LEDs, internal resistors, large Vf - Electronic Circuits Projects Diagrams Free

**3v0** · 10th June 2008, 10:39 PM

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 · 11th June 2008, 01:29 AM

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** · 11th June 2008, 04:05 AM

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 · 11th June 2008, 05:29 AM

Maybe they included diodes or resistors on that same substrate?

mneary · 11th June 2008, 08:24 PM

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** · 19th July 2008, 12:33 PM

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 · 19th July 2008, 01:20 PM

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 · 19th July 2008, 05:13 PM

Quote:

Originally Posted by ericgibbs

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 · 19th July 2008, 05:18 PM

Quote:

Originally Posted by audioguru

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:

http://www.leddealer.com/Multi%20Vol...0Datasheet.pdf
http://www.leddealer.com/newproducts.html

Hero999 · 19th July 2008, 05:47 PM

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 · 19th July 2008, 05:47 PM

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 · 19th July 2008, 05:55 PM

Quote:

Originally Posted by 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.

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** · 19th July 2008, 06:08 PM

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 · 19th July 2008, 06:38 PM

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 · 19th July 2008, 07:21 PM

If you look at the Lumex catalog. http://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.

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10th June 2008, 10:39 PM	(permalink)
3v0	LEDs, internal resistors, large Vf 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. __________________ search engine for electronic parts Junebug USB PIC programmer kit., USB Bit Wacker, Homepage The 15 Minute Printed Circuit Board! (+drill time)

11th June 2008, 01:29 AM	(permalink)
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. __________________ Uncle $crooge

11th June 2008, 04:05 AM	(permalink)
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. __________________ search engine for electronic parts Junebug USB PIC programmer kit., USB Bit Wacker, Homepage The 15 Minute Printed Circuit Board! (+drill time) Last edited by 3v0; 11th June 2008 at 04:05 AM.

11th June 2008, 05:29 AM	(permalink)
DigiTan	Maybe they included diodes or resistors on that same substrate? __________________ The most-hated young engineer in America. * AVR Projects * Robot War: TI * About

11th June 2008, 08:24 PM	(permalink)
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.

19th July 2008, 12:33 PM	(permalink)
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. __________________ search engine for electronic parts Junebug USB PIC programmer kit., USB Bit Wacker, Homepage The 15 Minute Printed Circuit Board! (+drill time)

19th July 2008, 05:47 PM	(permalink)
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. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez

19th July 2008, 06:08 PM	(permalink)
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. __________________ search engine for electronic parts Junebug USB PIC programmer kit., USB Bit Wacker, Homepage The 15 Minute Printed Circuit Board! (+drill time)

19th July 2008, 06:38 PM	(permalink)
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. __________________ Uncle $crooge

19th July 2008, 07:21 PM	(permalink)
Mikebits	If you look at the Lumex catalog. http://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.