LED power supply.

[BrianG]

I have a few hundred high brightness white LEDs that I'd like to make some kind of lamp out of. To be portable, I'd like it to be usable on 12v, either battery or car powered.

Since both car and battery power fluctuates, I want to use an LM317T configured as a current regulator so the LEDs get the exact amount of current. I figure the 317T is good because it only "eats up" 1.25v (plus a little more) on the regulator.

But, I can only series-connect 3 LEDs at once with 12v. This would be expensive because I would need a lot of 317T's for each chain.

So, I was thinking about some type of simple step-up DC-DC supply to get something much higher, say 50v. Then, I can use one 317T for each chain of 14 LEDs. I was figuring to use 5 chains of 14 LEDs. If each chain is set to ~25mA, I would need a DC-DC supply capable of 50v @ 125mA (plus a little extra as a cushion). This would mean the primary side (at 12v) would have to supply a little over 0.5A, plus a little extra for losses right?

So, is there anything simple I can use to do this? The secondary doesn't have to be exactly 50v, so I don't need tight regulation. How about a 555 timer triggering a switching transistor that will pulse a small AC transformer to step up the voltage, and then rectifying that?

 

[Hero999]

Why are you using LEDs when a fluroescent tube will give you more light and use less power?

 

[justDIY]

get one of those 35-watt inverters that install in the cigarette lighter socket, it'll give you ~120 volts AC that you can rectify and filter back into high voltage DC. Should run a lot of leds in series with that much voltage.

 

[BrianG]

@Hero999: It's just something to do really. I have the LEDs and some time, so why not? Plus, LEDs don't break as easily as and glass tube.

@justDIY: That's a pretty good idea! lol, 120v is a tad more than I was looking for, but should work fine.

 

[eblc1388]

But, I can only series-connect 3 LEDs at once with 12v. This would be expensive because I would need a lot of 317T's for each chain.

No. You only need one LM317 for as many 3-LEDs chains you would connects(total current less than 1A). Just connect them in parallel. This is possible because the output of LM317 is regulated and additional LED strings would affect other already connected strings.

 

[BrianG]

No. You only need one LM317 for as many 3-LEDs chains you would connects(total current less than 1A). Just connect them in parallel. This is possible because the output of LM317 is regulated and additional LED strings would affect other already connected strings.

I thought of that but was thinking there might be an issue with doing this.

Let's say I have four strings of three LEDs each. And let's say I want 25mA for each string. So, that's 100mA total for all four strings. Everything should work fine if all the LEDs have exactly the same Vf rating. But if three strings take 9.9v (3 LEDs X 3.3v Vf), and the last one takes 9.6v (3 LEDs X 3.2v Vf), won't the 9.6v string try to take all or the majority of the 100mA?

Also, if one string dies because of one series LED, that same 100mA will be forced through the three remaining chains bringing the current up to 33.3mA for each chain right?

Ok, so what if I use a single 317T set to 100mA for all four strings, but use a low resistor, like 5 ohms, for each series chain. Would this protect against the issues above?

 

[justDIY]

a low resistor would help balance the current making up for slight differences in forward voltage, but it wouldn't help if an entire series string failed open.

 

[BrianG]

All right, I'll just use a seperate 317T for each string. No biggie if it's just 4 or 5. But if I stick to 12v, I'll need a LOT more of them since there will be more strings...

I figure I still need a 60v supply, so, I made a little circuit using a 555 in astable mode. I don't have an o-scope to verify the wave shape, but my Fluke meter says it's 60Hz and 50% duty cycle, which I'll have to assume is squarewave-ish. I hooked the output to a darlington NPN transistor in switch mode with a base current set for ~5mA. I found a small AC transformer that puts out ~25v AC with 120v input, so I figure if I feed 12v AC into the secondary, I should get ~60v at the primary. So, I hooked that to the collector of the transistor and +12v with a bypass diode across the winding. I get 34v AC on the output (normally the primary) or the transformer. When I load it with 200mA of load, it drops to 10v. Scratch that idea. I might go with the inverter idea, but that will give me ~170v once rectified. Wow!

 

[BrianG]

A little searching online found this. Seems to be exactly what I need and is small and cheap too.

I'll probably only need one LED chain once the output of that is rectified...

 

[mramos1]

Were did you get all these LEDs? I am looking for a lot of LEDs cheap, white high LUX..

I pan to swap out exit lights FROM flouresent to LED. LEDs do not die near as fast and seem to pull less power.

 

[justDIY]

white leds aside from their novelty are next to worthless ... I suspect a white fluorescent tube to last much longer than a white led

the phosphor rs in low-cost white leds are not high end, they'll wear out quickly. what you'll be left with is a dull blue led in a few thousand hours. not to mention the phosphor is very temperature sensitive and high ambient temperatures, operating or non-operating will degrade it further.

look at combining blue and yellow leds behind a diffuser for a basic white light source, or use rgb leds. Dichromatic leds (blue + yellow) have a pretty poor CRI, so if you're lighting stuff that needs to look good (retail product, food, etc), they'll do a piss-poor job of it, best off using RGB here.

 

[BrianG]

I got them off eBay. They were cheap but I took a chance. I got 100 5mm white 10,000mcd for $15 shipped IIRC.

I also have the same amount of larger 10mm white LEDs but they don't seem to be as bright even though they were rated for high mcd. Must be because of how they are rated with respect to the beam angle and size of the lens.

 

[BrianG]

white leds aside from their novelty are next to worthless ... I suspect a white fluorescent tube to last much longer than a white led

the phosphor rs in low-cost white leds are not high end, they'll wear out quickly. what you'll be left with is a dull blue led in a few thousand hours. not to mention the phosphor is very temperature sensitive and high ambient temperatures, operating or non-operating will degrade it further.

look at combining blue and yellow leds behind a diffuser for a basic white light source, or use rgb leds. Dichromatic leds (blue + yellow) have a pretty poor CRI, so if you're lighting stuff that needs to look good (retail product, food, etc), they'll do a piss-poor job of it, best off using RGB here.

Yeah, the light is almost a blueish light, but good for a portable lamp that will be subjected to some possible manhandling. Safer than any bulb that I can think of for durability (glass breakage). Still, a few thousand hours is nothing to scoff at if you want low power, high durability, and quality of light isn't too important IMO...

 

[justDIY]

here's something to look at if you go with the inverter

**broken link removed**

it's an ultra simple constant current LED driver designed to work straight off line-voltage AC.

 

[BrianG]

That is a simple and nice IC. If I already didn't have the 317T's, I'd take a look at that definitely. I figure two strings of 40 LEDs each will need a minimum of 128vdc each. At 20mA, the wasted heat on each current regulator is less than 1w. Maybe I'll go with 45 LEDs per string for less waste...

Thanks for looking though!

 

[mramos1]

look at combining blue and yellow leds behind a diffuser for a basic white light source, or use rgb leds. Dichromatic leds (blue + yellow) have a pretty poor CRI, so if you're lighting stuff that needs to look good (retail product, food, etc), they'll do a piss-poor job of it, best off using RGB here.

Looking to convert EXIT signs to LED. The fluorescent type in our building seem to go out a lot. Figured I would try LEDs.

I built a lottle board last night, an was able to power 6 with no problem off 110volts. .5uF cap to a 1N4002 FW bridge to strings of 2 with a 330 ohm each string (did not have 220 ohms). Was not bad, I just need more LEDS (bright ones) and I think it will work. Oh there was a fuse in there too. Just in case.

 

[justDIY]

Looking to convert EXIT signs to LED. The fluorescent type in our building seem to go out a lot. Figured I would try LEDs

use red, amber or green leds (depending on the sign) ... you'll get a lot better brightness instead of trying to filter white leds.

 

[Hero999]

The datasheet for that IC says it can only handle a maximum of 10 LEDs in series, you might be able to use the 50mA version and connect the LEDs in parallel with low value series resistors of course.

 

[justDIY]

where did you read there is a limit of ten. I see the example where they calculate numbers for a string of ten.

Only thing I can think that would limit the string size is power dissipation. 1600 mW for the SOT version, figure 50mW for a cheap 5mm led, that'd be 32 leds exactly, so probably 30 would be a safer number?

 

[audioguru]

where did you read there is a limit of ten?

The datasheet says it can regulate the current for up to tens of LEDs which is terrible English. It should say lots of LEDs in series.