To All, I need to understand a few things about LED's, such as amperage,
current limiting and light output.. Anyone an expert that can explain things
and answer my further rising questions in terms an electronics hack (myself)
can understand...

Thanks
Bob in phx...

bobinphx wrote:

To All, I need to understand a few things about LED's, such as amperage,
current limiting and light output.. Anyone an expert that can explain things
and answer my further rising questions in terms an electronics hack (myself)
can understand...

Thanks
Bob in phx...


Well, as a start it is generally assumed that an LED when under
forward bias and lit up has 2V across it with 20mA flowing through it.
This allows you to calculate a series resistor with any applied voltage.
Does this help? Any other questions?

Bob WB0POQ

bobinphx wrote:

To All, I need to understand a few things about LED's, such as amperage,
current limiting and light output.. Anyone an expert that can explain things
and answer my further rising questions in terms an electronics hack (myself)
can understand...

Thanks
Bob in phx...


Well, as a start it is generally assumed that an LED when under
forward bias and lit up has 2V across it with 20mA flowing through it.
This allows you to calculate a series resistor with any applied voltage.
Does this help? Any other questions?

Bob WB0POQ

The advice about current is a good one. Be particularly watchful of the
specs on odd colored LEDs. White and Blue LEDs often require more voltage
to begin emitting. Many of the brighter LEDs will draw somewhat more
current.

If you are limiting the current with a simple series resistor, presume the
LED will drop the specification voltage, and select the resistor to drop the
remainder of the voltage at the specification current. The problem with
this approach is that you often end up dropping a bunch of volts, and so you
can dissapate a lot of power in the resistor. It can be a little startling
to see a little LED fry a resistor.

Unlike incandescent lamps, which fail abruptly, an LED has a half-life which
is dependent (in a very non-linear way) on the current. A typical LED, run
at the manufacturer's specs, will loose half it's intensity after about 5
years of on time. Decreasing the current will improve this somewhat, but
not a lot. On the other hand, increasing the current will shorten the life
a lot. There is a sort of avalange effect where a little too much current
will quickly fry the LED. It can be quite surprising how much current an
LED can suck up, for a short period of time(!).

When you are close to the specification current, changing the current a
little one way or the other doesn't seem to affect the output all that much.
This is handy if you are concerned, for example, with battery life. By
sacrificing a little light output you can often save quite a bit of current.

...

"W7TI" wrote in message
...
On Wed, 27 Aug 2003 20:25:19 -0700, "bobinphx"
wrote:

To All, I need to understand a few things about LED's, such as amperage,

The advice about current is a good one. Be particularly watchful of the
specs on odd colored LEDs. White and Blue LEDs often require more voltage
to begin emitting. Many of the brighter LEDs will draw somewhat more
current.

If you are limiting the current with a simple series resistor, presume the
LED will drop the specification voltage, and select the resistor to drop the
remainder of the voltage at the specification current. The problem with
this approach is that you often end up dropping a bunch of volts, and so you
can dissapate a lot of power in the resistor. It can be a little startling
to see a little LED fry a resistor.

Unlike incandescent lamps, which fail abruptly, an LED has a half-life which
is dependent (in a very non-linear way) on the current. A typical LED, run
at the manufacturer's specs, will loose half it's intensity after about 5
years of on time. Decreasing the current will improve this somewhat, but
not a lot. On the other hand, increasing the current will shorten the life
a lot. There is a sort of avalange effect where a little too much current
will quickly fry the LED. It can be quite surprising how much current an
LED can suck up, for a short period of time(!).

When you are close to the specification current, changing the current a
little one way or the other doesn't seem to affect the output all that much.
This is handy if you are concerned, for example, with battery life. By
sacrificing a little light output you can often save quite a bit of current.

...

"W7TI" wrote in message
...
On Wed, 27 Aug 2003 20:25:19 -0700, "bobinphx"
wrote:

To All, I need to understand a few things about LED's, such as amperage,

The best tutorials are on Agilent's website. For controllers, National Semi
and Linear Tech have very interesting articles.

One thing which I will point out is that each LED (generally) needs its own
current limiting resistor. Don't try to parallel LEDs with only one current
limiting resistor.

LED brightness is also affected by ambient temperature.

Lastly, there are some good current regulation circuits out there -- LEDs
can be used in densitometric (where you need 60+dB of linear performance)
applications if you tightly control the current.



"bobinphx" wrote in message
news:3Ae3b.74955$kP.24778@fed1read03...
To All, I need to understand a few things about LED's, such as amperage,
current limiting and light output.. Anyone an expert that can explain
things
and answer my further rising questions in terms an electronics hack
(myself)
can understand...

Thanks
Bob in phx...

The best tutorials are on Agilent's website. For controllers, National Semi
and Linear Tech have very interesting articles.

One thing which I will point out is that each LED (generally) needs its own
current limiting resistor. Don't try to parallel LEDs with only one current
limiting resistor.

LED brightness is also affected by ambient temperature.

Lastly, there are some good current regulation circuits out there -- LEDs
can be used in densitometric (where you need 60+dB of linear performance)
applications if you tightly control the current.



"bobinphx" wrote in message
news:3Ae3b.74955$kP.24778@fed1read03...
To All, I need to understand a few things about LED's, such as amperage,
current limiting and light output.. Anyone an expert that can explain
things
and answer my further rising questions in terms an electronics hack
(myself)
can understand...

Thanks
Bob in phx...

.....[snip]....
One thing which I will point out is that each LED (generally) needs its own
current limiting resistor. Don't try to parallel LEDs with only one current
limiting resistor.
.....[snip]....

But, if you have a few extra volts, they CAN be placed in series.
Aren't the super-bright traffic-light LEDs usually 5-6 in series?

--Myron, W0PBV.
--
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTX). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

.....[snip]....
One thing which I will point out is that each LED (generally) needs its own
current limiting resistor. Don't try to parallel LEDs with only one current
limiting resistor.
.....[snip]....

But, if you have a few extra volts, they CAN be placed in series.
Aren't the super-bright traffic-light LEDs usually 5-6 in series?

--Myron, W0PBV.
--
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTX). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

voltage / resistance = current. simple ohm's law.

E
I R

voltage / current = required resistance


"Bob Liesenfeld" wrote in message
...


bobinphx wrote:

To All, I need to understand a few things about LED's, such as amperage,
current limiting and light output.. Anyone an expert that can explain
things
and answer my further rising questions in terms an electronics hack
(myself)
can understand...

Thanks
Bob in phx...


Well, as a start it is generally assumed that an LED when under
forward bias and lit up has 2V across it with 20mA flowing through it.
This allows you to calculate a series resistor with any applied voltage.
Does this help? Any other questions?

Bob WB0POQ