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.

Thanks that was informative.
here is my DMM

I measure current using ammeter by getting the probes in series between resistor and LED
purpose is to get constant current in all LED's 1.44 mA +/- 0.09mA

 

hi T,
Basic [very] Constant Voltage regulator.
E

 

hi T,
I guess you realize by using two White LEDs in series, for 5 pairs, you will reduce your current draw on the battery by approx half, compared to 10 off, single LED/Resistor circuits.??
E

 

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.

View attachment 273553

This may help you:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

 

You may be trying to achieve the impossible, or at least the very difficult. 1.44mA +/- 0.09 is a tolerance of about 6%.

The typical value of Vf for these LEDs is 3.6 volts. But from the data sheet it can be as high as 4 volts and they don't specify a lower bound. So there's a tolerance of 11%.

IF the supply voltage is 5.00 volts AND Vf is 3.60 volts, the series dropping resistor must be 972 ohms to have a current of 1.44mA. But 972 ohms isn't a standard resistance value. In the E96 resistor series, 976 ohms is the closest standard value, which would yield 1.434mA. Within your bounds if supply voltage = 5.00 and Vf = 3.6 volts.

But what if Vf is 4.0 volts, which is in the specified range for the LED? With the supply voltage = 5.00, and R = 976 ohms, current = 1.025 mA, outside your goal of 1.44 – 0.09 = 1.35mA minimum.

Vf of the LED has some tolerance (which varies with temperature), resistors with a 1% tolerance are common (also varies with temperature), voltage regulators have a tolerance (7805CT can vary from 4.8 – 5.2 volts). All of these play into what the measured current will be.

Oh. And the three LEDs will have some variation between them too.

Your specification of 1.44 +/- 0.09 mA can be met, but it will take some effort. Is there a reason it must be met?

 

How important is a slight sage in LED illuminance? Illuminance will be approximately proportional to current. If is for a visual indicator, it will not make any difference and be extremely difficult to detect.

By the way, #22 copper wire is 16 millimohms per foot, with a 6" jumper that would be 0.008 ohms. I think zero ohms would be a close approximation, especially at these currents.

 

Hi,
Providing the TS keeps the LED operating current less than 2mA, he can assume a Vfwd for the NSCW100 around 2.9V to 3.0V

I would be interested in hearing why he is using 1.4mA for the application in this project.

E

 

EDIT: The zener diode voltage regulator shown in post #15 has a few problems:

The zener diode voltage regulator shown in post #22 with a 7.5V zener, a transistor emitter-follower and two series white LEDs will soon not work because a 9V battery voltage quickly drops below 8.2V.

The circuit should use parts that work well when the battery has dropped to 6V.

 

Hi,
Providing the TS keeps the LED operating current less than 2mA, he can assume a Vfwd for the NSCW100 around 2.9V to 3.0V

I would disagree with that statement. The data sheet specifies a typical Vf of 3.6 volts and a maximum Vf of 4 volts at a current of 20mA. This is due to manufacturing tolerances, differences in chemistry, etc

The graph shows Vf vs current for a typical LED. But one at the upper end of the tolerance range would shift the curve up.

 

Hi agu,
The circuit as posted clearly states that it is a BASIC circuit, and it is suitable to demonstrate to the TS that it has limitations.

The circuit should use parts that work well when the battery has dropped to 6V.

Well, why don't you post a circuit that will drive the two series 3.0Vfwd nominal from a 6V battery.??

Instead of telling the TS what will not work.

E

 

I would disagree with that statement. The data sheet specifies a typical Vf of 3.6 volts and a maximum Vf of 4 volts at a current of 20mA. This is due to manufacturing tolerances, differences in chemistry, etc

hi,
Where in my post do I mention 20mA.????

Those clips I posted, for approx 2mA are from the datasheet

 

hi,
Where in my post do I mention 20mA.????

Those clips I posted, for approx 2mA are from the datasheet

The acceptable range for Vf of the LED is specified at 20mA. Typically, it's 3.6 volts, but it could be as high as 4 volts. There's a range due to manufacturing tolerances. Pick one out of a package, and Vf is probably near 3.6 volts but it could be 4 volts and still considered acceptable.

The plot you're referring to is for a typical specimen with a Vf of 3.6 volts at 20mA. But if you happen to grab one at the edge of the the acceptable range, that typical curve will be shifted.

The key point is the Vf is not a fixed number, and trying to get a current accurate to even one decimal place across all range of conditions isn't likely.

 

Thanks that was informative.
here is my DMM

I measure current using ammeter by getting the probes in series between resistor and LED
purpose is to get constant current in all LED's 1.44 mA +/- 0.09mA

So you put the meter in the circuit and, with only that LED switched on, measured 1.44 mA on that meter, but when you switched on four of the other LEDs the current on the one being measured dropped to 1.2 mA on the meter? Is this where these numbers came from? Or did the 1.44 mA number come from the simulation and the 1.2 mA number come from a measurement with the meter?

What range was your meter set to? This is important, because the burden resistance is a function of range setting.

The spec sheet for that meter doesn't give the burden resistances (or voltages), but that is not uncommon with lower end meters. But they tend to be based on the same internals, so on the 2 mA scale the you are probably looking at over 100 Ω of burden resistance.

Where does this 1.44 mA +/- 0.09 mA requirement come from? What bad thing happens if the current is 1.34 mA that doesn't happen at 1.35 mA?

What tolerances do your components have? The voltage across the LED will differ from one LED to the next? The actual value of your resistors will not be exactly 1 kΩ.

How sure are you of that 50 Ω series resistance for your supply? That 50 Ω is very significant. With just one LED turned on, it accounts for 5% of the passive resistance. With all of them turned on, it accounts for 1/3 of it.

What is the goal, here. What problem are you trying to solve with this circuit? I'm not asking about what you think you want the current to be, I'm asking why are you building the thing in the first place. What is it's purpose? What does it have to achieve in order to satisfy that purpose?

Until you step back and address that, there is a good likelihood that you will end up needlessly chasing a solution for a problem that doesn't exist.

 

hi JC,
The TS knows and has accepted that 2mA is the current limitation for his working Vfwd range

So why do you insist on quoting the figures for 20mA, which is not related to the TS's project.??

I am fully conversant with reading datasheets, I am also aware of the spread in component parameters due to manufacturing.

Please read the TS's simple requirement and post helpful information that is relevant to his project.

E

 

I guess you refuse to understand my point. Vf falls into a range of values due to various tolerances. The manufacturer tells us that range at a single point, namely 20mA. See the snippet of data sheet I posted in post #7.

We don't know what the tolerance range is at 2mA. Or 5mA. Or 1mA. All we can know is that acceptable LEDs have a Vf that can vary by about 10% at 20mA.

The plot you're looking at is for a TYPICAL LED of this type. But if Vf can vary by 10% for typical vs. LEDs that just meet the spec at 20mA, do you somehow believe at 2mA it suddenly is a fixed number?

 

A relevant question for his stated goal is what is the range of Vf for the LEDs he is using at the current he is trying to achieve?

Is he even actually using the NSCW100 LEDs in the first place? Or are those just the closest match he could find in his simulation libraries?

Let's assume that he has the somewhat mythical "typical" LED from the data sheet and that it turns out to have exactly 2.9 V drop at the 1.44 mA that the TS is shooting for.

Now let's say that the TS has a voltage supply that delivers a rock steady 5 V output, regardless of current draw.

Now let's say that the TS finds a 1458 Ω resistor that results in pretty much exactly 1.44 mA of current in that typical LED.

Now he goes and picks another LED and replaces the one that is in the circuit. What is the min and max that the Vf for THIS device must fall within in order to meet his stated 90 uA spec?

To be at the bottom end of 1.35 mA, the Vf would need to be 3.03 V.

To be at the top end of 1.53 mA, the Vf would need to be 2.77 V.

So the question is how reasonable it is to assume that these LEDs are going to have a Vf, at 1.44 mA, that is within this 260 mV range?

The data sheet doesn't give much information to go on for that. The only indication we have is that, at 20 mA, the Vf is typically 3.6 V, but could be as high as 4.0 V and we don't know how low it might go. But, given this much variability at 20 mA, I'm pretty skeptical that the actual Vf for these LEDs will all fall within the needed range at 2 mA.

Plus, we have to keep in mind that this is the allowable tolerance when every other component in the circuit is exactly what it needs to be and when the temperature is fixed.

Leaving all of that aside, there is the question of the inherent variability in the light output of the LEDs to begin with. The spec sheet would seem to indicate that the luminous intensity of the LEDs can vary wildly, even within a single delivery. It appears that +/- 15%, even within the same intensity rank, has to be allowed for.

So it really all still comes back to what the TS truly needs? What is it they are trying to achieve? What is the underlying problem are they trying to solve?

 

Based on the range given at 20mA, Vf could be as high as 3.4 volts at 2mA, but we can't know since it's not documented.

Again, the point is that the Vf for a particular LED out of the package isn't guaranteed and can vary widely.

 

Based on the range given at 20mA, Vf could be as high as 3.4 volts at 2mA, but we can't know since it's not documented.

Again, the point is that the Vf for a particular LED out of the package isn't guaranteed and can vary widely.

I'm not sure what the best way is to try to translate the tolerance range from 20 mA down to 2 mA, though I'm sure it has been studied and reported on extensively.

The approach that comes to mind is to look at scaling. At 20 mA, 4 V (for a typical device) looks like it corresponds to 40 mA. So, taking that as a starting point, we might surmise that the range of Vf might go from what it would be at half the current to what it would be at twice the current. This ignores factors related to why an upper limit is specified but not a lower one, which could indicate that devices are tested for compliance with the upper limit but devices below the corresponding lower limit are allowed to stay. No way to know without info from the manufacturer that we simply don't have, so I'll opt to continue ignoring it.

Assuming this is reasonable, that puts the range at 20 mA from about 3.3 V to 4.0 V.

Using the same approach, the range at 2 mA would be from about 2.84 V to 3.04 V. Assuming this has any merit, that puts it barely within the acceptable range when all other parameters are controlled perfectly, meaning that there is really too little margin for any reasonable change at success.

If the TS really needs the current to be controlled to that degree, then they need to use a current source per LED. That's actually not too difficult, but I still suspect that it is either needless complexity to meet a spec that doesn't matter, or that the spec, even if it's an attempt to achieve a goal that does matter, such as constant luminous intensity, isn't sufficient to actually do so.

 

Fair point – it should have been a percentage. Again, the bottom line is Vf is not a constant and may not be the expected value.

 

hi JC,
You are still missing the point of the TS's query/question.

Your posts #35 and #37 are not relevant to the TS's project.
With respect, you should address your input to directly helping the TS, rather than defaulting the posts from others, which have been based on the limited information the TS has provided.

Until he tells us otherwise, he is asking for an LED current of approx 1.4mA, which from the typical d/s value is possible, giving a Vfwd of approx 2.9/3.0 Volts.

E