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Is this a way to measure forward voltage or V-drop of a particular LED or diodes?

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Willen

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I use 'diode test/continuity test' feature to measure small red LED's forward voltage (maybe it's its input voltage rating) which is around 1.2V. But when I try to measure Yellow LED, Green, Blue or White LEDs then DMM won't response and shows just 1 or 0 ('N/A' maybe). So I need to measure its forward voltage manually. Is this a way to measure LED's or Diodes's forward voltage, posted schematic here?

I think I just get approx value from the method and won't be accurate as its manufacturer mentioned. Also probably I will get very wrong value if I tried to measure Power LEDs in the circuit. For now I just want to measure small LEDs I have around me (used as indicator LEDs).
(Accuracy is not very critical to me here.)
 

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Hello Willen,

First, very happy to see that you are still doing your electronics stuff in spite of the natural problems in your area of the world.

Second, for testing LEDs, the best way is to drive it at the current you expect to use it at. So even if it is a 20ma LED if you intend to use it at 10ma then test it at 10ma instead.
You can use a resistor and pot to set the current, then measure the voltage.
This is pretty much the best way.

If you want to get a little more into it, you can create a constant current source of maybe 10ma or 20ma and that will force the LED current so you can just measure the voltage.

I usually measure my LEDs at several current levels, like 1ma, 5ma, 10ma, 15ma, and 20ma, noting the forward voltage for each current level. That way when i go to use one, i know what it can and can not do.
 
Hello Willen,

First, very happy to see that you are still doing your electronics stuff in spite of the natural problems in your area of the world.
Hi MrAl,
I would not be an 'electronics addicted' if I was out from the discussion forum. Because you all experience engineers taught me to carefully handle each components. I scrap components from old kits and it's like a magic when these works amazingly in a new circuit again. Such thing became addiction to me so when I feel bore or upset then I start to do electronics. It makes mind so fresh and energetic! Same thing happening currently. (I am trying to make a 'Easy Pulse v1.1' from embedded-lab.com to detect heartbeat from finger tip. It's just a quad OpAmp and few component so felt exciting. I have no exact OpAmp used by designer, will use which I have though.)

Second, for testing LEDs, the best way is to drive it at the current you expect to use it at. So even if it is a 20ma LED if you intend to use it at 10ma then test it at 10ma instead.
You can use a resistor and pot to set the current, then measure the voltage.
This is pretty much the best way.

If you want to get a little more into it, you can create a constant current source of maybe 10ma or 20ma and that will force the LED current so you can just measure the voltage.

I usually measure my LEDs at several current levels, like 1ma, 5ma, 10ma, 15ma, and 20ma, noting the forward voltage for each current level. That way when i go to use one, i know what it can and can not do.
If voltage across LEDs depends on its current then why manufacturer rates its voltage like 3.2V white LED or 1.2V red LED or 4.7V zener diodes (zener's regulated voltage is also current depended). Does this mean even a 1N4007 has different forward voltage according to current through it?
 
Hi MrAl, ......

If voltage across LEDs depends on its current then why manufacturer rates its voltage like 3.2V white LED or 1.2V red LED or 4.7V zener diodes (zener's regulated voltage is also current depended). Does this mean even a 1N4007 has different forward voltage according to current through it?

Diodes are not linear in their I-V behaviour like resistors - the voltage drop does vary ( in mV) depending on the current. Whether this variation need be taken into account in the design of circuits would depend on the application.

The manufacturer's ratings of the LED voltages are at a specified current - normally the max permitted. The power dissipation is also a factor to be considered when designing driver circuits.

Zener diodes fall under a different category - they are Reverse Biased diodes and operate in the Zener breakdown range. The voltage variations w.r.t current is much smaller and they are used as voltage references unlike LED's.
 
I use 'diode test/continuity test' feature to measure small red LED's forward voltage (maybe it's its input voltage rating) which is around 1.2V. But when I try to measure Yellow LED, Green, Blue or White LEDs then DMM won't response and shows just 1 or 0 ('N/A' maybe). So I need to measure its forward voltage manually. Is this a way to measure LED's or Diodes's forward voltage, posted schematic here?

I think I just get approx value from the method and won't be accurate as its manufacturer mentioned. Also probably I will get very wrong value if I tried to measure Power LEDs in the circuit. For now I just want to measure small LEDs I have around me (used as indicator LEDs).
(Accuracy is not very critical to me here.)

Hola Willen

Hope things are improving over there. Surely it will take time.

I suspect that measuring forward voltage with that function, your DMM becomes out of range. Isn't it by chance maximum 2V? I would do simple succesive checks with one, two, three and four Si diodes in series.

Have to check that today on mine.

Post the outcome, please.
 
If the meter has a 9v battery it'l give you a ball park idea, however it wont be a accurate value unless the led has its intended current through it, which on you meter it probably wont have.
 
I confirm my post above. Checked right now and above 2 V, my DMM goes out of range.

In diode check function.
 
Developing on what others have said, try this:

Diode Plot.JPG

Adjust the variable resistor R1, and at various settings measure V1 and V2.
Divide the V1 measurements by 100 to get the current in mA.
Then you can plot a graph of V2 on the Y axis against Current on the X axis.
You should be able to see that the voltage remains fairly constant as the current is varied, but the voltage will never be completely constant as the current through the diode varies.

JimB
 
Better DMM's use ~1mA CC with >2V range for diode test so white LEDs (blue substrate) will measure 2.85V at low current .i.e. 10% If.

Since this approx 5% of 20mA rated If, it will read the Vth or threshold voltage which is always proportional to frequency if light. Since IR is the longest Wavelength or lowest frequency, then 660nmD Red then 630nmD Red , then Yellow then Green , Blue, UV etc , Vf= Vth + If*ESR when using approx 10% of If for Vth rather than 5% .


Also rated Power of LED is inverse with ESR so W*ESR ~1 at 25C using rated If not Imax as ESR drops slightly with rising current and rising temp lowers Vth. (NTC effect) Some variations due to quality vs heat capacity of chip exist.

Vth is universal for dominant wavelength Vth but apparent Vth' at 10% If is accurate enough for measuring ESR and using Ohms Law between Vth' and Vf, when Tjcn is considered


Easy test is 9V bat to LED with 2mA or (9-2)/2mA ~ 3.3k Ohms in series . You can also compute current source value of diode test from R test and V across series R.
 
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Hi MrAl,
I would not be an 'electronics addicted' if I was out from the discussion forum. Because you all experience engineers taught me to carefully handle each components. I scrap components from old kits and it's like a magic when these works amazingly in a new circuit again. Such thing became addiction to me so when I feel bore or upset then I start to do electronics. It makes mind so fresh and energetic! Same thing happening currently. (I am trying to make a 'Easy Pulse v1.1' from embedded-lab.com to detect heartbeat from finger tip. It's just a quad OpAmp and few component so felt exciting. I have no exact OpAmp used by designer, will use which I have though.)


If voltage across LEDs depends on its current then why manufacturer rates its voltage like 3.2V white LED or 1.2V red LED or 4.7V zener diodes (zener's regulated voltage is also current depended). Does this mean even a 1N4007 has different forward voltage according to current through it?

Hi,

The voltage rating of an LED is not unlike the voltage rating of most other things we see. If we go to buy a light bulb, we usually look for a given voltage, like 6v, 12v, 24v, or 120v, etc., but there are other things to consider too. LEDs are like that also. When we look for an LED we have to match the current more so then the voltage, but both are important. The LED voltage is actually called the "characteristic voltage" unlike light bulbs where the voltage is really called the "operating voltage". The characteristic voltage is just meant to be used as a quick reference so we can compare different LEDs on the fly, without being too accurate. As you have already seen, the blue LED has a higher forward voltage than most red LEDs for example, so you know that a red LED might work in applications where a blue LED wont work, because the characteristic voltage is higher for blue. A lower voltage available might be able to light a red but not a blue.

But light bulbs too have to be matched in many applications, by specifications beyond their voltage rating. For example, in a particular automobile we usually have to match up the bulb NUMBER, not just the voltage of 12 volts. That number implies not only a given voltage, but also a given base size, bulb glass size, and current rating.
Recall that if you buy a lamp for the house too, you will most likely want to get the right WATTAGE as well as the right voltage. Here in the US we use mostly 120v bulbs, but we choose between wattage ratings of 20 watts, 40 watts, 50 watts, 60 watts, 75 watts, and 100 watts just to name a few. So we need to know that it is for 120 volts, but also what wattage we want to use in that particular location.

So you start to see that a voltage specification isnt really enough for anything when we take a closer look at this, and the LED is no different in this respect. We have to know the characteristic voltage as well as the normal operating current before we know if we can use it in a given circuit. And in the hobby arena we sometimes even go one step further: we sometimes want to know what they can do at other less common current levels like 1ma for example.

Comparing a few bulbs to a few LEDs:
bulb: 120v, 50 watts
bulb: 120v, 100 watts
bulb: 230v, 100 watts
LED: white, 3.5v, 20ma
LED: white, 3.2v, 350ma
LED: blue, 3.6v, 20ma
LED: red, 1.8v, 20ma

Looking at the above we see again that we always have to know something else about either type of device as it is usually not enough to know just the voltage.
Maybe if we went to buy a radio we'd have to know it runs on 12v or 120v, but then we'd also have to know:
1. The type of current, AC or DC
2. The line frequency, like 60 Hz.

so there are almost always other things to consider. The bottom line is that if you went out to buy a bulb for a car you could not just go out looking for a "12 volt" bulb, because that is not enough information even to buy a bulb even though the operating voltage is 12v. Even more to the point, that bulb will sometimes be operating at 14.2 or so volts when the car engine is running because the alternator raises the battery voltage by at least 2 more volts. So it is called a "12v bulb" but it may actually run at something like 11.8v to 14.6v sometimes.

There are however certain LEDs that have a more specific voltage rating. These are LEDs that are made for a particular use such as an indicator lamp. You might find one that says "12v" and that means it runs at pretty close to 12v. But that is really a composite device made up of an LED and resistor, where the designer sized the resistor so the actual LED sees the right current during operation at 12v. So they take some of the guess work out of it. You might still want to know how much current it draws though so you know if your power supply can handle it.
 
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A silicon diode or transistor junction has a forward voltage rating at 0.7V. But only at a certain current and temperature. It could be from 0.4V to 1.8V at different currents and temperatures.
 
Hi,

Yes i forgot to mention the common silicon diode and related.
Thanks for bringing that up audioguru.

The diode and LED current and voltage actually follow an exponential relationship. This can be approximated near one operating point by placing a battery in series with a resistor (for forward current only which is usually more important).

So near one operating point, we can look at the LED or diode as a battery in series with a resistor. As the current though the resistor changes the total voltage drop changes:

"Linearized" versions:

o----Rs-----0.5v-----o DIODE

o----Rs-----1.6v---o RED LED

o----Rs-----3v----o WHITE OR BLUE LED

The left node of these above is the anode, and the right node is the cathode, and the batteries all have polarity plus on the left and minus on the right. The resistor values Rs will be different for each type of LED and at different operating currents.

For an example of the diode, if Rs=100 ohms and the current Id=1ma, then the total voltage drop of the diode above is 0.5+0.001*100=0.5+0.1=0.6 volts total.
With 2ma the total drop would be 0.5+0.2=0.7 volts, although this might already be out of the intended linearized range for that resistor value.
 
"All LEDs are diodes"
Which sounds silly but all diodes have a saturation curve that is falls within a 50% tolerance range for effective series resistance, I conveniently call ESR.

The rising decades of current leading up to saturation where the ESR falls sharply from 1000x the saturated ESR.

This is one way to characterize small LEDs with 20mA ratings when used less or pulsed more than ratings.

Keep in mind efficacy always drops with rising power levels, then faster above rated levels.

Note the rated power of any diode also drops the rated current ESR so this product tends to be constant and drops with large rises in junction temperature.

This ESR would included in your external R when using say a 5V supply and two Reds or a 12V supply and 3 or 4 whites.

WcbdL.jpg
 
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