LED downlight internal wiring.

[peter.mitchell]

I'm curious, inside the "globe" of a low voltage LED downlight, are the LED dies in series or parallel?

I know it is better to put LEDs in series as they all will recieve the same current but as they are all thermally connected, thermal runaway isn't as much of an issue because their temperatures should all be very similar.

[Short Circuit]

LED's cannot be connected parallel because small variations in Vf can cause relative large variations in current and brightness.
So each would need at least a resistor, which is a waste of power. Connected in series, only one current control is needed, allowing for a much more efficient lamp.

[David_AVD]

LEDs can be connected in parallel.  There are design constraints to be aware of however.  You can't just parallel any old LEDs you like.

Pure parallel arrangements are not all that common however.  The more usual method is several series strings put into parallel.

[Psi]

LED's cannot be connected parallel because small variations in Vf can cause relative large variations in current and brightness.

That doesn't stop companies doing it in cheap products though :|

[peter.mitchell]

You havent really answered my question but more confirmed what i initially thought. Thanks! 

Also, i just realized i posted this in the wrong forum, can a mod move this for me, i should check better next time.

[David_AVD]

Peter, down lights would almost certainly use a series-parallel arrangement.  Something like this:



The design in question may or may not have a resistor in each series string.

[peter.mitchell]

Yeah, the ones in particular i was looking at were 3 LED or 5 LED units so they'd probably have the 3 in series, but for the 5 all series is a pretty hefty voltage drop, and splittimg for a series - parallel arrangement would be un-even.

I guess i should just get one and test the forward voltage.

[David_AVD]

A low voltage light with several LEDS could even have a boost converter.  National (now TI) make some cool driver chips.

[mux]

In the ones I tore down recently there was a little 12->80V boost converter with all LEDs in series. Seems like the cheapest option, because you only need one control loop for current regulation, and boost converters are very easy and cheap to make (no need for level translation, just one N-channel FET, low-side current measurement, etc.).