[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

 

[Chiron]

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.

 

[Adam Funk]

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!

 

[Jamie]

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

 

[Kaz Kylheku]

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!

 

[petrus bitbyter]

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

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]

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

Brilliantly worded!

Yes it was, and right to the point.


Jamie

 

[Adam Funk]

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?

 

[[email protected]]

It's usually the other way around: exponential at low currents, ohmic
at higher currents.

Some people still think an exponential has a "knee".

 

[Adam Funk]

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.

Ha ha, thanks!

 

[Adam Funk]

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.

Thanks.

 

[[email protected]]

---
There's no quibbling about the word "really", the quibble is about
your assertion that a diode junction is ohmic at vaguely described
qualitative "higher currents".
---


---
"As noted"???

I don't really think an "arbitrary" definition is necessary, since the
location of the knee has been with us for decades.

John, I told you there are people who believe there is a "knee" in an
exponential function. ;-)

What do you think constant dI/DV means?

<...>

 

[[email protected]]

I executed a red LED to see what it would withstand. The LED just
starts to glow at about 1.47 volts and drops 2 volts at 160mA. At 2.7
volts the current is 200mA and brightness is fairly constant over the
range of 20Ma to 200mA. At 230mA the voltage rises to almost 5 volts
and the brightness falls off. The LED dies a little above that, but it
lived a happy life.

Your eye is "fairly constant" over that range. Measure it with a light meter.

 

[josephkk]

And there are people who think that the only kind of resistance is
E/I.

Some famous person once said "When all you know is Ohm's Law,
everything looks like a resistor."

Well, that was me, actually.

Switch famous to infamous and the description of yourself is all too apt.

?-)

 

[DarkMatter]

How come you never post anything with content?

Is that what you call the crap you post?

 

[Numer0 Un0]

Well, that was a great technical contribution too.

Hey, idiot. YOU are UNqualified to make a valid assessment about
anyone else.

You got that, boy?

 

[[email protected]]

After almost a week of brooding, that's all you can come up with?

Week? He's been on the rag for years.