# 100 LEDs powered off mains



## LED Boatguy (Oct 29, 2008)

I'm putting together an exposure box using 100 5mm UV LEDs. For products I sell, I normally make an array of series-connected LEDs powered off automotive (boat) voltages, but for giggles (and since nobody but myself will be using it), I thought I'd try doing a mains-run setup. I'm either going to use an HV9910B reconfigured from constant frequency mode (limits you to 1/2 available voltage) to constant off time mode or an NUD4011.

The NUD4011 looks pretty darn simple. Three of those rascals running 30-33 LEDs each should do the trick.







This setup would allow me to use PWM and/or timing with the right timer chip. Looks like it's worth a try.

Am I re-inventing the wheel here?


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## Oznog (Oct 29, 2008)

Well, it's a linear regulator. That means a lot of heat if your voltage gap is wide. With a FWB if you shoot for a low difference in input and output voltage and low current it should be manageable, except there are variables- the supply ripple, the forward voltage, the line voltage. You don't want to run out of voltage and thus dim the LEDs or have too much voltage and thus overheat the NUD4011.


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## LED Boatguy (Oct 29, 2008)

I know the drill with linears. They are happiest when Vin is close to the LEDs' Vf plus the overhead. Ya, there's ripples and surges, but that's what capacitors and surge supressors are for.

OTOH, switchers bring their own baggage to the party. They get cranky when LED Vf is close to Vin. Their duty cycles jump to the 90s and up goes the heat. They don't care for "regular" silicon FWBs. Switchers need ultra-fast diodes in the FWB or they can start interacting and MAJOR gremlins pop up. To top it off, you're risking introducing EMI. The wife doesn't like it when one of my creations stomps on What Not to Wear . Been there, done that.

Whatever driver I choose, it'll be sitting on a 2oz cu plane with a heatsink straddling it and just for yucks, there will be a thermistor next to it ready to fire up a small fan if the chip heats up. Hell, the $1 fan controller will even give an alarm if the fan can't keep up at full RPMs. Love those fan controllers!

I buy from Mouser every week. Do they carry the NUDs? Noooooooooooo! Crap.

PS Hey Oznog: Wait till you see the $20 hot air soldering pencil I made.


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## James Jackson (Oct 29, 2008)

" I buy from Mouser every week. Do they carry the NUDs? Noooooooooooo!"

Time to place an order with Digikey.

Great idea you have.

Regards,

James


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## LED Boatguy (Oct 30, 2008)

James Jackson said:


> " I buy from Mouser every week. Do they carry the NUDs? Noooooooooooo!"
> 
> Time to place an order with Digikey.


 
I've been with Mouser since their catalog was 1/4" thick--though they usually had (or could get) what I needed. I feel like I'm cheating on them, though I find myself importing more and more of the bulk regular inventory. Heh, I got my first overseas package yesterday (0805 caps) that was opened buy Customs/HS. Wonder if i'm now on some watch list :wave:.

I need to have a chat with the OnSemi tech folks on another issue. Maybe I'll mooch a few samples of the 4011 and 4001 from them. I always wanted to torture test those 4001s anyway. They can handle [edit] 24 volts all day (so long as LED Vf is near that) and can handle a 60 volt spike which could make them a good choice for automotive applications for noobs--running 3 1-watters in series. Man, you just don't get much simpler than them for 1 watt applications--500ma max w/o external FET.






I see there's a 1-amp version that just came out--the NUD4101 or CAT4101. Major league heat sinking on that rascal. It needs a +5V Vin though . It's only got a .5V dropout at 1000ma. This thing looks pretty robust in the TO-263 package. I wonder if 20 or so of them, each running a series string, could be throttled by a single PWM source?


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## Oznog (Oct 30, 2008)

Something else to know..

When you rectify 120V AC line voltage, the caps have an average voltage close to peak voltage if they're large versus the load current, peak is about 170v at 120v and 155v at 110v. So to keep the heat at the bare minimum you might try to use 40x 3.7v LEDs=148v. That still makes 660mW @ 30mA @ 170v which is... (more if the Vf decreases) well, it's within spec, but pretty warm.

Now if you run off an inverter, they use a Modified Sine Wave:
http://www.pcguide.com/ref/power/ext/ups/funcOutput-c.html

I've never measured this but it looks pretty clear you get a substantially lower peak voltage since it's just a square wave, and peak voltage determines cap voltage. The cap voltage may be insufficient to meet the Vf needs of the string. On the positive side, AFAIK the DC voltage level of that sine is well regulated, unlike the AC line voltage, so the design doesn't have to deal with the uncertainty. 

I would never use a fan there myself. It's a lot of room, requires venting which lets in dust and whatever, fans are expensive and not guaranteed reliability. Someone could cover up the vents too. In fact I'm not sure where you get the low voltage to run a DC fan in this config. I prefer of course to keep the part's temp well below its Absolute Maximum. A lot of thought needs to be given as to what Absolute Max actually means, in general it means a number you cannot normally run anywhere near in normal operation, not for extended periods. So you need another method to design with. You might go for a max device case temp, figure wattage based on the thermal transfer coefficient in the datasheet, and consider the temp inside a closed housing is likely greater than 25C.


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## LED Boatguy (Oct 30, 2008)

Oznog: I'm realy just doing the fan thing for the practice. I'm using this technique on a LiIon charger I'm building and want to get it right. This is just a UV exposure box for exposing resist film. All the complexity was just because I can . If I wanted REALLY simple, I'd do some math (yeeeeech) and put a capacitor in series before the strings to limit the current and blow off the rectifier altogether--that's how some of the LED EXIT signs are wired--strings of 20, with every other string wired backwards. Half of them are on at any given time and there's no visible flicker--and they're UL!

Anyways, I ordered 40 samples of the chips so I can do some torture testing. I'm really interested in the CAT4101s as they seem really robust and that +5Vin thing is no big deal when they're all mounted on a board anyway. So, there's another bus. I'll probably need it anyways to power the PWM chip(s).

Back to PWM: Has anyone tried to PWM multiple LED drivers? I'm looking to run (to start off with at least) 20 3-series strings of 3W LEDs. That means I need to gang up 20 PWM pins and drive them somehow. Either that or put a small signal transistor on each one and drive them in lieu. This might be worth its own thread.


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