LED Circuit Question

[TKM]

At the residential lighting market in Dallas last week, there were numerous
new fixture designs using LEDs for general lighting including pendants,
downlights, task lights, under-counter and in-cabinet strips, landscape
floodlights and some decorative designs. A couple of manufacturers showed a
type of strip or track system which utilized clip-on modules containing
several LEDs which could be positioned anywhere on the strip. The
interesting thing was that the lights had the same output whether they were
at the beginning of the strip (near the transformer/driver) or at the end
which could be 10-20 meters away.

I thought that a simple constant-current circuit was being used for the
strip with the LEDs in series; but the modules were connected accross the
strip and there should have been a voltage drop at the far end.

Too bad I didn't have a meter with me.

So, the question is, what does the circuit look like?

Terry McGowan

 

[TKM]

At the residential lighting market in Dallas last week, there were
numerous
new fixture designs using LEDs for general lighting including pendants,
downlights, task lights, under-counter and in-cabinet strips, landscape
floodlights and some decorative designs. A couple of manufacturers showed
a
type of strip or track system which utilized clip-on modules containing
several LEDs which could be positioned anywhere on the strip. The
interesting thing was that the lights had the same output whether they
were
at the beginning of the strip (near the transformer/driver) or at the end
which could be 10-20 meters away.

I thought that a simple constant-current circuit was being used for the
strip with the LEDs in series; but the modules were connected accross the
strip and there should have been a voltage drop at the far end.

Too bad I didn't have a meter with me.

So, the question is, what does the circuit look like?

Terry McGowan

With Hans-Christian on this one, driver per module, either linear or
switch mode, they are getting pretty small nowadays, and seems to be a
move in general to 24V rather than 12V for lot of things lowers the
copper requirement.

Adam

Yes, the strip was 24 volts, so that fits.

If I understand what Hans-Christian is saying, there are two circuits
involved. One at the power supply (transformer) which feeds the strip and
another at the LED module. At the LED module, a driver circuit takes the
voltage available at that point in the strip and converts that into a
constant current source for the LED.

Where's the rectifier in the system -- at the LED module or at the power
supply?

Terry McGowan

 

[Andrew Gabriel]

Probably a D.C. rail and switch mode PSU at back of it, no real need
for A.C. bearing in mind if you have a 100W of LED off your rail ,main
problem is probably weight of the heatsinks ;-)
People like Supertex have been in LED driver business for a while,
things like CL2 linear driver 5 -90V in constant current 20ma out
http://www.supertex.com/products/selector_guides
now everyones catching on, major chip players used to have app notes
for contorting switch mode voltage controllers into current mode, now
theyre all keen to promote low component count current drivers.

I've been playing with designing and making LED-based lights
just recently (having been doing this with fluorescents for
years). Given the lack of constant current SMPSU's accessible
to me, I've found that modifying a conventional SMPSU to add
in current limiting to the voltage feedback circuit is only
an extra 5 components, so I've been doing that for now. (I
really want to avoid the extra heat from a linear current
regulator anywhere near LEDs.)

Koreans have got drive electronics down to about zero, SSC Acriche
line voltage LED all the way up to 230V
http://www.seoulsemicon.com/en/product/prd/acriche.asp

Very tiny high power high efficiency SMPSUs have been becoming
increasingly common on computer motherboards for some years
now to supply all the various components with their optimum
voltages. I could easily imagine this being extended to LED
lighting.