Things to consider when configuring LEDs in a series or parallel circuit.

 

Heh, my bad.

Adjuster: I have several sets of AC driven Christmas lights that don't flicker. You can see some flicker if you shake them fast, so as the doctor said; "don't do that!"

I thought Christmas lights are supposed to flicker.

 

I thought Christmas lights are supposed to flicker.

What do we have here, a comedian????

 

What do we have here, a comedian????

Perhaps, "Twinkle" would be more apropos!

 

Hey guys,

Thanks for all the replies.

@marshallf3, I like the heater idea, I think I will do that if I have the space.

@Ernie, I already have a 24VDC transformer and the voltage regulator, so I'd rather use that. Thanks for the idea though. I'll be sure to pick up some Acrylic caulk.

@Adjuster: I like the idea of the zener diodes to keep the series chain running in the event of one LED failure. I've included a revised schematic to show this. So what I have is a fuse to protect against short circuit failure of a chain and a zener to protect against open circuit failure of any LED. Does this configuration make sense? The 3 chains make sense because I have three separate parts to the display, so this way I can also keep the voltage requirements low.

@CDRIVE: I don't think the resistors are needed for each LED in parallel configuration because it's a constant current source, not a voltage source. Also each chain is fused in case something goes wrong, but under normal operation it should work correctly.


Thanks again for all the comments!

 

Connecting LEDs (singly or in multiple strings) in parallel without individual ballast resistors is not really advisable. Good current sharing cannot be guaranteed, especially if you are not sure that the LED forward voltage is very tightly standardised. Some manufacturers do make products which seem to break this rule: of these, some will employ carefully graded devices, others may be flaky.

The negative temperature coefficient of the LED forward voltage can lead to a risk of thermal runaway, where one LED or group of LEDs takes a larger and larger share of the current.

If you really want a reliable result, do yourself a favour and waste about 10%* of the total voltage in a resistor at the top of each chain. On reflection, a fuse might almost do that on its own, if it is a really low current rating. See link: http://www.cooperindustries.com/con...s/Data Sheets/Bus_Ele_DS_2052_S500_Series.pdf

Edit: *Some would suggest more than 10% resistor drop from a voltage source, but you are using a current driver.

 

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@CDRIVE: I don't think the resistors are needed for each LED in parallel configuration because it's a constant current source, not a voltage source. Also each chain is fused in case something goes wrong, but under normal operation it should work correctly.

Those three parallel strings of series LEDs should not be wired like that. Furthermore, the way that's wired you're not guaranteeing constant current though each LED.

Curious.... Where did you find 24mA Fuses? I need to get some.

EDIT: Eh! It appears Adjuster actually found some close to 24mA!
http://www.cooperindustries.com/con...s/Data Sheets/Bus_Ele_DS_2052_S500_Series.pdf