LED, serial or parallel, sink or source

[Keith Baxter]

Hi,

What is the popular type of LED connection when 2 led's are required for an annunciator? Serial or parallel? And of course why...pros and cons...
For me I would prefer parallel. Simply if one led go's pop then at least the other will still work. In series both will not work.

What is the resistor value if connected in parallel Still 180Ω or 82Ω and do I use one resistor for the pair or one each led?
Using 5V 18A PSU

Keith

[Ralph]

Depends on the forward voltage of the LED. If you have two LED's with a 2.6 V forward voltage, then they might not turn on.
I usually prefer series if it possible, that way you're burning less energy.

[Keith Baxter]

Ralph,

This is two LED's running on 5v with one 176Ω resistor. Resistor and led's are not even warm. Brightness is good

This is what the annunciator looks like from the back.

Keith

[Keith Baxter]

@Ralph

That combo I posted above only draws 15.5ma. Is that high? what should it be?
That means if the mega can handle 800ma then I can drive 50 LED's with comfort on a mega which has 52 pins available for LED's.


Keith

[jph]

Normal way is a constant current supply - you can use a 317. Then you can you any number of led's in different illuminations using a pass transistor / Mosfet.

As for a 800mA on a mega ????? - sorry my friend - not a snowball's chance in a feminist's knickers.
200mA - at a push. - 100mA more realistic.
Joe

[Keith Baxter]

Normal way is a constant current supply - you can use a 317. Then you can you any number of led's in different illuminations using a pass transistor / Mosfet.

As for a 800mA on a mega ????? - sorry my friend - not a snowball's chance in a feminist's knickers.
200mA - at a push.
Joe

Joe ,

I seem to find this in a few places.

With a normal Mega 2560, the maximum current output when connected via an external power supply is approx. 800mA.

Am I missing something??
Yes each pin has a 20ma limit.


Keith

[jph]

Normal way is a constant current supply - you can use a 317. Then you can you any number of led's in different illuminations using a pass transistor / Mosfet.

As for a 800mA on a mega ????? - sorry my friend - not a snowball's chance in a feminist's knickers.
200mA - at a push.
Joe

Joe ,

I seem to find this in a few places.

With a normal Mega 2560, the maximum current output when connected via an external power supply is approx. 800mA.

Am I missing something??


Keith

depends where you are reading Keith my firend. 800mA OUTPUT CURRENT ? ------- VCC = 5V ? - W=I*V = 5 * 0.8 ? = = four WATTS - I mean -- 4 watts DISSIPATION ? ................... ?????
You are reading the wrong websites mate. A current sink or source is one thing - per pin ... BUT - and this is REALLY important ----- doesn't in any way, shape, or form mean that every pin can be used at the same time.

[jph]

Have a look at CCS with an LM317 and a pass mosfet for LED CC supply in parallel.

[Ralph]

It depends on the number of lights/buttons you need. But it makes quite a difference. For example if you have an LED with 2.4V VF, you're burning away 4 mW, place those two in parallel and you're burning 104 mW, quite a difference.

But this also depends on the LED's you're going to use.

[Keith Baxter]

Hi,

Ok so I have 6 x arduino mega's driving the fore and aft overhead panels. there are 120 annunciator LED boxes each with 2 x 2.7v high power LED's. They do not all light up at once except when in test mode.

The way the led set count per mega is configured the maximum number of sets is 26 . With an average of 20 on each board.

there are ,
120 x led annunciator sets.
9 x 28byj steppers.
7 x MAX7219 drivers
5 x 10K pots
3 x encoders
the rest are rotary and toggle switches.

Arduinos and usb hub driven by a 5v 18A PSU


These LED's need to be able to have their intensity/brightness adjustable.

What do I use to drive all those LED's without over driving the megas?

@) the back-lighting will probably drive about 200 LED's. This also needs to be driven by one arduino pin.

Keith