Figuring out LED specs

I have a bunch of loose LEDs. Some of them are completely clear. Is
there anyway that I can figure out the specs for the LEDs? I no longer
have the packaging?

i.e. how many volts they use, and their current?

I am afraid if I hook them up to check them out, I will burn them out.

mike c

 

[Mark Fergerson]

Mike Chambers wrote:

I have a bunch of loose LEDs. Some of them are completely clear. Is
there anyway that I can figure out the specs for the LEDs? I no longer
have the packaging?

i.e. how many volts they use, and their current?

I am afraid if I hook them up to check them out, I will burn them out.

First you have to sort by color. Put the leads directly across a 3V
lithium watch battery (no resistor) which will safely light red LEDs,
and won't kill them if you reverse-connect them. If it stays dark, stack
two lithium cells for 6V; same lack of reverse-voltage concern. Been
sorting LEDs this way for some time. As for ultimate current limits per
device, well, suck 'em and see.

IRLEDs will be a tad more difficult...

Mark L. Fergerson

 

[Anthony Fremont]

"Mark Fergerson" <nunya@biz.ness> wrote in message
news:YQKHd.6100$0B.5475@fed1read02...

Mike Chambers wrote:
I have a bunch of loose LEDs. Some of them are completely clear. Is
there anyway that I can figure out the specs for the LEDs? I no
longer
have the packaging?

i.e. how many volts they use, and their current?

I am afraid if I hook them up to check them out, I will burn them
out.

First you have to sort by color. Put the leads directly across a 3V
lithium watch battery (no resistor) which will safely light red LEDs,

You've been lucky. Without current limitation (perhaps your battery was
weak), pretty much any LED will burn out soon after you exceed the Vf
and get it turned on. Granted that a quick hit probably won't do it
(I've done dangerous things too ;-) I do find that many LED's are quite
durable to this kind of thing, but there are plenty that will just go
poof in the blink of an eye.

and won't kill them if you reverse-connect them. If it stays dark,
stack
two lithium cells for 6V; same lack of reverse-voltage concern. Been
sorting LEDs this way for some time. As for ultimate current limits
per
device, well, suck 'em and see.

AFAIK, 6V is likely to destroy just about any LED without some kind of
current limiting. Most LED's will light dimly if you supply as little
as 1 or 2mA.

 

Mike,

You've gotten some conflicting advice here. Just wanted to add another
vote to the ones who are saying "put a current-limiting resistor in
series with the LED". Do not hook it up directly to a battery or other
voltage source without a resistor.

Mark

 

[Mark Fergerson]

Anthony Fremont wrote:

"Mark Fergerson" <nunya@biz.ness> wrote in message
news:YQKHd.6100$0B.5475@fed1read02...

Mike Chambers wrote:

I have a bunch of loose LEDs. Some of them are completely clear.
Is there anyway that I can figure out the specs for the LEDs? I
no longer have the packaging?

i.e. how many volts they use, and their current?

I am afraid if I hook them up to check them out, I will burn them
out.

First you have to sort by color. Put the leads directly across a 3V
lithium watch battery (no resistor) which will safely light red
LEDs,

You've been lucky. Without current limitation (perhaps your battery
was weak),

Dumb Luck has always played a large part in my life; I always use
batteries from "dead" or otherwise defective watches (I suppose I might
have mentioned that), so maybe you're right.

pretty much any LED will burn out soon after you exceed the Vf and
get it turned on. Granted that a quick hit probably won't do it
(I've done dangerous things too ;-) I do find that many LED's are
quite durable to this kind of thing, but there are plenty that will
just go poof in the blink of an eye.

Haven't had any die from testing so far.

and won't kill them if you reverse-connect them. If it stays dark,
stack
two lithium cells for 6V; same lack of reverse-voltage concern.
Been sorting LEDs this way for some time. As for ultimate current
limits per device, well, suck 'em and see.

AFAIK, 6V is likely to destroy just about any LED without some kind
of current limiting. Most LED's will light dimly if you supply as
little as 1 or 2mA.

Again, Dumb Luck on my part. I'll go with the other suggestion about a
current-limited supply and voltmeter if the OP can manage it, but I felt
comfortable recommending watch batteries for quick 'n' dirty sorting
since I inferred the OP was more interested in not throwing the unknowns
away than assembling "proper" test gear. Might be wrong again...

Mark L. Fergerson

 

[Anthony Fremont]

"Mark Fergerson" <nunya@biz.ness> wrote in message
news:m2RHd.6381$0B.2822@fed1read02...

Anthony Fremont wrote:
"Mark Fergerson" <nunya@biz.ness> wrote in message
news:YQKHd.6100$0B.5475@fed1read02...

Mike Chambers wrote:

I have a bunch of loose LEDs. Some of them are completely clear.
Is there anyway that I can figure out the specs for the LEDs? I
no longer have the packaging?

i.e. how many volts they use, and their current?

I am afraid if I hook them up to check them out, I will burn them
out.

First you have to sort by color. Put the leads directly across a 3V
lithium watch battery (no resistor) which will safely light red
LEDs,

You've been lucky. Without current limitation (perhaps your battery
was weak),

Dumb Luck has always played a large part in my life; I always use
batteries from "dead" or otherwise defective watches (I suppose I
might
have mentioned that), so maybe you're right.

Me too.

pretty much any LED will burn out soon after you exceed the Vf and
get it turned on. Granted that a quick hit probably won't do it
(I've done dangerous things too ;-) I do find that many LED's are
quite durable to this kind of thing, but there are plenty that will
just go poof in the blink of an eye.

Haven't had any die from testing so far.

Like I said, you've been lucky. I've killed a few small ones by just
juicing them for a split second with 5V.

and won't kill them if you reverse-connect them. If it stays dark,
stack
two lithium cells for 6V; same lack of reverse-voltage concern.
Been sorting LEDs this way for some time. As for ultimate current
limits per device, well, suck 'em and see.

AFAIK, 6V is likely to destroy just about any LED without some kind
of current limiting. Most LED's will light dimly if you supply as
little as 1 or 2mA.

Again, Dumb Luck on my part. I'll go with the other suggestion
about a
current-limited supply and voltmeter if the OP can manage it, but I
felt
comfortable recommending watch batteries for quick 'n' dirty sorting
since I inferred the OP was more interested in not throwing the
unknowns
away than assembling "proper" test gear. Might be wrong again...

The only problem is that there is risk involved when not using a
resistor.

have fun

 

[Jonathan Kirwan]

On Thu, 20 Jan 2005 06:05:32 -0600, "Leon Sorokin" <lsorokin@tds.net> wrote:

actually if you put 3V across a red LED, you'll probably fry it. most reds
are 1.7V, high brightness are 1.9V. amber, and green are around 2.0-2.1,
whites and blues are 3.3-3.6 usually.

Mark mentioned using a lithium. They aren't going to deliver enough current to
fry much of anything.

Jon

 

[Jonathan Kirwan]

On Thu, 20 Jan 2005 12:31:14 GMT, "Anthony Fremont" <spam@anywhere.com> wrote:

First you have to sort by color. Put the leads directly across a 3V
lithium watch battery (no resistor) which will safely light red LEDs,

You've been lucky. Without current limitation (perhaps your battery was
weak), pretty much any LED will burn out soon after you exceed the Vf
and get it turned on.

Have you looked at the battery specs for a CR2016 or CR2025 or CR2032?

I don't think it would be possible to burn out a red LED with one.

Alkaline? Yup -- that would be bad. But these lithium button batteries used in
watches just aren't designed to deliver appreciable currents.

Jon

 

[Jonathan Kirwan]

On Thu, 20 Jan 2005 20:22:06 GMT, I wrote:

You can also achieve something very similar with a single 1.5 AA alkaline
battery, using a hand-wired transformer (easily done with a couple of feet of
magnet wire on a tiny toroid) and two resistors and a capacitor and a simple
1N4148 diode. If you are interested, I can post that, too.

Forgot to mention that it also needs the NPN transistor. Oh, well.

Jon

 

[Anthony Fremont]

"Jonathan Kirwan" <jkirwan@easystreet.com> wrote in message
news:hn20v0hj9sr97bnmu2vjffa3da69uinff9@4ax.com...

On Thu, 20 Jan 2005 12:31:14 GMT, "Anthony Fremont"
spam@anywhere.com> wrote:

First you have to sort by color. Put the leads directly across a
3V
lithium watch battery (no resistor) which will safely light red
LEDs,

You've been lucky. Without current limitation (perhaps your battery
was
weak), pretty much any LED will burn out soon after you exceed the Vf
and get it turned on.

Have you looked at the battery specs for a CR2016 or CR2025 or CR2032?

Honestly I haven't. It makes sense that being able to deliver high
current supply wasn't a primary design consideration, long shelf life
was.

I don't think it would be possible to burn out a red LED with one.

That may be, they're pretty tough (mostly).

Alkaline? Yup -- that would be bad. But these lithium button
batteries used in
watches just aren't designed to deliver appreciable currents.

I'm actually kinda surprised that they won't supply much current because
I have an SLR 35mm camera that uses a lithium battery (2cr5). Besides
physical size, does anyone know what the big difference is that allows
it to output, what must amount to, several amps of current when it
recharges the flash?

 

[Jonathan Kirwan]

On Thu, 20 Jan 2005 20:52:11 GMT, "Anthony Fremont" <spam@anywhere.com> wrote:

I'm actually kinda surprised that they won't supply much current because
I have an SLR 35mm camera that uses a lithium battery (2cr5). Besides
physical size, does anyone know what the big difference is that allows
it to output, what must amount to, several amps of current when it
recharges the flash?

The CR2016, according to Energizer's data sheet, is about 38 Ohms rising
offscale past 120 Ohms shortly after 70mAh of use. They point out that it can
be used in pulse applications and supply about 6.5mA.

Assume that 38 Ohms is fixed, independent of current being supplied (it's not,
it gets worse when you pull more), then if your LED needs 2V and the battery is
supplying a peak of about 2.9V (about right for most of its life) you will get
about 0.9V/38ohms or slightly less than 24mA. Which is about right for 2V on a
red LED (the old style red LED is about 1.55V+21*I, which in this case is about
2.05V at 24mA -- about 2V.)

I doubt the CR2016 would supply even that 24mA, though.

Now, if you look at the fatter CR2032, it's all the same. The spec shows about
38 ohms, again. They suggest a slightly higher pulse current as an example,
7.3mA, but the device is otherwise very similar except for its greater energy
storage.

None of these things deliver the energy very quickly.

The 2CR5 you mention is NOT a button battery. And it is designed to deliver a
20mA continuous current, an ability to actually provide 1.5A continuous, and
with peaks of up to 3.5A!! -- as compared to a button battery's design for about
0.1mA and ... maybe up to 0.5mA. That's quite a healthy difference. Notice
that the 2CR5 weighs some 38 grams as compared to about 2 grams for the button
batteries.

Comparing those two families is TOTALLY crazy-minded!!

Jon

 

[Robert Monsen]

Jonathan Kirwan wrote:

On Thu, 20 Jan 2005 20:22:06 GMT, I wrote:


You can also achieve something very similar with a single 1.5 AA alkaline
battery, using a hand-wired transformer (easily done with a couple of feet of
magnet wire on a tiny toroid) and two resistors and a capacitor and a simple
1N4148 diode. If you are interested, I can post that, too.


Forgot to mention that it also needs the NPN transistor. Oh, well.

Jon

Another simple scheme, if you can get a jfet, is to use it as a current
limiter:

.--------------.
| |
| |-+d
| | N-Channel JFET
| .-g->|-+s
| | |
| 9V | .-.
--- | | | R
- | | |
| | '-'
| | |
| '------o
| |
| .---------.
| | |
| | Your |
| | LEDS |
| | |
| | |
| | |
| '---------'
| |
'--------------'
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

The JFET automagically limits the current. Start with a 10k resistor,
work your way down to something that makes a normal red LED glow
appropriately.

N-JFET with a Vgs(off) of between -1 and -8, like a BF245A would work
well. Heck, with that JFET, connecting the gate to the source would
limit current to max 6.5mA, since that's the Idss. Using a resistor is a
good idea, however, with different JFETs.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

I agree with Jon, some time ago I worked with some Red, white and blue
LEDs using CR2016 and CR2032 (very common batteries in LED torches) The
batteries won't fry an LED as the batteries can't supply enough
current.

It's common to use the CR2032 for red ones, and two CR2016 for white
and blue ones, I found though that the last option will end with the
LED's life faster.

It's important to remember the LED's are current divices, so I think
that the simpliest thing is to use a resistor to limit the current
below 20mA.

 

[Matt J. McCullar]

Here's a circuit for testing LEDs that works very well.

Use a +5 volt source. Any power supply will do the trick.

Connect a 150-ohm resistor in series with the LED.

If you look at an LED from above, you'll note the circular shape; the flat
spot indicates the cathode.

So....

Cathode of LED to - terminal of power supply.
Anode of LED to 150-ohm resistor.
Other end of resistor to + terminal of power supply.

Turn on power supply, and you're in business!

If you get an LED that won't light up, there's a possibility that it's an
infra-red LED and its light won't be visible. Some video cameras can see
it, though, so try looking at it through one.

 

[The Mighty I]

I have a small bench power supply that I got from Circuit Specialists.
Model: CSI1802X. I start by turning the current knob all the way down and
the voltage knob gets turned up. I hook up the LED to the supply and turn
the current slowly up to 20 milliamps (this is my assumption for all LEDs,
but might not be the case). If there is no current I swap the leads. At
twenty milliamps the forward voltage is drawn and I read the value from the
display.

chris I.

<Mike Chambers> wrote in message
news:2005012000261416807%@news.giganews.com...

I have a bunch of loose LEDs. Some of them are completely clear. Is
there anyway that I can figure out the specs for the LEDs? I no longer
have the packaging?

i.e. how many volts they use, and their current?

I am afraid if I hook them up to check them out, I will burn them out.

mike c

 

[Don Klipstein]

In <dk20v05d59bi3cn4clkr2np74tpe9db5nr@4ax.com>, Jonathan Kirwan wrote:

On Thu, 20 Jan 2005 06:05:32 -0600, "Leon Sorokin" <lsorokin@tds.net> wrote:

actually if you put 3V across a red LED, you'll probably fry it. most reds
are 1.7V, high brightness are 1.9V. amber, and green are around 2.0-2.1,
whites and blues are 3.3-3.6 usually.

Mark mentioned using a lithium. They aren't going to deliver enough
current to fry much of anything.

In my experience, most "keychain" LED flashlights have lithium "coin"
cells and no resistors. It does appear to me that with fresh cells the
LEDs get noticeably overpowered, but the survival rate is impressively
high. I suspect the internal resistance of lithium coin cells limits
current enough for the LEDs to last long enough to avoid noticed problems
with cooked LEDs.

- Don Klipstein (don@misty.com)

 

[Leon Sorokin]

actually if you put 3V across a red LED, you'll probably fry it. most reds
are 1.7V, high brightness are 1.9V. amber, and green are around 2.0-2.1,
whites and blues are 3.3-3.6 usually.

as long as you get some current through the LED, u'l be able to tell what
color it is. 3V will destroy or damage anything other than blue or white. to
be safe, test everything with a 1.5V battery or a well drained 3V. most LEDs
wont be full brightness at 1.5V, but 85%-90% will at least glow.

regards,
Leon


"Mark Fergerson" <nunya@biz.ness> wrote in message
news:YQKHd.6100$0B.5475@fed1read02...

Mike Chambers wrote:
I have a bunch of loose LEDs. Some of them are completely clear. Is there
anyway that I can figure out the specs for the LEDs? I no longer have the
packaging?

i.e. how many volts they use, and their current?

I am afraid if I hook them up to check them out, I will burn them out.

First you have to sort by color. Put the leads directly across a 3V
lithium watch battery (no resistor) which will safely light red LEDs, and
won't kill them if you reverse-connect them. If it stays dark, stack two
lithium cells for 6V; same lack of reverse-voltage concern. Been sorting
LEDs this way for some time. As for ultimate current limits per device,
well, suck 'em and see.

IRLEDs will be a tad more difficult...

Mark L. Fergerson

 

[Roger Johansson]

"Leon Sorokin" <lsorokin@tds.net> wrote:

actually if you put 3V across a red LED, you'll probably fry it. most
reds are 1.7V, high brightness are 1.9V. amber, and green are around
2.0-2.1, whites and blues are 3.3-3.6 usually.

The right way to do this is to send a specific current through the led,
and then measure the voltage over the led.

5mA is a safe current.

Use a voltage source which gives significantly more volts than the led.

So a 6 - 12 volt DC source is suitable.

Calculate the resistor you need to put in series with the led to set the
current to 5mA.

Assuming a 12 V supply, 12 minus the led voltage which is max 3 or so,
12-3=9Volt.

9V/0.005=1800 = 1k8 resistor.

Ok, so you should connect the power supply 12 V in series with a 1k8
resistor and put the led you want to test in series with the 1k8
resistor.

Then measure the voltage over the led. Check visually if it lights up and
in what color.

You have built an "unknown led" tester, from only a battery and a
resistor, and a voltmeter.

What happens if you happen to connect the led backwards?

The led has a back voltage and you will find out in a safe way how much
it is. The 1k8 resistor limits the current to 5mA in both directions.
The led does not work as a lamp when turned the other way around.

So you can try each led twice, once in each direction.
That way you find out the polarity of the led too.

If you want to use a higher current, like 10mA, that is okay.
Most leds work well at currents up to 20mA.

To raise the current to 10mA you lower the resistor to 1kOhm


--
Roger J.

 

[Michael Black]

Roger Johansson (no-email@home.se) writes:

"Leon Sorokin" <lsorokin@tds.net> wrote:

actually if you put 3V across a red LED, you'll probably fry it. most
reds are 1.7V, high brightness are 1.9V. amber, and green are around
2.0-2.1, whites and blues are 3.3-3.6 usually.

The right way to do this is to send a specific current through the led,
and then measure the voltage over the led.

5mA is a safe current.

Use a voltage source which gives significantly more volts than the led.

So a 6 - 12 volt DC source is suitable.

For decades, I've had a 9volt battery with a 1K resistor soldered to one

terminal. The resistor limits the current, and the lead of the resistor
makes it easy to hook up the LED.

It's not only handy to make sure the LED is working (or at least a visible
light LED), but handy to get the polarity right.

Michael

Calculate the resistor you need to put in series with the led to set the
current to 5mA.

Assuming a 12 V supply, 12 minus the led voltage which is max 3 or so,
12-3=9Volt.

9V/0.005=1800 = 1k8 resistor.

Ok, so you should connect the power supply 12 V in series with a 1k8
resistor and put the led you want to test in series with the 1k8
resistor.

Then measure the voltage over the led. Check visually if it lights up and
in what color.

You have built an "unknown led" tester, from only a battery and a
resistor, and a voltmeter.

What happens if you happen to connect the led backwards?

The led has a back voltage and you will find out in a safe way how much
it is. The 1k8 resistor limits the current to 5mA in both directions.
The led does not work as a lamp when turned the other way around.

So you can try each led twice, once in each direction.
That way you find out the polarity of the led too.

If you want to use a higher current, like 10mA, that is okay.
Most leds work well at currents up to 20mA.

To raise the current to 10mA you lower the resistor to 1kOhm


--
Roger J.

 

[Jonathan Kirwan]

On Thu, 20 Jan 2005 00:26:14 -0800, Mike Chambers wrote:

I have a bunch of loose LEDs. Some of them are completely clear. Is
there anyway that I can figure out the specs for the LEDs? I no longer
have the packaging?

i.e. how many volts they use, and their current?

I am afraid if I hook them up to check them out, I will burn them out.

If you are willing to use 6 AA alkalines, you can arrange things so that you use
a current limited driver:

,-------+---------- VP2 (+)
| |
| --- ~ LED
--- \ / ~ under
B1 - --- test
6V --- 4 |
- AA +---------- VP2 (-)
| |
| |/c Q1
+-----| 2N2222
| |\e (NPN)
| |
--- +---------- VP1 (+)
B2 - |
3V --- 2 \ R1
- AA / 120 Ohms
| \
| |
'-------+---------- VP1 (-)

If you now place the LED into the circuit, it will limit the current to it.
Typically, below 19mA. If you now measure the voltage for VP1 (setting your
test leads as indicated) you can compute the current through the LED as:

(VP1/120)

If you measure the voltage for VP2 (again, as indicated), you will have already
measured the operating voltage of your LED. It's just VP2, at the above
computed current.

The 6V of battery B2 should allow this to work for a wide variety of LED colors.

You can adjust the current to something else by changing R1. For example, by
using a 100 Ohm for more current or a 180 OHM or 270 Ohm for less current.

You can also achieve something very similar with a single 1.5 AA alkaline
battery, using a hand-wired transformer (easily done with a couple of feet of
magnet wire on a tiny toroid) and two resistors and a capacitor and a simple
1N4148 diode. If you are interested, I can post that, too.

Jon