AC or DC that is the question?

[stuartiannaylor]

I have an application with HB LEDs that I am working on.

The system works much like MR16 low voltage lighting circuit.

I require multiple constant current drivers to control groups of 3 HB LEDs with a individual forward voltage of 3.2V giving approx a 3W total.

Like a standard low voltage system I have a electronic transformer to supply up to 32 devices (100 watt max) and my question is the choice of an output 12Vac or 24Vdc in respect to voltage drop.

The 12Vac is a more standard option in terms of lighting which will require a boost current controller @ 350ma. This solution requires rectification so diodes and smoothing caps are needed increasing cost and circuit footprint.

The 24Vdc not really standard would require a buck controller @ 350ma. This solution allows the rectification at the electronic transformer and cuts the cost of buck input rectification diodes + smoothing capacitors and provides a much smaller footprint for the buck circuit.

In terms of problems with voltage drop over cable lengths of 10m + which do you think would be the best method of supply?

 

[itsmoked]

Hi stuartiannaylor.

(Since you moved this here I've moved my response here so others don't repeat my response.)

That is almost exactly the problem I am working on at the moment.

BTW this is NOT an "Electric power" forum question. This would be better in "Electrical/Electronic" as we are NOT talking kW or MW.

Generally the answer is 24V but as you point out there are trade offs lurking everywhere. Suffice it to say that if you come right down to it, there will be other more important criteria to decide with.

If I were you I would sketch out both designs,(you probably have seeing your mentioned design aspects). Make a table and list the benefit/hassles, and costs in a tabular form for each method. This renders the requirement for wire that can be 30-40% smaller just one of the design items as it should be.

You may decide that if 12V track style transformers are more common then that might greatly override a little wire diameter issue. Or, the failure mode of one type switcher may be more pathological that the other. Or some other aspect may leap out at you from the table. Baring that assign a value to each point and total them up. Find the winner.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MacGyverS2000]

Where do you want to spend the money, in the power supply or in the downline components? Over 30' the voltage drop is minimal for heavier gauge wires, such as those found in outdoor lighting systems (therefore DC would be the choice), but if it's meant for smaller gauge wire it could present a problem (and AC would be the way to go).

I have to ask, though... will the entire system be limited to 350mA (sounds like a single LED), or will there be a number of strings in parallel, increasing the current requirements? Something to consider when making your decision.

Also, your terminology seems a bit skewed. I'm assuming you're coming from 120VAC, which means the 12VAC would be a simple step-down transformer. The 24VDC would be either a buck controller from the 120VAC, or a boost controller from the 12VAC. If you have access to 120VAC, no reason to change it to 12VAC and then to 24VDC, go straight to DC and remove some inefficiency in the system.

I do not know the format of your system, but if it were me (Keith?)? I would go with 12VAC since it is standard and guarantees an easy find on UL-listed transformers. Each LED series strip would have its own current controller, since the cost for even a basic/naive solution (i.e., LM317 in current mode) is extremely minimal. You may even be able to get away with quite small-valued smoothing caps if you set the LED voltage drop a bit low and just eat the extra losses int the current controller (if it's an active controller, the extra losses would be negligible anyway).


Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

So Dan. Rectifying the AC at each strip?

There is a COTS controller that runs from AC to 350mA... It was in a MAXIM design note in that last month or two.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MacGyverS2000]

Yeah, that would be my way out, but again, your particular system requirements may determine otherwise. A few FETs to rectify and maybe a small cap for smoothing (the cap may not even be needed, depending upon light levels). Controlling two or three LEDs with a single voltage reg and some FETs seems a pretty cheap way to go, particularly considering you can go with COTS power supplies and power tracks. You can even mix LED and incandescent lighting on the same tracks without issue that way, and I would consider that a big bonus in many applications.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[stuartiannaylor]

Many Thanks,

Just so everything is clear...

Starting with a mains voltage (120 / 230).

With the minefield of worldwide regulations 12Vac or 24Vdc are suited to practically any install and don't require professionals to install on SELV power lines.

Each LED cluster 3x 350ma HBLED forward voltage approx 3.4v

Small buck / boost drivers are included with each LED cluster so that you have a 12Vac or 24Vdc lower line bus and then up 32 clusters of 3 LEDs with included driver.

Each cluster is approx 3watt so 32 x 3 = 96 watts total.

Does anyone think the 24Vdc will cause any confusion to electricians who are used to 12Vac non polarised power.

Due to having a single electronic transformer and multiples of the driver and LED cluster I am trying to keep the costs and footprint down on the LED Clusters.

Thanks for your input

Stuart

 

[MacGyverS2000]

There's no need for the extra complexity of a buck/boost converter, stick with a simple buck and lower parts count/cost. Wattage is only for the LEDs, I assume, and doesn't include efficiency drops in the converters... you'll need to bump that figure up a good 10-20%, depending upon your converter. Anything new is bound to cause confusion, YMMV.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

Hi Stuart.

Two things. Are you absolutely sure those HBLED clusters are only 3.4V? All of them I have ever seen are put in series to allow the controller to put out one single fixed current to safely drive three LEDS. This mean your drive voltage is 3 x 3.4V = 10.2V

Next, are these "included drivers" already designed? If they are and are not designed to take in AC I would go with DC.

Heck I would go with DC anyway. Higher voltage, lower current, and no rectification issues. Think of the harmonics issues with 32 rectifiers strung on one supply line. You would probably need a transformer vastly larger than you think due to the form-factor. The resulting switching AC/DC powersupply to run the entire chain will be much smaller than the transformer you would need.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com