On 20 Feb, 15:14, "Dave Plowman (News)" wrote:
In article ,
* *Geoff Lane wrote:

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;
http://tinyurl.com/albmpj*Voltage driver

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.

http://tinyurl.com/atfp7m*Current controller

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.


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

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.

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


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

Simplest and most effective way with little LEDs usually.

Adam


--
*If at first you don't succeed, destroy all evidence that you tried *

* * Dave Plowman * * * * * * * * London SW
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