Variable Voltage DC power supply question

I think you'll find what you're looking for in this thread from a week or two ago: https://ogrforum.com/t...ighting-transformers

My recommendation is to use a salvaged computer power supply and use some of the cheap PWM controllers from china discussed in that thread, if the 3.3v, 5v, or 12v outputs are not enough variation in voltages.   There is also discussion of a 0-30 VDC 10A bench power supply that can be had for something like $55, if you are not the sort to want to have to buy a box to stuff a couple parts into.  

JGL

Let's see; 10 amps supply, 20 ma per LED. That works out to 500 LEDs. How many did you say you wanted to power up?

Most newer 500+ watt PC power supplies can put out 30+ amps @12 VDC and 20+ amps each @ 3.3 VDC and 5 VDC. So you have lots of options. If you are feeding 10 or 20 LED's in one building say, or something similar, you can work out a resistor value to feed them as a group, depending on which supply voltage you use. Otherwise you will be in the market for a boatload of 20 ma current limiters.

Rod

With a variable voltage power supply set to less than 12 volts I don't think I would need any current limiting devices. Also, the higher amperage would give me the ability to place LEDs  as ambient lighting. But, my question remains, there are many choices, many vendors all selling what appear to be (judged from the advert photos) the same item at different prices. I realize we're not talking about lab-quality here but "good enough." That is good enough not to burn down your house. As for lower voltage devices, such as Woodlan Scenics 4.5V  buildings, that would work from a PC power supply.

Rick

LED lighting almost always needs current limiting at some level.  I seriously doubt all your lights will have identical LED bulbs and the same wiring scheme!  LEDs are current mode devices, for a given LED a 10% change in voltage can result in many times the current flow.  While and blue LED's have similar voltage curves, here's a plot that includes a blue LED.  Note that at 3.6-3.7V, the LED current is at 20ma, a normal value.  However, by the time that the voltage is at 4V, the current has over doubled to way over the rating of a common LED.

Rick posted:

With a variable voltage power supply set to less than 12 volts I don't think I would need any current limiting devices. Rick

 

 

 

Respectfully; totally wrong. Refer to grj's post. At anything over 3 volts you will likely smoke any of those LED's.

Rod

Rick posted:

 

...I would like to power some LEDs so critical voltage regulation is not essential although I don't want a unit with  a high initial surge...

There may be some misunderstanding on what you kind of LEDs you have.

Are you referring to those 12V DC strips of LEDs?  Are you referring to single pre-wired LEDs that operate on 12V DC?  Are you referring to 12V LED bayonet/screw-in bulbs that are meant to be drop-in replacements for incandescent type?

OR are you talking about bare LEDs that operate at 2-3 Volts DC and, as is being pointed out by others, would need additional components such as resistors to operate directly from a 12V DC supply - even a variable/adjustable one.

To answer the OP's original question, I have an Astron SL-11a that I acquired years ago.  I use it to power many accessories.  It has an internal adjustment from 11 to 15 VDC.  However, unless you can find a used one, they are a bit expensive at around $100.  A used PC power supply would be a good option but they are not adjustable.

Regarding LED's, as others have pointed out, you can not "voltage" limit a bare LED as they are current devices.  A PN junction drops roughly .7 volts (unless the spec says differently, and they often do) and they all have a current rating.  For bare LEDs you would have to figure out the resistor value to properly limit the current through the LED based on the voltage and current rating of the LED and the voltage of the power source.

Tony

I was referring to the 5 meter reels of 5050 LEDs. They seem to operate fine on 12V DC but they are way too bright.  I thought I could cut along the cut lines and power them with reduced voltage to dim them. Then I would connect  to terminal blocks. If I had a source of variable DC voltage I thought this would be a simple solution. Yes, I realize there would be a lot of wiring involved. Yes I can solder. Only now, I'm more confused than ever.

Also,  a bench type power supply would allow me to test components. So if anyone has a recommendation for any particular model say $50- $75 range please post it.

Thanks,

Rick

If you're using ONLY the 12V strips, variable voltage will do the trick, they do have some current limiting already included.  However, if there are other LED types in the mix, then it gets more complicated.

30V 5A Variable Adjustable Precision Digital DC Power Supply, $39.45 shipped from a US seller.  eBay: 222181019956

grj; slick looking power supply. Looks like a good replacement for my tired old 1 amp DC bench power supply. May have to order one.

Rod

There are many similar models.  I actually bought a different style that has a 10A capacity, I figured I'd not run out of juice with that one.

This one is $65 with free shipping.

Rick posted:

I was referring to the 5 meter reels of 5050 LEDs. They seem to operate fine on 12V DC but they are way too bright...

If you are using 5m LED reels, and they are "way too bright" I'd guess you probably need to cut the brightness in half to start.  LEDs have the interesting property that brightness as measured in light output is exactly proportional to current.  Double the current, double the brightness.  Halve the current, halve the brightness.  Here's a plot from one section (3-LEDs) of a strip from an earlier thread showing the current vs. voltage relationship.

In this case, you apply 12V DC and it turned out to be about 18 mA of current.  So to cut the brightness in half (to about 9 mA current) you'd need to apply just above 10V DC.  This kind of gives you an idea of the voltage range you need to manage; in other words no point in going below 8V since there will be no current and hence no light at all.

The other comment that others have made perhaps indirectly is do you really need a 10 Amp power supply?   A typical 5m LED strip is 100 cut-able sections of 5cm length with 3-LEDs per section.  At 12V DC, each section draws about 20mA so the total strip draws 2000mA or 2 Amps.  That's 300 LEDs.  So 10 Amps could drive 5 full strips or 1,500 LEDs!!!  I'm not questioning the value of having current in reserve, but do you really have that big a layout?!   And if applying 12V DC is "way too bright" and you dial your power source down to 10V DC (or so) to cut the current in half, that same 10 Amp power supply can now drive 3,000 LEDs!!!

So you have many options.  Since you'd be using the bench supply at 12V DC (or less), you really are buying a 12V, 10 Amp supply.  That's 120 Watts or about 50 cents per Watt.  Yes, it's all in one neat tidy package, but as mentioned previously you can get fixed-output 12V DC bricks (wall-warts) for closer to 10 cents per Watt.  Then you can use low-cost DC light-dimmer modules to chop this voltage down to between 8-12V DC and lower the overall cost.  Everyone has their opinion on building lighting but I think having a bit more control over different buildings, areas, etc. is worth the extra wiring of multiple knobs/controls...rather than just one global control knob controlling all of those thousands of LEDs.  You can then use the money you save and buy a 3 Amp (or whatever) bench variable supply to "test components" on the bench.