Based on the "output" statements on the pictures you provided:
The LED power supply puts out 12 volts DC, 5 watts, or about .4 amps. The
spotlight transformer puts out 12 volts AC, 60 watts, or
about 5 amps.
This makes sense as a very simple low voltage supply for an incadescent
spotlight strip would be a stepdown transformer and a very
simple low voltage supply for an LED strip would be a cheap DC power supply. [I
can think of other simple ones, but ...]
So your spotlights are expecting AC and your LED's are expecting DC. This is
part of the "why don't the work together" issue.
The spotlight transformer can supply enough current to light the LED's, its
going in the correct direction only half the time.
They may not light up at all but even if you are lucky enough to get them to
light, they will never put out more than half the
light they were designed for.
As John Puckett pointed out, a full wave bridge rectifier would resolve the
'needs to be DC' problem, so lets move on the the
dimming part.
LED's are to some extent a binary device. The do not put out any light until
the voltage reaches a certain level. Then they
conduct and put out light. But unless something is done to limit the current
while they are in the conducting/light giving mode,
they will be destroyed a few seconds later.
I'm not sure how much light they put out as burn up. To prevent the LED from
burning up, the manufacturer has probably installed a
current limiting resistor in the circuit.
Given that the current limiting resistior is fixed, and the LED will continue
to live over a small variance in the current range,
a small range of dimming can be done by varying the voltage. Usually a full
range LED dimmer is implemented by employing a
constant current circuit that will vary the current from 0 to a little bit
below self-destruct and allow the LED to go from off to
its maximim light output. If pictures help,
http://www.mikesflightdeck.com/led_dimmer.htm seems to be pretty straight
forward, but
a goole for "led dimmer circuit" gets plenty of other options.
Arvid
> Hi All,
>
> Here's what I'm trying to do: I have a built-in wall unit that has existing
>12V spotlights using a 300W transformer (see
12VTrans pic link below) that has a lot of excess capacity (only using 60W of
it). I'm adding LED strip lights that are also 12V
and come with their own little power supply (see LED pic links below). I'd
like to cut off the LED strips power supply and just
wire them into my existing transformer that is controlled by a dimmer on the
wall.
>
> The problem: I wired the LED strips into one of the open 60W transformers
>(the 300W 12V unit is actually 5 individual 60W
magnetic transformers). It's a magnetic transformer so it dims nicely with the
wall dimmer. However, the LED lights will not
work. At full power on the dimmer the LEDs come on at normal brightness for a
fraction of a second, then they dim to maybe 10%
brightness and stay that way. As I adjust the dimmer they don't get more than
maybe 20% brightness. Using the LED power supply,
they are full brightness; they only come with an on/off switch, no dimmer.
Does it sound like the LED light strips have some sort
of electronic circuit on them that doesn't like my 12V magnetic power supply,
as even at full power (no dimming) they aren't
working?
>
> I'd love to have these strip lights wired into my existing transformer, but
>in lieu of that my plan is to just use their
original power supplies and give up on dimming them. If anyone knows how I can
make them work or can even just explain why they
don't work, I'd appreciate the insight.
>
> http://i17.photobucket.com/albums/b98/ianmcfetridge/LEDstripsps800x600.jpg
>
> http://i17.photobucket.com/albums/b98/ianmcfetridge/LEDstripsbox800x600.jpg
>
> http://i17.photobucket.com/albums/b98/ianmcfetridge/12VTrans800x600.jpg
>
>
> Thanks, Ian
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