LED Current Controller or Voltage Driver

[Geoff Lane]

I've posted a couple of questions recently on GU5.3 12v LED lamps.

Another thing I cannot quite get to grips with is the power supply.

In the screwfix catalogue it shows the following;

Voltage driver Current controller

What would be the reason for using either one or the other of these PSUs.

Geoff Lane

[Dave Plowman (News)]

I'd say it depends on the connection of the lamps - series or parallel.

It's normal to series LEDs and use a constant current driver. Assuming the LEDs are well matched.

If they are connected in parallel and each has its own current limiting resistor etc then constant voltage would be the thing.

[Arfa Daily]

Agreed

Arfa

[Adam Aglionby]

LED lamps rated at 12V will have internal electronics , either resistor or something more complex intside. But the load is too low for most electronic trafos and wound trafos will deliver considerably more than 12V at light loading.

Hence need for a compact electronic trafo that will tolerate small loads.Taken market while to actually supply the need.

More subtle and as far as can see Screwfix don`t sell any lamps that would require this kind of driver.

350mA is the current a 1W , big chip LED, Luxeon Star is a well known brand, need. A 5mm standard LED takes 20mA by comparison. Thats nearly half an amp at 3.2V or so that the LED likes, but not all LEDs are the same and some will go to nearly 4V before taking that much current, but if you fed an LED that takes 350mA at 3.2V too much voltage , even a couple of tenths of a volt, say 3.4V the current drawn will double, triple or even more.The result is a very hot , slightly brighter and extremely short lived LED.

Big LEDs are almost exclusively run with current drivers exactly because of this. The driver might be internal and use a 12V or

120/240V base or an external driver might be used to power a series string, this has advantages on long runs as V drop can almost be ignored, the driver will boost voltage to maintain current round the loop.Also allows for the LED to be subtly mounted as long as it has sufficient heatsink it needs no directly adjacent driver electronics.

Constant current gets round the matching problem to an extent. It ignores differing forward voltages.

Simplest and most effective way with little LEDs usually.

Adam

=A0 London SW

[Geoff Lane]

Thanks for comprehensive explanation.

Geoff Lane