120W switching power supply designed to run a 280 LED pannel asking a question

[sam_sam_sam]

120W switching power supply designed to replace most laptop computer power supplies currently available.


Voltage adjustable between 15 and 24VDC. 15-20Vdc / 6A max. 20-24Vdc/ 5A max. Also has a 5V USB jack.


Here is what I want to I want to make this adj-able from 17 volts to 24 volts is there any thing I should watch out for doing this

Thank You

Here is the link to where I am going to buy a few of them
http://www.allelectronics.com/make-a...L-REPLACEMENT/
1.html

I have made these in to a adj-able(by removing the selector switch and putting in a small pot) power supply before and one them I fried the 12 volt zener diode and pop a cap because the voltage when to
30 volts but after I removed the diode and replaced the cap the power supply was working again

Model# PHC-1000K. Perfect for small appliances and breadboard projects. Provides 3 / 4.5 / 6 / 7.5 / 9 or 12 Vdc, and it's regulated,
so the voltage doesn't fluctuate when the load varies.

http://www.allelectronics.com/make-a...-OUTPUT/1.html

[mariushm]

PHC-1000K is too expensive. I'm not sure how "regulated" it is, no matter what the description says.

You can build one yourself for much cheaper and it would be very regulated but it won't look as nice.

You get a 12v 1.5-2A transformer, use a bridge rectifier to convert to DC and then get a simple linear regulator or a switchng regulator on a heatsink to go down to whatever you want.
It won't be enough to run those leds.

The panel leds have 2-3v voltage drop, so it depends how those leds are connected - it looks like they're about 8-9 leds vertically. If that's how those are connected, I can see why the panel would need 18-24v.
The one in the link looks ok.

[sam_sam_sam]

PHC-1000K dose regulated fairly well I do agree that it is a little costly but I need a small board to a job

[999999999]

The LEDs don't appear to be heatsunk very well (just a guess, I can't see the back) so you probably want to run them at no more than 20mA per. They are going to need a lot closer to 3.0V than 2.0V being white LEDs, maybe slightly more than 3.0V but let's call it 3.0V for now...

280 * .02A * 3V = 16.8W

$40 for a 120W PSU seems overkill. Mariushm mentioned the issue of how the LEDs are arranged, have you determined they are actually wire as parallel runs of 8 LEDs in each series? If so, you should aim for at least a 26V PSU to give a little margin then limit current to 20mA with a series resistor on each series of LEDs. Trying to adjust the PSU to some exact voltage to cause 20mA or whatever your target current, may not work very well as their forward voltage drop decreases as they heat up, unless you can accept having them at lower brightness until they are warm which shouldn't take very long.

However things get easier if you can determine the forward voltage needed at the target current, which is something you can test and measure. If a 24V PSU would do the job things get a lot easier and cheaper as you can get 24V laptop PSU off eBay and elsewhere for about $10 to $20. Even the crude generics should be capable of more than 16.8W, though as always the generics are most likely to have poor capacitors in them. I found about 150 listed on ebay by just searching for "24V laptop".

It all comes back to how the LEDs are wired. If there are 8 in each series, then 35 parallel series will have about 0.7A total current. There are a few 24V 1A switches I saw on ebay for about $6 delivered when I searched "24V switching".

[mariushm]

Exactly, figure out how that panel is made first.

With so little loads, you could actually even make your own.

A 20 V AC 1.5A transformer is 15$ : http://search.digikey.com/us/en/prod...1052-ND/952922

With a 0.5$ bridge rectifier you get about 28v DC, 1A+ from it. That's close enough, you can just play with the resistor value to limit the current through the chains of leds. It's not regulated, but leds don't really care about it. if you really want to... you can use a linear regulator to drop the ~28v to a fixed 24-26v. Something like LM350 should work : http://search.digikey.com/us/en/prod...0TFS-ND/458688

For such low loads, there's no need for switching power supplies unless you need the psu to be very thin.

[ReverendJones]

The rating on the LED panels is 24vdc @ 0.4 amps.
http://www.goldmine-elec-products.co...?number=G17973

[999999999]

If they are going to be used as general lighting that stays on several hours a week or longer, I'd definitely go with a switching PSU, IF it's not going to run outside where temperature extremes could be a problem, to save power.

Thanks for the link RJ. It does seem to be 8 LEDs per series, or at least that would make the most sense. I'd just get the best quality fixed 24V / 1A switching PSU I could find on ebay that doesn't have a permanently sealed casing so if I ever wanted to open it, I'd only need a screwdriver instead of a vice and prying tool.

I'm still confused though, with the panel spec'd to run from 24V and 24V PSU available, what need is there to buy new PSU that the plan is to hack them or is this meant to be some sort of dimmer circuit for the light? If so, it seems as though you might need a fixed value resistor in series somewhere with the POT so that the value never goes out of bounds to cause the voltage to get too high as it did previously. Without knowing exactly how the circuit is arranged that's all I can suggest, that I am assuming it's just taking the feedback through a voltage divider made of resistors you are replacing with the pot and with the pot at one end of its rotation, there is not enough resistance in the voltage divider section of the circuit to keep voltage from rising too high.

[sam_sam_sam]

Thanks to all that reply to this post

Originally Posted by 999999999

The LEDs don't appear to be heatsunk very well (just a guess, I can't see the back) so you probably want to run them at no more than 20mA per

....> No there is no heat sink

They are going to need a lot closer to 3.0V than 2.0V being white LEDs, maybe slightly more than 3.0V but let's call it 3.0V for now...

280 * .02A * 3V = 16.8W

$40 for a 120W PSU seems overkill. Mariushm mentioned the issue of how the LEDs are arranged, have you determined they are actually wire as parallel runs of 8 LEDs in each series?...> Yes they are set this way

If so, you should aim for at least a 26V PSU to give a little margin then limit current to 20mA

.....> (with a series resistor on each series of LEDs.)

<.......( they are on the board)

Trying to adjust the PSU to some exact voltage....>

to adj brightness of the leds with out removing the series resistor on the board <......

to cause 20mA or whatever your target current, may not work very well as their forward voltage drop decreases as they heat up, unless you can accept having them at lower brightness until they are warm which shouldn't take very long.

However things get easier if you can determine the forward voltage needed at the target current, which is something you can test and measure. If a 24V PSU would do the job things get a lot easier and cheaper as you can get 24V laptop PSU off eBay and elsewhere for about $10 to $20. Even the crude generics should be capable of more than 16.8W, though as always the generics are most likely to have poor capacitors in them. I found about 150 listed on ebay by just searching for "24V laptop".

It all comes back to how the LEDs are wired. If there are 8 in each series, then 35 parallel series will have about 0.7A total current. There are a few 24V 1A switches I saw on ebay for about $6 delivered when I searched "24V switching".