LEDs in parallel

[Adam Funk]

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."

 

[Tom Biasi]

On Tue, 24 Jan 2012 13:08:48 +0000, Adam Funk <a24061@ducksburg.com>
wrote:

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

necessary in your circuit what is your point?

Tom

 

[fungus]

On Jan 24, 2:08 pm, Adam Funk <a24...@ducksburg.com> wrote:

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

So you took a working circuit and added
an extra resistor? What does that achieve?

 

[Chiron]

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

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

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?

In this case you seem to believe that having low values for R2 is an
advantage. Why would it be? You need to limit the current through the
diodes, and because of the way that semiconductors work, a resistor in
parallel would not reliably do this. The reason for this is that a diode
that heats up begins to offer lower resistance, which allows more current
to flow through it, which causes it to heat up. You can wind up with
thermal runaway.

With diodes in parallel, you can wind up with most of the current flowing
through one diode. Since the resistor is valued for twice the desired
current per diode (since it's in parallel), the diode can easily be
stressed into failure.

With individual resistors, the current is limited for each diode,
avoiding this problem. Adding yet another resistor in parallel would
offer no advantage that I can see.


--
I'm always looking for a new idea that will be more productive than its
cost.
-- David Rockefeller

 

[Adam Funk]

On 2012-01-24, Chiron wrote:

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?

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


--
Bob just used 'canonical' in the canonical way. [Guy Steele]

 

[Jamie]

Adam Funk wrote:

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

is uneven forward current. One will dominate the other, which ever one
has the lowest forward voltage.

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.

Trying to regulate the voltage with out over doing the current rating
on the LED is very hard to do, unless you have a current source that is
regulated or simply linearly limited like an R.

Jamie

 

[petrus bitbyter]

"Adam Funk" <a24061@ducksburg.com> schreef in bericht
news:04n2v8xja8.ln2@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

 

[Jamie]

Kaz Kylheku wrote:

On 2012-01-24, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@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!

Yes it was, and right to the point.


Jamie

 

[Kaz Kylheku]

On 2012-01-24, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@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!

 

[Bill Bowden]

On Jan 24, 3:29 pm, "petrus bitbyter" <petrus.bitby...@hotmail.com>
wrote:

"Adam Funk" <a24...@ducksburg.com> schreef in berichtnews:04n2v8xja8.ln2@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

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

-Bill

 

[Tom Biasi]

On Tue, 24 Jan 2012 21:02:16 +0000, Adam Funk <a24061@ducksburg.com>
wrote:

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.

Tom

 

[Tom Biasi]

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.

 

[John Larkin]

On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi <tombiasi@optonline.net>
wrote:

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.

LEDs have a current:voltage slope that's not a brick wall. They can be
run at constant voltage, and often are.

The dynamic impedance of a flashlight-type LED is a lot higher than
the impedance of an AA battery.

John

 

[fungus]

On Jan 25, 2:13 am, Tom Biasi <tombi...@optonline.net> wrote:

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.

Yep, not all of us have an extensive parts bin
or single-click access to all those goodies in
the electronics catalogs.

Wouldn't be the first time I've twisted two or
three higher value resistors together to get
a lower value...

 

[fungus]

On Jan 25, 12:56 am, Bill Bowden <bper...@bowdenshobbycircuits.info>
wrote:

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

Doesn't mean it's a good design that should be emulated...

The problem with LEDs is that their resistance isn't constant,
it varies with voltage.

There's a point where the resistance of a LED drops off
a cliff, allowing enough current through to burn them out.
That voltage needed for that is usually very very close to
their 'optimum' voltage. If you try to run them anywhere
near that voltage without current limiting you're living
dangerously.

 

On Tue, 24 Jan 2012 17:02:49 GMT, Chiron <chiron613@gmail.com> wrote:

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.

You are 100% correct but...

I've run into places where I'm switching on and off tri color leds
from the same supply with a common anode (in my case) then a limiting
resistor in the supply as well as the cathodes may come in handy to
balance light output among different colors.

 

[Tom Biasi]

On Tue, 24 Jan 2012 21:11:23 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi <tombiasi@optonline.net
wrote:

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.

LEDs have a current:voltage slope that's not a brick wall. They can be
run at constant voltage, and often are.

The dynamic impedance of a flashlight-type LED is a lot higher than
the impedance of an AA battery.

John

True, but the slope drops off sharply. Not good design.

 

[John Larkin]

On Wed, 25 Jan 2012 02:07:02 -0800 (PST), fungus <tooby@artlum.com>
wrote:

On Jan 25, 12:56 am, Bill Bowden <bper...@bowdenshobbycircuits.info
wrote:

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


Doesn't mean it's a good design that should be emulated...

The problem with LEDs is that their resistance isn't constant,
it varies with voltage.

There's a point where the resistance of a LED drops off
a cliff,

The typical small (like, 20 to 50 mA rated) LED has the classic
exponential current:voltage curve at lower currents, and gets ohmic at
higher currents. Actually, almost all diodes do this. Check the data
sheets on specific parts.

allowing enough current through to burn them out.

That voltage needed for that is usually very very close to
their 'optimum' voltage.

Not usually true. Again, check the curves on a specific part.

Constant-current drive is better of course, but CV can be usable.

John

 

[fungus]

On Jan 25, 5:54 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

Constant-current drive is better of course, but CV can be usable.

In the context of the thread:

Using CV to run LEDs in parallel near their
optimum current is risky (the first circuit in
the thread).

If you're using CV then at least make sure
each LED has its own resistor. If you're
really paranoid then design for a couple
of mA less then spec.

 

[Adam Funk]

On 2012-01-25, fungus wrote:

On Jan 25, 12:56 am, Bill Bowden <bper...@bowdenshobbycircuits.info
wrote:

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


Doesn't mean it's a good design that should be emulated...

The problem with LEDs is that their resistance isn't constant,
it varies with voltage.

There's a point where the resistance of a LED drops off
a cliff, allowing enough current through to burn them out.
That voltage needed for that is usually very very close to
their 'optimum' voltage. If you try to run them anywhere
near that voltage without current limiting you're living
dangerously.

Doesn't that flashlight design cause inconsistent behavior when you
switch between alkaline & rechargeable batteries?


--
It is probable that television drama of high caliber and produced by
first-rate artists will materially raise the level of dramatic taste
of the nation. (David Sarnoff, CEO of RCA, 1939; in Stoll 1995)