LEDs in parallel

I understand the points about calculating the series resistance for an
LED and the battery voltage, and about using a small resistor just to
be safe even if the battery voltage is "about right" for the LED.

The general advice "do not connect LEDs in parallel" refers to this
sort of circuit:

D1
R1 /---|<---\
ground------\/\/\/\/--| |-----+V
\---|<---/
D2

where even if D1 and D2 are the same model, slight variations could
cause one to burn out.


Of course, the following circuit is OK, provided R1 is correct for V
and D1, and R2 is correct for V and D2. And it can be extended for
additional Rs and Ds.

R1 D1
/----\/\/\/\/-----|<----\
ground-----| |-----+V
\----\/\/\/\/-----|<----/
R2 D2



What about the following variation on the first circuit? Both LEDs
are the same model, R1 + R2 is calculated for V and D1 and R2 is quite
small.

R2 D1
R1 /----\/\/\/-----|<----\
ground---\/\/\/--| |-----+V
\----\/\/\/-----|<----/
R2 D1
R1 will limit the current in both branches. Besides not being

                              R2       D1
                R1     /----\/\/\/-----|<----\
     ground---\/\/\/--|                       |-----+V
                       \----\/\/\/-----|<----/
                              R2       D1

I understand the points about calculating the series resistance for an
LED and the battery voltage, and about using a small resistor just to be
safe even if the battery voltage is "about right" for the LED.

The general advice "do not connect LEDs in parallel" refers to this sort
of circuit:

D1
R1 /---|<---\
ground------\/\/\/\/--| |-----+V
\---|<---/
D2

where even if D1 and D2 are the same model, slight variations could
cause one to burn out.


Of course, the following circuit is OK, provided R1 is correct for V and
D1, and R2 is correct for V and D2. And it can be extended for
additional Rs and Ds.

R1 D1
/----\/\/\/\/-----|<----\
ground-----| |-----+V
\----\/\/\/\/-----|<----/
R2 D2



What about the following variation on the first circuit? Both LEDs are
the same model, R1 + R2 is calculated for V and D1 and R2 is quite
small.

R2 D1
R1 /----\/\/\/-----|<----\
ground---\/\/\/--| |-----+V
\----\/\/\/-----|<----/
R2 D1

On Tue, 24 Jan 2012 13:08:48 +0000, Adam Funk wrote:

What about the following variation on the first circuit? Both LEDs are
the same model, R1 + R2 is calculated for V and D1 and R2 is quite
small.

R2 D1
R1 /----\/\/\/-----|<----\
ground---\/\/\/--| |-----+V
\----\/\/\/-----|<----/
R2 D1


This circuit is more complicated than the others, without offering any
benefit. You've already got two R2's to limit the current through the
diodes. R1 serves no additional purpose. If you want to lower the
current through the whole circuit, simply raise the values of the R2
resistors.

So you should ask yourself what advantage this circuit offers over the
other. How is it better, or what can it do that the others can't?

I understand the points about calculating the series resistance for an
LED and the battery voltage, and about using a small resistor just to
be safe even if the battery voltage is "about right" for the LED.

The general advice "do not connect LEDs in parallel" refers to this
sort of circuit:

D1
R1 /---|<---\
ground------\/\/\/\/--| |-----+V
\---|<---/
D2

where even if D1 and D2 are the same model, slight variations could
cause one to burn out.


Of course, the following circuit is OK, provided R1 is correct for V
and D1, and R2 is correct for V and D2. And it can be extended for
additional Rs and Ds.

R1 D1
/----\/\/\/\/-----|<----\
ground-----| |-----+V
\----\/\/\/\/-----|<----/
R2 D2



What about the following variation on the first circuit? Both LEDs
are the same model, R1 + R2 is calculated for V and D1 and R2 is quite
small.

R2 D1
R1 /----\/\/\/-----|<----\
ground---\/\/\/--| |-----+V
\----\/\/\/-----|<----/
R2 D1



the last two will work.. and the problem is not some much burn out, it

I understand the points about calculating the series resistance for an
LED and the battery voltage, and about using a small resistor just to
be safe even if the battery voltage is "about right" for the LED.

The general advice "do not connect LEDs in parallel" refers to this
sort of circuit:

D1
R1 /---|<---\
ground------\/\/\/\/--| |-----+V
\---|<---/
D2

where even if D1 and D2 are the same model, slight variations could
cause one to burn out.


Of course, the following circuit is OK, provided R1 is correct for V
and D1, and R2 is correct for V and D2. And it can be extended for
additional Rs and Ds.

R1 D1
/----\/\/\/\/-----|<----\
ground-----| |-----+V
\----\/\/\/\/-----|<----/
R2 D2



What about the following variation on the first circuit? Both LEDs
are the same model, R1 + R2 is calculated for V and D1 and R2 is quite
small.

R2 D1
R1 /----\/\/\/-----|<----\
ground---\/\/\/--| |-----+V
\----\/\/\/-----|<----/
R2 D1



--
"Gonzo, is that the contract from the devil?"
"No, Kermit, it's worse than that. This is the bill from special
effects."

On 2012-01-24, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa__at_charter.net> wrote:

LED's work with current, not voltage, and have a forward starting
point of voltage before they even start to exert current, which is the
reason for the R, a simple way to get sufficient voltage but limiting
the current.


LEDs "don't work with voltage" but have a "starting point of voltage".

Brilliantly worded!

LED's work with current, not voltage, and have a forward starting
point of voltage before they even start to exert current, which is the
reason for the R, a simple way to get sufficient voltage but limiting
the current.

"Adam Funk" <a24...@ducksburg.com> schreef in berichtnews:04n2v8xja8.ln2_at_news.ducksburg.com...



I understand the points about calculating the series resistance for an
LED and the battery voltage, and about using a small resistor just to
be safe even if the battery voltage is "about right" for the LED.

The general advice "do not connect LEDs in parallel" refers to this
sort of circuit:

                              D1
                  R1      /---|<---\
    ground------\/\/\/\/--|        |-----+V
                          \---|<---/
                              D2

where even if D1 and D2 are the same model, slight variations could
cause one to burn out.

Of course, the following circuit is OK, provided R1 is correct for V
and D1, and R2 is correct for V and D2.  And it can be extended for
additional Rs and Ds.

                       R1        D1
               /----\/\/\/\/-----|<----\
    ground-----|                       |-----+V
               \----\/\/\/\/-----|<----/
                       R2        D2

What about the following variation on the first circuit?  Both LEDs
are the same model, R1 + R2 is calculated for V and D1 and R2 is quite
small.

                             R2       D1
               R1     /----\/\/\/-----|<----\
    ground---\/\/\/--|                       |-----+V
                      \----\/\/\/-----|<----/
                             R2       D1

--
"Gonzo, is that the contract from the devil?"
"No, Kermit, it's worse than that. This is the bill from special
effects."

LEDs are current drive devices so you will always need to limit the current
somehow. You will sometimes meet LEDs driven directly from a battery but
then the internal resistance of that battery accounts for the current
limiting. Sometimes you see LEDs directly in parallel which may do for LEDs
from the same batch as long as they are not driven to their limits. It is
nevertheless considered bad practice.

Usually you will not use three resistors if you can do with two of them. If
the LEDs differ widely they may also influence each other. Nevertheless
there may be some use for this circuit. For instance you may want to spread
the dissipated heat.

petrus bitbyter

That's what I thought after I posted it. I guess the only advantage
is if you're short of higher valued resistors!

I've seen LED flashlights with 2 white 3 volt LEDs wired directly in
parallel across two AA batteries.

-Bill

On Tue, 24 Jan 2012 15:56:48 -0800 (PST), Bill Bowden

I've seen LED flashlights with 2 white 3 volt LEDs wired directly in
parallel across two AA batteries.

-Bill

They rely heavily on the battery's internal resistance and luck.

That's what I thought after I posted it.  I guess the only advantage
is if you're short of higher valued resistors!

That would come under "Making what you need with what you have."
Stay in there Adam, electronics is fun and never be afraid to ask for
help.

I've seen LED flashlights with 2 white 3 volt LEDs wired directly in
parallel across two AA batteries.