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25 Volt LED?

H

Holden Bonwit

Jan 1, 1970
0
I was curious if anyone knew of a vendor who makes/sells high voltage
LEDs. I have a 25 volt power supply, and for various reasons don't
want to step down the voltage to ~12 volts with a voltage regulator.

If it helps, or makes a difference, I'm looking for an IR led with a
wavelength of about 830 nanometers, but would like to hear of any
suggestions that people have for LED vendors.

Thanks,

Holden
hbonwit04 at yahooDOTcom
 
J

John Larkin

Jan 1, 1970
0
I was curious if anyone knew of a vendor who makes/sells high voltage
LEDs. I have a 25 volt power supply, and for various reasons don't
want to step down the voltage to ~12 volts with a voltage regulator.

If it helps, or makes a difference, I'm looking for an IR led with a
wavelength of about 830 nanometers, but would like to hear of any
suggestions that people have for LED vendors.

Thanks,

Holden
hbonwit04 at yahooDOTcom


All IR led's have about the same inherent voltage drop, 1.2 volts or
so. Just add a series resistor.

John
 
R

Roger Hamlett

Jan 1, 1970
0
Holden Bonwit said:
I was curious if anyone knew of a vendor who makes/sells high voltage
LEDs. I have a 25 volt power supply, and for various reasons don't
want to step down the voltage to ~12 volts with a voltage regulator.

If it helps, or makes a difference, I'm looking for an IR led with a
wavelength of about 830 nanometers, but would like to hear of any
suggestions that people have for LED vendors.

Thanks,
Seriously, even if such a high voltage LED existed, you would need to
current regulate, since the forward drop will change with temperature.
Ideally, you need to get an LED with drive electronics built in. 25v units
are quite common, since trucks use nominally 24v electronics, and many now
use LED rear lights, and instrument panel lights like this. Many are made
by SLi http://www.sli-ml.com/led.htm, and Agilent, but these companies
rarely do IR LEDs, so it does make a large difference that this is the
frequency band needed.
You don't say what sort of brightness you require.
Remember also, that you can series connect several LED's, and minimise the
resistance that has to be used. So (for instance), you could series
connect perhaps 9 IR LEDs, and then have perhaps a 25R resistor to limit
the current (you would need to check the typical forward voltage at the
rated current, but normally this is about 2.4v, for the AIGaAs/GaAS
Technology normally used at this frequency). This would be for a 100mA
rated LED, like the QLD-830-100S.
Some of the LED manufacturers do offer small switch mode drive modules
that will cover this voltage range.

Best Wishes
 
M

Michael A. Covington

Jan 1, 1970
0
Quantum physics dictates that LED voltage is related to wavelength -- about
1.8 volts for red, 3.6 volts for blue, 4 volts for ultraviolet (the white
ones are ultraviolet plus a phosphor).

All LEDs require current-limiting resistors. There are no 12-volt LEDs
either; LED lamps that run on 12 volts have the resistors built in.
 
P

Pooh Bear

Jan 1, 1970
0
Holden said:
I was curious if anyone knew of a vendor who makes/sells high voltage
LEDs. I have a 25 volt power supply, and for various reasons don't
want to step down the voltage to ~12 volts with a voltage regulator.

If it helps, or makes a difference, I'm looking for an IR led with a
wavelength of about 830 nanometers, but would like to hear of any
suggestions that people have for LED vendors.

The operating voltage of a single LED is typically around 1.6 Volts. This
is determined by semiconductor physics.

You can run a single LED by adding a series resistor which sets the
current. It wastes most of the power in the resistor though.

You could run, say, 12 LEDs in series ( operating voltage approx 19.2 V )
and lose less power in the current limiting resistor.

'LED lamps' use multiple LEDs like this. A.C. versions also require a
rectifier to provide DC to the LEDs. LEDs only work with current flowing
in one direction, so need D.C. current.

In short, there is no *high voltage LED* ( except several LEDs in series
) - you have to work within the constraints of science.

'Clever circuitry' can be used to optimise efficiency - but you're
clearly not operating in this region of expertise.


Graham
 
P

Paul Burridge

Jan 1, 1970
0
Quantum physics dictates that LED voltage is related to wavelength -- about
1.8 volts for red, 3.6 volts for blue, 4 volts for ultraviolet (the white
ones are ultraviolet plus a phosphor).

All LEDs require current-limiting resistors. There are no 12-volt LEDs
either; LED lamps that run on 12 volts have the resistors built in.

I got some ultrabright whites the other day. Not much info on 'em,
though. Typical values only given; no maximums. Vfwd was 3.4V; Ifwd
was 30mA. Can anyone hazard a guess as to what the *do not exceed*
figures might be? They tend to draw more current as they get hotter
and when bunched close together and they do generate quite a bit of
heat. I'm worried I might exceed the 30mA (typ.) figure by too much
and blow 'em.
 
K

Ken Smith

Jan 1, 1970
0
Quantum physics dictates that LED voltage is related to wavelength -- about
1.8 volts for red, 3.6 volts for blue, 4 volts for ultraviolet (the white
ones are ultraviolet plus a phosphor).

At least some white ones are blue and a phosphor and have a drop of about
3.5V.
 
D

Dave_S

Jan 1, 1970
0
Paul Burridge said:
I got some ultrabright whites the other day. Not much info on 'em,
though. Typical values only given; no maximums. Vfwd was 3.4V; Ifwd
was 30mA. Can anyone hazard a guess as to what the *do not exceed*
figures might be? They tend to draw more current as they get hotter
and when bunched close together and they do generate quite a bit of
heat. I'm worried I might exceed the 30mA (typ.) figure by too much
and blow 'em.

The formula for calculating the resistor to use for current limiting is:

R = Supply voltage - Forward voltage drop of LED
--------------------------------------------
Desired current

Example:

R = 5V - 1.6V = 3.4 = 340 Ohms (330 Ohms will do fine [standard value])
---------- ----
10 MA .010
 
P

Paul Hovnanian P.E.

Jan 1, 1970
0
Michael A. Covington said:
Quantum physics dictates that LED voltage is related to wavelength -- about
1.8 volts for red, 3.6 volts for blue, 4 volts for ultraviolet (the white
ones are ultraviolet plus a phosphor).

Maybe an X-ray LED will run at 24V. ;-)
All LEDs require current-limiting resistors. There are no 12-volt LEDs
either; LED lamps that run on 12 volts have the resistors built in.

Or a combination of series LEDs and a resistor, or (if efficiency is
critical) a switched mode regulator. I've seen some app notes for LED
drivers, but not having done any LED work recently, I can't lay my hands
on the part numbers as quickly as a good search engine might.
 
W

w_tom

Jan 1, 1970
0
Wow! So many responses saying LED is not possible. Any low
voltage incandescent indicator lamp - any voltage AC or DC -
is sold as an LED from www.ledtronics.com and from other
companies. They require the lamp number and whether power
will be AC or DC. That's it.

Many decades ago, an L1011 crashed into the Everglades when
the flight crew, so busy trying to determine if an indicator
bulb had gone out, accidentally disconnected the automatic
pilot. (It also resulted in the book Ghost of Flight 401).
As a result, indicator lamps on aircraft even way back then
were replaced with LEDs. Did they change the voltage? Of
course not. They simply changed incandescent indicator lamps
with LED equivalents. The technology is that old - decades
old.
 
C

cpemma

Jan 1, 1970
0
Paul said:
I got some ultrabright whites the other day. Not much info on 'em,
though. Typical values only given; no maximums. Vfwd was 3.4V; Ifwd
was 30mA. Can anyone hazard a guess as to what the *do not exceed*
figures might be?

It's worth getting Kingbright's LED Application Note, it has current/voltage
curves & max specs for most led chemistries, plus tips on fitting,
soldering, etc.
http://www.kingbright-led.com/Data/Spec/ApplicationNote.pdf

The 3 white types they produce are each 30mA max If, though you can go
considerably higher by pulsing them. But heat is still the problem.
 
J

John Miller

Jan 1, 1970
0
w_tom said:
As a result, indicator lamps on aircraft even way back then
were replaced with LEDs. Did they change the voltage? Of
course not.

Not the voltage of the aircraft, and probably not the voltage going to the
socket, but they certainly had to change the voltage going to the diode(s)
proper (said circuitry would likely be part of the LED assembly, if they
wanted it to be a "drop-in" replacement).

--
John Miller
Email address: domain, n4vu.com; username, jsm

A political man can have as his aim the realization of freedom, but he has
no means to realize it other than through violence.
-Jean Paul Sartre
 
D

Don Klipstein

Jan 1, 1970
0
I got some ultrabright whites the other day. Not much info on 'em,
though. Typical values only given; no maximums. Vfwd was 3.4V; Ifwd
was 30mA. Can anyone hazard a guess as to what the *do not exceed*
figures might be? They tend to draw more current as they get hotter
and when bunched close together and they do generate quite a bit of
heat. I'm worried I might exceed the 30mA (typ.) figure by too much
and blow 'em.

You should also have thermal resistance data and maximum junction
temperature. At least most of these figures are similar enough for
"usual" 5 mm LEDs - the thermal resistance from the junction to 5 mm
outside the case on the lead that the chip is mounted to is generally
around 250 degrees C per watt. Temperature rise of the chip above the
temperature at that point is thermal resistance times the amount of power
being dissipated in the chip.
If you want really long life, have the junction temperature not exceed
85 degrees C. If you can tolerate a shorter life, it's OK to go up to
around 100, maybe 110 degrees C or so.

Note that the junctions in a cluster lamp can sometimes get that hot at
less than 30 mA through each LED.

- Don Klipstein ([email protected])
 
D

Don

Jan 1, 1970
0
Foe real bright LEDs look at "Luxeon Star LED" they are super.
 
W

w_tom

Jan 1, 1970
0
The point is that LED assemblies have long been made as a
replacement for any incandescent indicator lamp. Same even
for traffic lights whereby LEDs now replace incandescent
lamps.

The original poster wants an LED light that can operate at
25 volts. Do they make same incandescent lamp replacements
for 830 nm LEDs? I don't know. But back when we were first
purchasing these incandescent lamp replacements, they were
willing to do any existing LED color as a replacement for any
incandescent lamp. Often these assemblies contain four or
more LEDs meaning they also have good intensity.

LED replacement have long existed for most every
incandescent indicator lamp.
 
D

Dr Engelbert Buxbaum

Jan 1, 1970
0
Holden said:
I was curious if anyone knew of a vendor who makes/sells high voltage
LEDs. I have a 25 volt power supply, and for various reasons don't
want to step down the voltage to ~12 volts with a voltage regulator.

LEDs are current, not voltage driven. In other words, you need a current
limiting resistor in series with the LED. You can calculate the
resistance required by (Vin - Vled)/ current, current is usually 20 mA.
If the voltage is likely to change, or if you are to lazy to do the
calculation, use a FET as constant current source, I have posted the
schematic here a couple of times, use google.

You can also buy LEDs with that FET integrated on the chip, these work
on any DC source up to 30 V.
 
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