Hello,

I have connected 10 LED's in parallels with switches (not shown in circuit diagram) and I want 1.44 mA of current flowing through any of the LED that I switch ON, but I have noticed that when I switch on 5 or more then 5 LED's then current them is reduced to from 1.44 mA to 1.2 mA (actual measurement from DMM).

I searched for the reason on internet and reason came because of resistance inherent in voltage source the voltage doesn't stay same in circuit.

Is there any solution to this problem.

 

Yes, use a more capable power source.

 

Use regulated psu

 

Well, just one LED segment consists of a 5V source (amp rating unknown at this time) and a 1KΩ resistor. That alone translates to 5mA per seg. You're reporting 1.4mA, so to get that your LED must have a forward voltage drop of approximately 3.6Vf. For us to fully calculate the current & voltage we'd need to know if that's accurate. I can't spot the error, but I believe something is wrong with your numbers. But even if you're running 5mA per seg and there are 10 of them then a supply capable of a minimum of 50mA would be required. In that case I'd opt for one capable of delivering 5V @ 100mA minimum to avoid the issue of insufficient power. Power is measured in watts and is (in a DC circuit) Volts X Amps. If when you switch all 10 of them on and they're dimming then either your supply is insufficient for the task or there's an error somehow diminishing the available current.

Standard LED's run at 1.4mA would be quite dim to begin with. AND standard LED's typically don't have that high a Vf. So there may be an "Error of Assumption". Please let us know more about the LED's and the power source.

[edit] if you're depending solely on the numbers from the sim you might not be getting the whole story. Sim's can be in error. [end edit]

 

And if this is an actual circuit on a breadboard, BB's can have notoriously poor connections.

 

Is there any solution to this problem.

Use a higher current power source.

You have your schematic drawn upside down. Ground always points down and the preferred flow in schematics is left to right and top to bottom.

current them is reduced to from 1.44 mA to 1.2 mA (actual measurement from DMM).

Note that your measurement technique is perturbing the circuit.

 

Well, just one LED segment consists of a 5V source (amp rating unknown at this time) and a 1KΩ resistor. That alone translates to 5mA per seg. You're reporting 1.4mA, so to get that your LED must have a forward voltage drop of approximately 3.6Vf. For us to fully calculate the current & voltage we'd need to know if that's accurate. I can't spot the error, but I believe something is wrong with your numbers. But even if you're running 5mA per seg and there are 10 of them then a supply capable of a minimum of 50mA would be required. In that case I'd opt for one capable of delivering 5V @ 100mA minimum to avoid the issue of insufficient power. Power is measured in watts and is (in a DC circuit) Volts X Amps. If when you switch all 10 of them on and they're dimming then either your supply is insufficient for the task or there's an error somehow diminishing the available current.

Standard LED's run at 1.4mA would be quite dim to begin with. AND standard LED's typically don't have that high a Vf. So there may be an "Error of Assumption". Please let us know more about the LED's and the power source.

[edit] if you're depending solely on the numbers from the sim you might not be getting the whole story. Sim's can be in error. [end edit]

Not to be that guy....ok, I am that guy.

The "Error of Assumption" is on Tony's part. First, white and blue LEDs typically have a Vf in the 3.6v range (and some green LEDs too depending on the chemistry). But even lacking that knowledge, the LED model is shown in the simulation picture, so the datasheet is a click away. Guess what? It's a white LED with typical Vf = 3.6v. (At the desired current, Vf will be somewhat lower.)



Modern LEDs, especially non-diffused ones, can be "spots in your eyes" bright even at 1mA.

 

On its datasheet, only a "typical" NSCW100 white LED has a forward voltage of 3.6V at 20mA. The maximum voltage is 4.0V and the minimum voltage is not listed but might be 3.2V. The datasheet shows a graph of forward voltage at various currents for a "typical" one so at 2mA the voltage is about 2.9V then the current is 2.1mA.

10 LEDs typically draw 21mA then the horrible 50 ohms series resistance of the 5V power supply reduces its voltage to 3.95V and the calculations must be done over again.

The simulator DONT NO DEES important things.

 

The "Error of Assumption" is on Tony's part.

Not to be that guy - OK, I am. I don't know everything, and I AM fallible. I found it difficult to discern the part number on the drawing. I looked closer and can see it now, but - that guy (um - me) can be lazy. Also, the results of a sim can be askew from lack of information as Audioguru again points out:

The simulator DONT NO DEES important things.

The TS states his physical measurement of the current is 1.44mA. I pointed out

if this is an actual circuit on a breadboard, BB's can have notoriously poor connections

Which I stand by. Part of the reason for the dimming or dropping off of current can be attributed to poor connections. I've personally experienced that myself. What works on a BB sometimes doesn't end up working on a fully soldered circuit. And vice versa, a BB might not work whereas a fully soldered circuit might. This isn't always true, just that sometimes you don't get the same results from a BB as you do from an actual circuit.

And we still don't know anything about the PS. We can "Assume" but you know how that makes you and me look.

 

The simularion shows 10 LEDs with 1k series resistors. Each LED is shown operating at 1.44mA then the total current from the supply is 14.4mA.
BUT Tejasvi said the current in each LED drops to 1.2mA maybe because the supply is from an old battery that is slowly running down.
I agree that breadboards frequently have poor connections.

 

You are using the LEDs you call out in the diagram? You can see the data sheet information others have posted. The only reason I can see for a voltage drop or sag is if the supply can't supply the needed current. The only other possible cause is as mentioned if you have poor connections on your breadboard. I have experienced the latter. While total current is not much is your supply up to it?

Ron

 

So, 5V - 3.6 (Vf) = 1.4V/1K yields your 1.4 ma per led.
But why are you modeling Rser in the 5V source as 50 ohms?
If driving LEDs from a regulated power supply your Rser should essentially be 0, unless you have some really poor or long wiring.
Now battery would be different.
With 5 LEDs on your 5 1K yield 200 ohms + 50 Rser gets 250 ohms. 1.4V/250 = 5.6mA div by 5 LEDs = 1.2 to 1.2ma.
In other words, the more LEDs on, the more impact the Rsers (50 ohm) has on the LED current.

 

WARNING: THREAD DERAILER AHEAD

You have your schematic drawn upside down. Ground always points down and the preferred flow in schematics is left to right and top to bottom.
Note that your measurement technique is perturbing the circuit.

The ground symbol means exactly the same no matter what the orientation is. While I prefer the traditional orientation I see no issue putting it up down or sideways, and have done that when convenient.

 

So, 5V - 3.6 (Vf) = 1.4V/1K yields your 1.4 ma per led.
But why are you modeling Rser in the 5V source as 50 ohms?
If driving LEDs from a regulated power supply your Rser should essentially be 0, unless you have some really poor or long wiring.
Now battery would be different.
With 5 LEDs on your 5 1K yield 200 ohms + 50 Rser gets 250 ohms. 1.4V/250 = 5.6mA div by 5 LEDs = 1.2 to 1.2ma.
In other words, the more LEDs on, the more impact the Rsers (50 ohm) has on the LED current.

I used 50 ohm resistance in simulation for wires like these jumper wires
My power supply is this Breadboard power supply capable of delivering current <700 mA
I get it from above post that Breadboard can have loose connections which may induce resistance.

 

The simularion shows 10 LEDs with 1k series resistors. Each LED is shown operating at 1.44mA then the total current from the supply is 14.4mA.
BUT Tejasvi said the current in each LED drops to 1.2mA maybe because the supply is from an old battery that is slowly running down.
I agree that breadboards frequently have poor connections.

I get that I can't reduce resistance to zero in Breadboard or wires,
I read somewhere that I need constant voltage source to get constant current in all LED's no matter how many LED's I light up.
can I use this circuit for constant voltage.

 

hi T471,
What is the purpose of this project, why is it necessary to have1.44mA per LED.?

Have you considered two White LED nominal 3.2Vfwd, in series.?

E

 

hi T,
This is a basic Constant Current drive circuit.
E

 

I have connected 10 LED's in parallels with switches (not shown in circuit diagram) and I want 1.44 mA of current flowing through any of the LED that I switch ON, but I have noticed that when I switch on 5 or more then 5 LED's then current them is reduced to from 1.44 mA to 1.2 mA (actual measurement from DMM).

Need some more details on your numbers. When you say "actual measurement", are you talking only about the 1.2 mA, or was the 1.44 mA also the measured value?

How did you make the measurements? By inserting the DMM as an ammeter in series with one of the LEDs? Or by measuring the voltage across the resistor in series with the LED? If the former, your meter can easily add a good portion of the needed ~170 Ω if burden resistance to account for the difference in the measurement (with breadboard connections not helping matters, either). What model DMM are you using and what range is it on?

To give an example, the MY-64 meter, on the 2 mA range, has a nominal burden resistance of 110 Ω. High quality meters (and, thus, much higher priced) have burden resistances about two orders of magnitude less than this.

 

I have connected 10 LED's in parallels with switches (not shown in circuit diagram) and I want 1.44 mA of current flowing through any of the LED that I switch ON, but I have noticed that when I switch on 5 or more then 5 LED's then current them is reduced to from 1.44 mA to 1.2 mA (actual measurement from DMM).

Let's take the results at face value.

Why does it matter? Most people are going to be hard pressed to notice a drop of roughly 15% in current in an LED.

So be sure you aren't chasing a problem that doesn't exist.

 

hi T,
This is a basic Constant Current drive circuit.
E
View attachment 273627

Thanks, can you give me a constant voltage drive circuit