. I don't know if it has an exact Continuous-Current-Rating.
 

Thread Starter

khazan

Joined Nov 23, 2022
6
First off, all of your LEDs are backwards, so I'm surprised you are even getting that much current.

Having said that, if these are blue LEDs intended for forward currents in the 100 mA range, it may be possible that you are seeing 40 mA reverse currents as some of these LEDs have quite low reverse breakdown voltages.

Second, it's not a good idea to run LEDs in parallel, and that's particularly true for higher power devices (unless they are in the same package and specifically intended to be operated that way). You risk inducing thermal runaway if you do.
Thank you.
I'm sorry to make you confuse. I reversed LEDs when I was soldering.
I wanna as much as possible LEDs can work with a 5V 1A power bank.
 

Thread Starter

khazan

Joined Nov 23, 2022
6
Hello,

As said, the leds are reversed biased.
Do you have a datasheet of the leds?
As said, use a resistor for each led.
Can the powersupply really give 1 Amp?
You would need some overhead, a powersupply of 1.5 Amp could do.

Bertus
Thank you.
I reversed LEDs when I was soldering.
I use the 5730 LEDs. The seller didn't provide a datasheet for me.
I use a 5V 1A power bank.
Now I changed the circuit and use a resistor for each led.
 

LowQCab

Joined Nov 6, 2012
2,646
The number "5730" refers to the physical-size of the LED,
NOT the Current-Rating or Voltage-Drop.

Don't purchase Electronic-Parts from any source that doesn't provide a proper Specification-PDF file.
( unless You don't care if it smokes )

100mA is going to generate a lot of HEAT,
which will destroy the LEDs if the Heat is not adequately removed and dissipated.

100mA X 8 = 800mA, or 0.8-Amps
this is very close to the Maximum-Output-Rating of your Power-Supply.
The Voltage of the Power-Supply may "sag" significantly with a full-Load.

Is your Power-Supply a USB-Socket ???
USB Power-Output-Ratings are often grossly over-exaggerated, ( like most things from China ).
You will be lucky to get ~500mA from a generic USB Power-Supply
before the Output-Voltage starts to drop significantly.

You may overheat and damage a cheap USB Voltage-Regulator,
by running it with an extremely heavy-Load.
.
.
.
 
Unless you are committed to using a 5V power supply I'd recommend using something like a 19V laptop charger (most of us hang on to them when the laptop dies) and wire four LEDs in series along with a 68 Ohm 1 Watt (at least) resistor. I'm basing this on your assumed 3.3V drop per LED with a current of 100mA, although I believe the 5730 is happy with a current of up to 150mA and that the corresponding voltage drop can be from 2.8 to 3.4 Volts. In case it's only 2.8 Volts you'd be driving around 114mA and the 1W power rating of the resistor would be marginal.

Probably better to connect five LEDs in series with one resistor and put this in parallel with five more (and a resistor). Each leg will then need a 25 Ohm resistor - 2 x 47 ohm in parallel and ordinary 1/4 resistors will do.

Your actual LEDs are likely to be from the same batch but I recommend plotting their current/voltage curve, or at least establishing what the voltage drop is when you put each one in series with a 18 Ohm resistor connected to your 5V supply in order to optimise your series resistor. The nice thing about this approach, in case you are using a processor to control the brightness you can keep the resistors as low a value as you dare and PWM the LEDs legs on and off with a transistor to vary the brightness.
 
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