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/diy/ Do-It-Yourself

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wat 2012-11-14 13:01:34 No.331323

[Missing image file: IMAG0336.jpg]

So, I need to drop 12v from my moped to 8v to power this LED light.
I was thinking diodes in series but that would take a lot of diodes and they would get really hot.
Then I thought of an LM7808, but that can only handle 1A, the light will take 10-12 watts at 8v which is 1.25 to 1.5A, so that wont work unless I run 2 in parallel, but I don't know if you can do that.

Somebody said I could use a 7808 with a 2n3055 transistor, but I have no clue how to design a circuit to utilize this.

Here is a link to the light:

Halp please.

Anonymous 2012-11-14 13:03:12 No.331324
Use a resistor? Or am I missing something here?

wat 2012-11-14 13:08:50 No.331329
Would this work?
How do I calculate the required resistance and resistor wattage capacity?
Also, efficiency is not mandatory, but would be nice, so if you have a link to a cheap swich mode power supply or something let me know.

wat 2012-11-14 13:14:26 No.331334
After thinking for a minute I don't believe this would be wise, if the load on the stator changed and the voltage sagged or rose, the voltage to the light would fluctuate.
I need something more precise, like an LM7808, but those aren't high enough current.

Pinzfag. 2012-11-14 13:14:52 No.331335
voltage regulator?

wat 2012-11-14 13:16:11 No.331336
LM7808 is a linear voltage regulator that regulates to 8vdc, but it can only manage 1a load, which isn't enough.

Pinzfag. 2012-11-14 13:17:44 No.331338
They don't come in higher amperage's? strange

wat 2012-11-14 13:19:24 No.331341
Don't know of any other parts that regulate to 8v with a higher current capacity, but if anybody reading this does, help a nigga out.

Anonymous 2012-11-14 13:44:19 No.331350
Your problem is you're going to radioshack.

Anonymous 2012-11-14 13:47:18 No.331351
Use a resistor. You can use any online calculator. Just use 14v as your maximum supply voltage.

wat 2012-11-14 13:51:13 No.331354
check out

wat 2012-11-14 13:51:44 No.331355
Fuck meant to quote >>331334

wat 2012-11-14 14:32:50 No.331362

[Missing image file: mfw.png.jpg]
Holy fuck.

wat 2012-11-14 14:41:50 No.331366

[Missing image file: 1351186804528.jpg]
This seems too good to be true.

Anonymous 2012-11-14 15:35:01 No.331382
ignition ballast resistor
a few 10watt resistors

You might need more than one resistor if they are small and heat up. This should be a $2 max project. It is just LEDs after all.

wat 2012-11-14 16:18:41 No.331396
These aren't your average LEDs these are high powered as fuck.

Anonymous 2012-11-14 16:22:44 No.331399
>These aren't your average LEDs these are high powered as fuck.

What are they then?

>10-12 watts at 8v which is 1.25 to 1.5A

Use a larger resistor or more resistors. I have a bag of big resistors I've taken off various things. Those work pretty well for an application like what you'd need. Mine are the size of quarters. Mouser, digikey, and that 3rd one I can't ever remember the name of are good places to order the stuff you need to make this very simple.

I suggest coming back tomorrow to give some of the resident EE's a chance to see the thread. You have been BUMPING WAY TOO MUCH THIS ISN'T /b/.

Anonymous 2012-11-14 16:30:54 No.331403

wow, nice find!! yes, little buck converters are thew way to go. Probably 80 - 90% efficiency.

the IDEAL thing would be a buck converter that output a constant cURRENT, not voltage. LEDs run at a fixed voltage only because they usually have a series resistor chosen to make it so. With constant-current out you could eliminate all that wastefulness.

But the DC DC buck converter (buck == DC down, boost == DC up) is a super great choice.

wat 2012-11-14 16:31:06 No.331404
2 other people in the thread have already told me to try resistors, it's not the proper way to do this and if you don't know what you're talking about why are you posting like you do?
I know this isn't /b/, I post here frequently.
Also, I'm not bumping, I'm replying to people.

Now fuck off.

Anonymous 2012-11-14 16:31:56 No.331405

in 1975, the 7808/2n3055 combo would be the solution. its wasteful as fuck today. the DC DC buck thing would have been exotic tech in '75, now its ordinary as hell.

wat 2012-11-14 16:32:13 No.331406
Yeah, the bike light has a built in driver, but it has a max input voltage of 8.4v, while my moped puts out 12v.
So I need to drop the 12v to a 8 or it'll blow the LED driver.
Looks like those will work well though.

Anonymous 2012-11-14 16:33:03 No.331407
You don't want to use resistors for the amperage you are drawing. It would be hard on the resistors and wasteful of the limited output of a moped alternator. You could wind up running your battery down riding around town.
The off-the-shelf solutions posted here are not automotive grade. If you're relying on this light as your only headlight, you probably want a voltage regulator rated for the harsh environment of an automotive application.

wat 2012-11-14 16:38:09 No.331408
Once I get the regulators all dialed in I could dip them in potting material for durability, so that's not really an issue.

Anonymous 2012-11-14 16:42:06 No.331412
It's got nothing to do with potting. You need to do a bit more research on automotive grade power supplies before spending money on this. On the other hand, it's not my money and it's not my ass that's going to be riding around in the dark, so whatever.

wat 2012-11-14 16:43:25 No.331415
It's not a big deal, most people ride mopeds with flashlights taped to the handlebars, I'm a rare exception.
There are no "automotive grade" power supplies to do what I need, so this is the only option.

Anonymous 2012-11-14 16:46:21 No.331416

not trying to start a fight, but you seem to be familiar with electronics, and yet you say "the amperage...would be hard on the resistors".

properly rated resistors last virtually forever as long as they do not overheat.

Anonymous 2012-11-14 16:48:41 No.331418

[Missing image file: moped_headlight.jpg]
If the lighting isn't that important why don't you use a inexpensive, reliable 12V headlight?

Anonymous 2012-11-14 16:49:58 No.331420
>as long as they do not overheat

wat 2012-11-14 16:51:32 No.331421
I am currently, but I'd like more power for a water pump/fan, and LEDs are much brighter and more efficient.
I'm using a 12v headlight right now, look at the pic.
No battery in this thing BTW.

Anonymous 2012-11-14 17:10:13 No.331428

EE to the rescue.

OP, in your particular case, the series resistor is the way to go. I seconded the DC DC buck converter another smart anon found. That is a great thing.

But because your app is so specific, a single resistor adequate.

I'm assuming that your 8.4V LED is made of two 3.6V white LED "engines" wired in series.

The voltage across an LED is an "accident" of it's physics (red=1.34V, white 3.6V...) and the real rating on an LED is the CURRENT passing through it. A small white LED is 3.6V and a large white LED engine is 3.6V, but one wants .03amps thru it the other might have 1.5amp...

So assuming you have an LED module here's the whole design breakdown:

12V supply, 8.4V LED drop, assuming 1.375A (halfway between yr 2 numbers) will give a good margin of error (for 4chan).

12 - 8.4 = 3.6V. you need to drop 3.6V.

Ohms Law: R= E / I. E is 3.6, I is 1.375,

R= 2.6 ohms. hold that thought a second.

Power. The resistor is gonna be dumping that waste energy as heat. Ohm's Law for power is P= I * E. Easy: 3.6 * 1.375

P= 4.95 Watts.

Resistors are usually found in integer increments, so you'd think a 5-watt resistor would be OK. I would get a 10 watt resistor. it wont run so hot, will cost 10% more and not much larger.

Its unlkely you will find a 2.6 ohm resistor. A 3-ohm resistor (two 1.5 in series) would be fine; hardly a loss of brightness (1.2 amps for 3 ohms). Also each 1.5 ohm R could be a 5-watt job.

the buck converter is better for sure, but the resistor is doable.

Personally, in my case, if i had the power resistor(s) in my stocks, I'd use that; if i had to mailorder, I'd get the buck converter!

Everything is a compromise especially technology.

'online calculator' seriously it's ONE DIVISION.

Anonymous 2012-11-14 17:10:58 No.331429
Buy a 12volt led light.

Anonymous 2012-11-14 17:15:00 No.331432
If you want to avoid lots of waste heat you need to use either a switching power supply circuit or a PWM regulator designed for white LED use.

Since it's a moped and is either generating 12V when it's running or charging a 12V battery, I'd just use a standard incandescent headlight and be done with it. Besides which if your LED headlight isn't approved for road use you could end up with a fix-it ticket and end up changing it to a standard headlamp anyway.

Anonymous 2012-11-14 17:15:18 No.331433

your bike's alternator is voltage-regulated. I'm sure it varies + or - 10%, and maybe drops lower when idling slowly, but low voltage wont hurt your LED. "12V" vehicle supplies are usually closer to 13.7V nominal to deal with lead acid battery chemistry.

samefag here, with the math. there;s a lot of tolerance built into this stuff.

If it is vehicular 12V, eg. really 14V, then use a 4 ohm resistor (10 watt).

14 - 8.4= 5.6V to drop
R= E / I
5.6V / 1.375A = 4.07 ohms

4 ohm 5W. Hell, 5ohm 5W is fine. 5 ohms would give you 1.12 amps. I doubt you could tell the difference by eye, 1.1 vs. 1.4 amps.

wat 2012-11-14 17:15:35 No.331435
The LED in in the OP is actually a contained LED/driver in an Al housing that doubles as a heatsink, the forward voltage of the LED is 3.3v and it operates at 2.8a.
The driver in the housing supplies the LED with 3.3v at 2.8a, but it has an input voltage maximum of 8.4vdc, and my moped puts out 12vdc, so I have to drop the voltage before it reaches the driver, otherwise it would be damaged as the capacitors on board are only rated for 10v.
That other anon that posted the dc-dc converter is me, and I'm pretty sure that's the way I'm gonna go, as I want max efficiency.

Anonymous 2012-11-14 17:16:24 No.331436
>white LED
>no battery
You need some extensive circuitry then because that "12VDC" isn't going to be filtered or even remotely regulated to 12VDC. Just stick with incandescent and forget all this other nonsense about running other things, it's a freakin' moped!

wat 2012-11-14 17:16:49 No.331437
Actually at idle the voltage sags quite a lot, my friend has a similar setup and when he's sitting at a light the LEDs blink with every rotation of the flywheel, which is quite annoying.
I may have to rig up a battery or large capacitor to deal with this.

wat 2012-11-14 17:17:29 No.331438
It is regulated, I stole a regulator/rectifier off a chinese scooter.

Anonymous 2012-11-14 17:24:51 No.331444
Why not retain the light's battery pack and use an off the shelf 12V charger to keep the batteries charged?

Anonymous 2012-11-14 17:25:05 No.331445

Th eDC DC converter by itself wont work you STILL need a series resistor for the LED, since it is constant VOLTAGE and the LEDs want a maximum CURRENT.

Without one, if the DC DC converter varies even 1% the current could double i nthe LEDs. Yo ucan work it out with ohms law, tl;dnr.

But lets look at the efficiency anyway. Assume 14V (worst case):

14V * 1.375A= 19.25 watts

CONVERTER AND RESISTOR (note below):assume the converter is 85% efficient; probably damn close. assuming R is 0, which it cannot be, P is 8.4V * 1.375A * efficiency = 13.28 watts.

that's only a 6 watt difference. PLUS you really need thr series resistor anyway! it could be much smaller, like 1 ohm (which wastes 1.375W).

it's tl;dnr on why you need a series resistor. it's knowable. outside the scope of /diy/ i'm afraid atm.

Anonymous 2012-11-14 17:26:34 No.331447

nonsense. LEDs dont need filtering, they are happy with pulsing voltages. Just dont exceed max. current. its not magic.

wat 2012-11-14 17:32:14 No.331454
That's a good idea, but the light comes with a 120v charger, not a 12.
A charger is a lot more complicated than one of those DC-DC converters I linked to.
Don't think you read my posts or looked at the light in the OP, but it's got a built in regulator, these aren't your average LEDs, they are as bright as a car headlight.
All I'm trying to do is drop 12v to 8v because the built in LED driver has a max voltage of 8.4.

Anonymous 2012-11-14 17:40:47 No.331463
>A charger is a lot more complicated than one of those DC-DC converters I linked to.

The light probably uses a standard rechargeable battery pack. There are probably chargers available for that pack for charging from 12V car systems. Look at the output of the AC charger and find a 12V DC charger with the same output specs.
If you can't find a 12V DC charger comparable with that light select a light that you can find a 12V charger for.

wat 2012-11-14 17:41:54 No.331467
But a DC-DC converter costs 3 dollars, that's hardly an issue.
A charger would cost much more.

wat 2012-11-14 17:43:12 No.331468
Not really sure why this thread is still going, I found a solution like 30 posts ago.
Unless somebody could come up with an even better solution, that would be pretty kick ass.

Anonymous 2012-11-14 18:02:37 No.331481
> didnt read

lol, i did, but did not look at link. that was dumb.

then the DC DC converter is the best bet, no question.

Anonymous 2012-11-14 18:03:47 No.331482
Properly rated and used resistors won't overheat unless you wire them incorrectly.

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