LEDs - Simple Wireing Questions

[ShadowTek]

I just bought 50 3.2V 20mA white LEDs for about $30US total off of
EBay.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=66952&item=3847009435&rd=1&ssPageName=WDVW
I tried to read up on electrical stuff but there is a lot I dont
understand.
For instance, if I use 2 1.5V AA betteries in a series that are 3 Amps
each, then is the output 3 Amps or 6 Amps?
I know voltage doubles when connecting cells in a series but what
about current?
Assumeing that the total current of the two batteries were 3 Amps,
does that mean that these 2 batteries alone could power 150 of these
3V 20mA LEDs if the LEDs were connected in series?
That doesn't sound correct.
Or would that need to be parallel?
Maybe that would work but only for like a few minutes before the cells
were completely drained?
How many of these LEDs could I hook up to 2 AA (wired how?)cells
without using any other form of resistor other than the LEDs
themselves?

 

[Lord Garth]

"ShadowTek" <psistormyamato@cs.com> wrote in message
news:81655c49.0411031531.2678fa25@posting.google.com...

I just bought 50 3.2V 20mA white LEDs for about $30US total off of
EBay.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=66952&item=3847009435&rd=1&ssPageName=WDVW
I tried to read up on electrical stuff but there is a lot I dont
understand.
For instance, if I use 2 1.5V AA betteries in a series that are 3 Amps
each, then is the output 3 Amps or 6 Amps?
I know voltage doubles when connecting cells in a series but what
about current?
Assumeing that the total current of the two batteries were 3 Amps,
does that mean that these 2 batteries alone could power 150 of these
3V 20mA LEDs if the LEDs were connected in series?
That doesn't sound correct.
Or would that need to be parallel?
Maybe that would work but only for like a few minutes before the cells
were completely drained?
How many of these LEDs could I hook up to 2 AA (wired how?)cells
without using any other form of resistor other than the LEDs
themselves?

First problem is the batteries in series are 3 volts and the voltage drop
across one LED is 3.2 volts. This is with no consideration toward the
voltage drop across any form of current limiting. If you don't limit the
current, your LED will overheat and burn out.

 

[John G]

"Lord Garth" <LGarth@Tantalus.net> wrote in message
news:Iceid.15222$bP2.931@newssvr12.news.prodigy.com...

"ShadowTek" <psistormyamato@cs.com> wrote in message
news:81655c49.0411031531.2678fa25@posting.google.com...
I just bought 50 3.2V 20mA white LEDs for about $30US total off of
EBay.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=66952&item=3847009435&rd=1&ssPageName=WDVW
I tried to read up on electrical stuff but there is a lot I dont
understand.
For instance, if I use 2 1.5V AA betteries in a series that are 3
Amps
each, then is the output 3 Amps or 6 Amps?
I know voltage doubles when connecting cells in a series but what
about current?
Assumeing that the total current of the two batteries were 3 Amps,
does that mean that these 2 batteries alone could power 150 of these
3V 20mA LEDs if the LEDs were connected in series?
That doesn't sound correct.
Or would that need to be parallel?
Maybe that would work but only for like a few minutes before the
cells
were completely drained?
How many of these LEDs could I hook up to 2 AA (wired how?)cells
without using any other form of resistor other than the LEDs
themselves?

First problem is the batteries in series are 3 volts and the voltage
drop
across one LED is 3.2 volts. This is with no consideration toward the
voltage drop across any form of current limiting. If you don't limit
the
current, your LED will overheat and burn out.

You cannot say the voltage drop across a led is 3.2 volts.
There are many different leds and some are less than 2 volts.
The proper limiting resistor is absolutely necessary.
3 amps is a lot, maybe the short-circuit current for an AA cell.
A lot of study would seem to be indicated here before short answers are
meaningful.
--
John G

Wot's Your Real Problem?

 

[dB]

psistormyamato@cs.com (ShadowTek) wrote

How many of these LEDs could I hook up .....
without using any other form of resistor other than the LEDs
themselves?

You MUST use a series resistor. Otherwise your l.e.ds are likely to
have a very bright life but a very short one.

 

[ShadowTek]

OK thanks.
I thought 3 Amps for a AA battery sounded strange but I was confused
by the markings on the battery.
They were "LRG AM3 1.5V".
I guess LRG just means large and AM3 means short curcuit current.
I was further thrown off by trying to read the current with my
multimeter from terminal to terminal.
I guess that I was createing a short curcuit which was trying to empty
the battery.

(BTW, my multimeter only reads up to 250mA so do you think reading a
3A short curcuit for no more than 2 seconds would have damaged the
MM?)

The exact specs for the LEDs were 2.8V-Min. 3.2V Typ. 3.8V-Max..
So if intend to run the LED at 2.9V then I should still be able to run
1 LED off 2 1.5V cells right?
..1 divided by .02 = 5 Ohm reistor?
So 2 AA batteries and a 5 Ohm resistor will run 1 of these LEDs for
150 hours?
Or I could wire several LEDs in parallel so long as a 5 Ohm resistor
prefixed every led?

Do LEDs suffer any damage from being underpowerd?
Since we are talking about batteries then the constant drain would be
a problem.
I have a 3 white LED headlamp and I have run the batteries in it all
the way down several times.
It uses 2 flat 3V lithium batteries.
It still seems to work ok.

BTW, radio shack sells their white LEDs for 6$ a peice.
If you are used to payin that much then you should check out the link
I posted earlier.

 

[Bill Bowden]

"John G" <Greentest@ozemail.com.au> wrote in message news:<Bweid.35$At6.1304@nnrp1.ozemail.com.au>...

You cannot say the voltage drop across a led is 3.2 volts.
There are many different leds and some are less than 2 volts.
The proper limiting resistor is absolutely necessary.
3 amps is a lot, maybe the short-circuit current for an AA cell.
A lot of study would seem to be indicated here before short answers are
meaningful.

I have a little dual white LED flashlight that uses 3 AA batteries.
The leds are in parallel, connected directly to the battery,
with *no* resistors. These LEDs apparently have a voltage range
of 3 to 4.7 volts or so.

-Bill

 

[peterken]

could use a voltage regulator and a resistor...
but why not use just a current source ? They are easy to come by...
gets you the same result (stable current through led's while dropping supply
voltage) but cheaper circuit

one more thing.... led voltage is depending on color type, from say 1.2V
upto say 2V
best define setpoints of leds using current i.s.o. voltages, suince
calculations with led voltage might get you faulty resistor values




"BobGardner" <bobgardner@aol.comma> wrote in message
news:20041105072924.12288.00000149@mb-m28.aol.com...

I tried to read up on electrical stuff but there is a lot I dont
understand.
=======================

Most of it is derived from Ohms Law.... E=IR (voltage=current x ohms). The
gist
of it is, voltages in series add, but the current in a series circuit is the
same in all series elements of that circuit... makes sense if you think
about
it. The trend is to use special led driving voltage regulators that drive
strings of several (6 or 7) leds in a string, 20ma each string. The
regulator
keeps the right voltage and current even as the battery voltage starts
dropping. You could use a 12v battery and drive 3 leds in series with a
resistor... 12V from batt - 9v across leds leave 3v/20ma for the resistor to
soak up... 150 ohms. You could drive as many of these 3 diode and a 150 ohm
resistor strings as you can afford.