Generating DC voltage

[Cheater]

How do some of you guys generate DC for running LEDs?

Right now I've got one LED string which seems to like somewhere between 14v and 18v (wall wart says 24v - it isnt).
5 more in the mail which probably will want something different.

I intend to be getting only LEDs, and I will more than likely be buying them opportunistically so different sets with different requirements.
No I'm not making it easy on myself I know. :p

Right now my idea is computer psu supplying 12v. Cheap and simple.
Then I'm thinking of making a generic adjustable buck/boost switching regulator board which I can power off the psu, and adjust it for each string of lights.
That way I can just make a bunch of them, put the regulator board out where they are being switched and just run 12v everywhere.

I am starting from scratch (0 lights at the beginning of this year) and I have the knowledge required to design something like this.
The question is are there other ideas people have for supplying DC?
I know the vast majority of the stuff here is AC based.

[Tabor]

Before answering your question are you sure the plug pack is not producing 24V AC. Most light strings in Australia run a 24 V AC.

How are you measuring the voltage from the plug pack?
If with a multimeter are you on the AC setting?

[aussiephil]

Cheater

Same question, how are you measuring that voltage, i'd be suprised if the unloaded voltage was lower than the rated voltage on the outside.

As to your question, I just buy SMPS from here
http://cgi.ebay.com.au/27V-DC-13A-350W-Regulated-Switching-Power-Supply_W0QQitemZ370311936942QQcmdZViewItemQQptZLH_ DefaultDomain_0?hash=item56384fbbae
they have various voltages.

If your going DC then some advice, settle on one or two voltage levels and either buy all your lights to suit or be prepared to make them suit the voltage. trust me it is far easier in the long run.

I run 14k of lights, mixed inc and LEDs all from DC at 28v and 48v this year with the other 2.5k i had being 240v AC

The rare 18v stuff is not used to i can modify the strings.

Oh btw, nearly all xmas LED's sold this year at 24v seemed to require 26v to 30v DC to achieve 20mA per section of LED's.


Cheers
aussiephil

[Cheater]

How are you measuring the voltage from the plug pack?
Actually you just made me go re-read the wall wart. I read 24 the first time but its actually 18.

As to your question, I just buy SMPS from here
$50 AUD for 350W is surprisingly good actually. I'll keep that in mind.
A computer psu is cheaper but you do need to boost it back up.

I'll see what the next batch of lights use and figure it out then. :)
Its just good to know that DC is used a bit since nearly everything here is about AC.

[Cheater]

Attached is a picture of a computer PSU with a boost regulator attached.

$20 350 watt power supply gives 16A of 12v and 20A of 5v which is more than enough to power lights and the logic circuitry respectively. :)
A few bucks worth of parts gives me a range of 15v - 50v with a few amps.
Mind you it started whining at 45v. I'll mainly be using it at 24v.
Also the inductors I've got cant exactly take a lot (hence why there are 3 of them). I'll be making a few boards of these and sourcing proper inductors from Mouser.

[aussiephil]

Glad to see someone using boost convertors, keep having fun and developing.

For many who may read this thread - an alternative view.
Whilst mindful of the DIY nature we all love, i've been down the road of computer supplies, i had seven of them set up, including a nice series connection to have 50v and ultimately gave it up as a too big a pain if you wanted any voltage other than 5v or 12v.
I have even done a PCB for easy connection of ATX power supplies including a switch so that you didn't need to modify any connectors, just plug and play.

The 350w supplies i've been buying, as linked above, have been fabulous and are available in most of the voltages we need for christmas lights. This is not a promotion for that vendor as a few vendors have the same thing. It's about simplicity.

Cheers
aussiephil

[smartalec]

im biting the bullet big time,
i thought of going lower with the amps,
but in the end it's cheaper to buy a big gun at 20amp's:D, cause then 2 would be 40amps;)
not 26a(2x13a) amps:(.
http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=170393975109&ssPageName=STRK:MEWAX:IT
im also got analog panel meters to see whats going on.
30volt meter and a 20amp meter,
along with some auto reset circuit breakers rated at 4a @ 24v
hopefully that protects my investment with the switchmode power supply.

[Cheater]

There is the easy way, and the cooler, slightly cheaper and more flexible way.
I chose the latter cause I can. ;)

I've been wondering about circuit protection.
If you use a good power supply (e.g. a decent computer one) then it has fairly good short circuit/overload/etc... protection.
But its desirable to have a second line of defence, and possibly only take out a portion of the display rather than all at once.
Polyfuses look attractive.

[smartalec]

well when the tranformer costs $90bucks with postage, having meters an trip switchs just add's the protection when i run the display in the rain.
no 240v stuff on my display,
all 24v an 36v bulb lights, an now 24v leds for my arch's.
it will feel good putting this old thing i made to rest. (24v 6amp dc)
http://doityourselfchristmas.com/forums/picture.php?albumid=134&pictureid=662
at the moment i'll leave the 36v one as it is.

[aussiephil]

SM - nice find, pity they don't have 27v versions, i'll have to ask.
I'm looking at buying 600w-800w ones if i can find a decent price.

the picture shows my set from 2009.
4 x 350w at 27v - 52A total and most of it used :)
1 x 350w at 48v

I need nearly 35A extra for my new megatree alone this year, a single 1000w would be good, but 2 x 600w will work just as well :)

Cheers
Aussiephil

[aussiephil]

im also got analog panel meters to see whats going on.
30volt meter and a 20amp meter,
along with some auto reset circuit breakers rated at 4a @ 24v
hopefully that protects my investment with the switchmode power supply.

Just to add, the only protection i worry about input fuses on the controllers, these should be rated to blow before the supply cables melt :)

The Meanwell suplies i'm using are rated to be short circuit and overload protected and all good SMPS should be the same. I haven't personally tested it and keep a spare just to be safe.
I doubt the average $20 computer supply is short circuit proof.

The current meter is a good idea and highly recommended.

Cheers

[westom]

How do some of you guys generate DC for running LEDs? LED lightning is not about voltage regulation. It is about current regulation. LEDs typically require 20 mA. Some LEDs are 3.x volts at 20 mA. Some are at 1.2V. But the typical current through all LEDs - one or a string - is 20 mA.

So you don't need a voltage regulator. You need a current regulator.

Any diode or diode bridge can convert AC to DC. But the trick is to regulate that DC current. Also necessary is galvanic isolation - for human safety reasons. Galvanic isolation means low voltage insulation is sufficient for an LED string.

And finally - no power supply 'blows' to provide safety. Any and all minimally acceptable supplies must be fully shorted - and not fail. Shorting a power supply without damage has been routine and required for too many generations - long before PCs even existed.

[aussiephil]

LED lightning is not about voltage regulation. It is about current regulation. LEDs typically require 20 mA. Some LEDs are 3.x volts at 20 mA. Some are at 1.2V. But the typical current through all LEDs - one or a string - is 20 mA.

So you don't need a voltage regulator. You need a current regulator.

Any diode or diode bridge can convert AC to DC. But the trick is to regulate that DC current. Also necessary is galvanic isolation - for human safety reasons. Galvanic isolation means low voltage insulation is sufficient for an LED string.



Whilst this is all technically correct you still need to have a DC voltage somewhere close to and above the combined series Vf of you LED string otherwise they won't light up.

A string of LED xmas lights will draw more than 20mA due to series-parallel connections of sections for the nominal voltage of the string.

The cheapest way to get constant current is to pick a DC voltage (real filtered DC) and set the current with the correct resistor value. Simple cheap and easy to calculate.

Now if you have 350mA 3W leds or even 100mA 1W leds then a constant current source will be far better.

A typical 120 LED string at nominal 24v will have 12 sections in parallel for a total current draw of 240mA.

a 70 count US string will have 2x35 count sections each drawing 20ma for a total of 40mA. a half wave or full wave bridge will be present to rectify the 110v AC but i challenge you to find a current limit resistor, most xmas light string rely on increasing Vf as current increases to limit max current - especially AC powered strings.

[westom]

Whilst this is all technically correct you still need to have a DC voltage somewhere close to and above the combined series Vf of you LED string otherwise they won't light up.

A string of LED xmas lights will draw more than 20mA due to series-parallel connections of sections for the nominal voltage of the string.
A single string will draw 20 mA. Two parallel strings will draw 40 max. But then must be carefully voltage adjusted so that 40 mA is shared equally by both strings.

aussiephil describes a Thévenin equivalent circuit as taught to all first semester engineers. Even described in Wikipedia. One method of converting a voltage source into a current source. Does not change what was posted. LEDs require a current source - not a voltage source.

[David_AVD]

I think you may have missed Phil's point. The series resistor is what turns the voltage supply into the current source as required by the LEDs.

As for the Galvanic isolation you mentioned, this is already provided by the low voltage power supplies that are used in Australia. We do not use mains (240V) in general for outdoor decorative lighting.

[westom]

I think you may have missed Phil's point. I think you need to read again - and first read the hyperlink citations. Learn what a Thévenin equivalent circuit is. Then post back. LEDs require a current source - not a voltage source.

[David_AVD]

That's right. What we were talking about was the LED string including the series resistor. ;)

[aussiephil]

Hopefully and thoughtful Mod will split this off as it's way off topic.

A Led does NOT require a Current source.

It requires
x current
at
y voltage

x = design target current, lets say 10mA
y = (Vf) Forward voltage drop at x current

How do we supply that.
1. use a Constant current source
2. use a Fixed Voltage equal to Vf
3. use a Higher Voltage than Vf and drop required Voltage across resistor.

LED's a BEST driven with a Constant Current and the only practical way to drive high power LEDs is with a constant current source.

btw "Learn what a Thévenin equivalent circuit is"
Yeah, i learnt all that theory nearly 30years ago and i have the equivalent of an Electronics Engineering degree from my original employer.
Most of it bubbles to the surface every now and again, sometimes it's wrong :)

oh and further i'm not really disagreeing with westom about constant current.


That's right. What we were talking about was the LED string including the series resistor. ;)
Yep

cheers
Aussiephil