LEDs, internal resistors, large Vf

[3v0]

PHAnderson sells LEDs with a built in current limiting resistor so you can connect them directly to 5V. These are very handy for use with solderless breadboards. I have been looking for another source as he only has them in red.

The Newark site shows LEDs with Vf ranging from 1V to over 100V. If I buy one with a Vf of 5V am I getting a LED that does not require an external resistor to work at 5V?

If the large Vf is not due to an internal resistor what is happening? A link would be sufficent on this last question.

Thanks.

 

[audioguru]

Some white, blue and green LEDs have a forward voltage of 5V at high pulses of current. They need a higher voltage and a current-limiting resistor.
Red LEDs are about only 1.8V.

 

[3v0]

This LED is red. The datasheet lists Vf as 5V and does not say anything about using an internal resistor to get from about 1.8V which is normal for RED to the listed Vf of 5.

So either REDs can have a Vf over twice what is normal or these have a internal resistor that drops the other 2.2V. Does anyone know which it is?

I could order one and see if 5V with no resistor blurns it out.

 

[DigiTan]

Maybe they included diodes or resistors on that same substrate?

 

[mneary]

Since the same LED is available in three versions (1.8V, 5V, and 12V) I would expect to see either a resistor or active current regulator.

 

[3v0]

I have confirmed what mneary said.


SSL-LX3044GD-5V — LUMEX — LED - Standard
Forward Current:12mA
Forward Voltage:5V

Hooked directly between +5 and Gnd draws 11 mA
according to my 20+ year old RS DMM.

You can get them for about 25 cents at mouser or newark.

Why do you care ? These things are great for breadboarding.

 

[ericgibbs]

hi,
Its most likely a internal CC source of 12mA.

You can also buy LED's that will work over a range of voltages without an external resistor.

 

[audioguru]

You can also buy LED's that will work over a range of voltages without an external resistor.

I disagree.
The range of voltages is what you can expect to see.
If you feed 2.65V to an amber LED that has a forward voltage of 1.9V then it will burn out.
If you feed 1.9V to an amber LED that has a forward voltage of 2.65V then it won't light.

If you connect an LED with a forward voltage of 2.65V in parallel with an LED with a forward voltage of 1.9V then only the 1.9V one will light.

 

[ericgibbs]

I disagree.
The range of voltages is what you can expect to see.
If you feed 2.65V to an amber LED that has a forward voltage of 1.9V then it will burn out.
If you feed 1.9V to an amber LED that has a forward voltage of 2.65V then it won't light.

If you connect an LED with a forward voltage of 2.65V in parallel with an LED with a forward voltage of 1.9V then only the 1.9V one will light.

Hi,
You can disagree, BUT I have bought and used them in my products.
They have an inbuilt constant current source, so do not require an external resistor.

IF you took the care to read my post carefully, you can see, I am not talking about standard LED's.

EDIT:

**broken link removed**
**broken link removed**

 

[Hero999]

audioguru is talking nonsense, you can get LEDs with buit-in resistors so i can't see why you can't get them with inbuilt current regulators.

It's easy to make a current regulator, just tie a JUGFET's gate to its source and the manufacturer could easilly integrate this into the same package as the LED.

 

[audioguru]

Hi Eric,
You posted a list of the standard typical forward voltage range of ordinary LEDs that need a current-limiting resistor added. Tese ordinary LEDs do not have a constant current source because their voltage range is the same as ordinary LEDs that do not have a constsnt current source.

Here is a graph showing an ordinary red LED and what happens if it does not have a current-limiting device.

 

[ericgibbs]

Hi Eric,
You posted a list of the standard typical forward voltage range of ordinary LEDs that need a current-limiting resistor added. Tese ordinary LEDs do not have a constant current source because their voltage range is the same as ordinary LEDs that do not have a constsnt current source.

Here is a graph showing an ordinary red LED and what happens if it does not have a current-limiting device.

hi agu,
I posted that small chart of conventional LED's, as part of the ongoing discussion, for reference only.

I appreciate that standard LED's will not stand a lot of abuse, I have used them a 'zillion' ways on 'zillions of projects.

The easy way, is to use a low cost pnp transistor, a bog standard diode and a couple of resistors and make a 15mA CC source.

It will drive the full colour spectrum of LED's quite happily from 12V thru 30v.

 

[3v0]

If they listed internal CC regulation or internal resistor in the datasheet or the catalog I would not have started this thread.

It looks like if the Vf is outside what you expect for the color it must have one or the other.

 

[audioguru]

The LUMEX datasheet is written in an old font like they use in Taiwan.
It is very dim like old green LEDs used to be and then their forward voltage was half of this one.
Therefore it has two LED chips inside (it has two bubbles on its bottom), each with a typical forward voltage of 2.5V (3.5v max). It might work from 5.0V without a current-limiting circuit.

 

[Mikebits]

If you look at the Lumex catalog. https://www.lumex.com/mastercatalog2007.aspx
The SSL-LX3044xD family of LED's can be ordered with or without an internal resistor which is an option. The -5V suffix most likely has internal resistor. There is also a -12V option.

 

[3v0]

The LUMEX datasheet is written in an old font like they use in Taiwan.
It is very dim like old green LEDs used to be and then their forward voltage was half of this one.
Therefore it has two LED chips inside (it has two bubbles on its bottom), each with a typical forward voltage of 2.5V (3.5v max). It might work from 5.0V without a current-limiting circuit.

I need some basic understanding. I was under the impression that when voltage supplied equaled forward voltage you still needed external resistance to limit the current. Is this wrong ?

 

[3v0]

If you look at the Lumex catalog. https://www.lumex.com/mastercatalog2007.aspx
The SSL-LX3044xD family of LED's can be ordered with or without an internal resistor which is an option. The -5V suffix most likely has internal resistor. There is also a -12V option.

As in
Products > LEDs > Thru-Hole > Round > 3mm > Electrical Features > Resistor Chip LED

 

[audioguru]

I need some basic understanding. I was under the impression that when voltage supplied equaled forward voltage you still needed external resistance to limit the current. Is this wrong ?

Yes, you need to have something to limit the current.
You don't know the exact voltage of an LED unless you measure it. The spec for an LED is a range of voltages. Each LED has a different voltage.

In order for the LED to conduct current, the series resistor needs to have a voltage across it that is in addition to the forward voltage of the LED:
2.0V red LED.
3.0V supply.
For a current of 25mA then the series resistor must be (3V - 2V)/25mA= 40 ohms. A 39 ohm resistor is a standard value and would limit the current to 1/39= 25.6mA.
If the forward voltage of the LED is actually 2.5V then the current with the same resistor is only half.

If the supply has a higher voltage then the current variation is less:
2.0V LED.
4.0V supply.
An 82 ohm resistor limits the current to 24.4mA.
If the LED is actually 2.5V then the current is reduced to 18.3mA which is much more than half.

If the forward voltage is exactly 3.0V and you applied a 3.0V supply witjout a curremt-limiting resistor then the current could be a few mA or a few A. a little keychain LED flashlight uses the internal resistance of the button battery cells to limit the current.

 

[saturn1bguy]

I was under the impression that when voltage supplied equaled forward voltage you still needed external resistance to limit the current. Is this wrong ?

It is always wise to limit the current. Theoretically, if one could maintain the precise Vf of a particular device within a very tight tolerance, it might operate nominally. But this is not practical. Always limit the current in an LED, whether by dropping resistor or CC source. It's easy and cheap, and then Mr. LED can live it's nominally long lifetime

Most LEDs do not sport internal dropping resistors or current limiting devices because it leaves the device more open to specific operating requirements of the end user. The only real trend to this is where LEDs are used as pilot lights, etc, at 5V, 12V, etc.

 

[3v0]

I understand everything but the last paragraph.

If the forward voltage is exactly 3.0V and you applied a 3.0V supply witjout a curremt-limiting resistor then the current could be a few mA or a few A. a little keychain LED flashlight uses the internal resistance of the button battery cells to limit the current.

Are you saying when supply voltage = forward voltage ohms law still applies but we can not depend on an internal R for the LED so I is undetermined?

For most knowing how pick a current limiting resistor is good enough. But this has always bugged me. I would like to have a good answer when one of the kids asks me why. Up to now I have made do with saying that LEDs are greedy and without the limit resistor they may burn out. More of a what will happen then a how or why.